- Email: [email protected]
An investigation of the formation patterns of the ionospheric F3 layer in low and equatorial latitudes
Journal of Atmospheric and Solar-Terrestrial Physics 102 (2013) 48–58
Contents lists available at ScienceDirect
Journal of Atmospheric and Solar-Terrestrial Physics journal homepage: www.elsevier.com/locate/jastp
An investigation of the formation patterns of the ionospheric F3 layer in low and equatorial latitudes Jie Zhu a,b,c, Biqiang Zhao a,b,n, Weixing Wan a,b, Baiqi Ning a,b a
Key Laboratory of Ionospheric Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China Beijing National Observatory of Space Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China c University of Chinese Academy of Sciences, Beijing, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 20 October 2012 Received in revised form 21 March 2013 Accepted 22 April 2013 Available online 18 May 2013
Ionogram traces with the F3 layer in different latitude do not always seem similar. In our work, we tend to describe morphological features of traces with the F3 layer in magnetic low-latitude region and near magnetic equator through the quantitative investigation of the diurnal variation and latitude dependence of two morphologically characteristic parameters – the foF2-to-foF3 ratio and the difference between h′F3 and h′F2 – in geomagnetically quiet period. The distribution of two formation patterns (pattern A and pattern B are defined with increasing F3 peak density and with nearly constant or decreasing F3 peak density respectively as the peak moving upward around the onset of the F3 layer’s occurrence) of the F3 layer is also investigated based on statistics of formation patterns of the F3 layer in Sanya and Kwajalein in 2011. The ideal equinoctial distribution (without the summer-to-winter neutral wind) of those patterns is symmetrical about magnetic equator with pattern A in magnetic low-latitude region and pattern B near magnetic equator. When taking the summer-to-winter neutral wind which resists (enhances) the plasma diffusion to higher latitude in the windward (leeward) into consideration in a solstice, pattern A could be observed near magnetic equator in summer hemisphere and pattern B in magnetic low-latitude region in winter hemisphere compared with the ideal distribution in the equinox. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Ionosphere Low latitude F3 layer
1. Introduction The F2 layer stratification has received much attention since the additional layer of F region was first observed in the middle of 20 century distinguished as a ‘spur’ structure near the highfrequency end of the virtual height–frequency ionogram records from ground-based ionosondes (e.g., Sen, 1949; Ratcliffe, 1951; Skinner et al., 1954; Huang, 1974, 1975). This new layer above regular F2 layer was called ‘G layer’ at first (Balan and Bailey, 1995), and then renamed to ‘F3 layer’ because of the same chemical composition as that of the F region (Balan et al., 1997). During several decades, the stratification of the F2 layer at low and equatorial latitude has been investigated extensively (e.g., Jenkins et al., 1997; Balan et al., 1998, 2000; Lynn et al., 2000; Hsiao et al., 2001; Batista et al., 2002; Rama Rao et al., 2005; Zain et al., 2008; Uemoto et al., 2007, 2011; Sreeja et al., 2010; Chaitanya et al., 2012). Generally, the features of the F3 layer under geomagnetically quiet conditions are as follows [Klimenko
n Corresponding author at: No. 19, Beitucheng Western Road Chaoyang District, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China. Tel.: +86 10 82998310; fax: +86 10 82998332. E-mail addresses: [email protected] (J. Zhu), [email protected] (B. Zhao).
1364-6826/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jastp.2013.04.015
et al., 2012a]: (1) the F3 layer can occur during daytime (0800– 1700 LT) within about 7 101 magnetic latitudes, (2) the occurrence of the F3 layer can be frequent on the summer side of the geomagnetic equator at solar minimum, (3) the F3 layer is observed to be dependent on the magnetic field inclination with lower occurrence at the equator and higher far off the equator. Besides the daytime F3 layer, Zhao et al. (2011a) investigated characteristics of the sunset F3 layer using a solar cycle of ionosonde data (1995–2010) from the magnetic equatorial station Jicamarca and evidence shows that the local time distribution of the occurrence of the F3 layer can extend to the postsunset time (18:00–21:00 LT). Unlike the daytime F3 layer, the occurrence of the sunset F3 layer clearly increases with increasing solar activity. The F3 layer not only occurs during quiescent ionosphere periods but also during magnetic storm periods. Recent observations show the existence of storm time F3 layer occurring at the magnetic equator in response to a strong penetration electric field (e.g., Zhao et al., 2005; Paznukhov et al., 2007; Balan et al., 2008, 2011; Lin et al., 2009a, 2009b; Sreeja et al., 2010; Klimenko et al., 2011), or to a non-uniform in height zonal electric field generated by the disturbance dynamo electric field (Klimenko and Klimenko, 2012). Lynn et al. (2000) discussed the problem of the nomenclature to describe F2 stratification. They proposed that the additional
J. Zhu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 102 (2013) 48–58
layer be referred to as F3 or F1.5 depending on whether the transitory layer moved above or stayed below the F2 layer peak which maintained continuity with the prestratification and poststratification F2 layer. The stratified layer is commonly seen as an F3 layer near magnetic equator but as an F1.5 layer away from magnetic equator. Besides the traditional ground ionosonde measurement, new techniques have been used to investigate the F3 layer characteristics. For example, the position of the appearance of the F3 layer is also derived through the single station total electron content (TEC) measurements using radio beacon transmissions from Low Earth Orbiting Satellites (LEOS) (Thampi et al., 2007). They have found a hump structure in the latitude variation of vertical TEC centered at magnetic latitude 7–81N at longitude 801E. Zhao et al. (2011b) infer the global F3 layer structure directly from COSMIC/FORMOSAT-3 radio occultation (RO) data. Statistical results show an accurate magnetic latitude dependence of the occurrence of the additional layer and reveal that the highest occurrence of F3 layer appears at dip latitude 781 during summer months—a few degrees inside the equatorial ionization anomaly (EIA). The spatial distribution of stratification of the F2 layer can also be observed by the satellite equipment. It is identified as the ‘topside ledge’ on the topside electron density profiles recorded from in-situ and the topside sounding ionograms (e.g., Sayers et al., 1963; Lockwood and Nelms, 1964; Raghavarao and Sivaraman, 1974; Sharma and Raghavarao, 1989). Unlike the F3 layer being observed mainly during the daytime, the topside ledge at both the daytime and night was observed by Depuev and Pulinets (2001) using Intercosmos-19 (IK-19) satellite ionograms. Uemoto et al. (2006) reveals that the ionization ledge is observable in almost all local time sectors except the period from 03 to 08LT sector based on several hundred topside ionograms from ISIS and Ohzora satellites. Recently, Karpachev et al. (2012) demonstrated that the topside ledge is most often observed in the afternoon and evening hours, less often at night and rarely in the morning based on large dataset (about 3600 passes across the equator) of the IK19 topside sounding data. Uemoto et al. (2006) examined the possible link between the topside ledge and the F3 layer as regard to the seasonal and the local time dependences of those two structures. The coexistence of the F3 layer and topside ledge, and the statistical maximum occurrence of the topside ledge observed 1–2 h delay with respect to the F3 layer, suggest these two phenomena have close relationship. However, the seasonal dependence of the occurrence probability of the ionization ledge shows contradict manner to the F3 layer. Meanwhile, Karpachev et al. (2013) reveals the latitudinal variation of topside ledge from the IK-19 topside sounding data and showed that the distribution differs from that latitude distribution observed from the ground observation and COSMIC radio occultation result of Zhao et al. (2011b). They suggested that the discrepancy could result from the fact that the bottomside measurement cannot observe the F3 layer over magnetic equator because of smaller foF3 than foF2 and much higher hmF3 than
49
hmF2, but can do in regions away from the magnetic equator due to increasing foF3 and decreasing hmF3. That raises a question: is the F3 layer in regions away from the magnetic equator formed by the transportation of the topside ledge from lower latitude along magnetic line or by local photochemical and dynamical processes? To answer this question, simultaneous ground-based and satellites observations are needed. According to Balan et al. (1998), the physical mechanism of F3 layer formation is the combined effect of the upward E B drift and neutral wind. The F2 peak is transported by upward E B drift to much higher altitude and could be prevented from diffusion along the magnetic field line if the neutral wind is equatorward. Then the F3 layer forms while the normal F2 layer develops at lower altitude through the usual photochemical and dynamical processes (Balan et al., 1998). Uemoto et al. (2007, 2011) improved the mechanism of Balan et al. (1998) by claiming that the fieldaligned diffusion of plasma acts to make the F3 layer prominent in the magnetic latitude far off the magnetic equator region (more than 71). Fagundes et al. (2007) reported for the first time the daytime F2 layer stratification over an equatorial anomaly crest location. This type of F2 layer stratification seems to be associated with a possible manifestation of middle scale travelling ionospheric disturbances (MSTIDs) due to the propagation of atmospheric gravity waves (AGWs) in the middle latitudes. Later, Fagundes et al. (2011) discussed the occurrence of the F3 layer as a function of solar cycle and season near the EIA southern crest in Brazil. Klimenko et al. [2012a,b] demonstrated that the F3 layer is formed as a result of the nonuniformity of vertical E B plasma drifts in height at the geomagnetic equator. So the mechanism of the F2 layer stratification in the low latitudes is complicated, and we do not know which is dominant that well explained the observed phenomenon of different latitude. Thus a detailed analysis is needed from a comprehensive study based on both the observation and modeling [Klimenko et al. 2012a]. In this paper, we tend to describe morphological features of traces with the F3 layer in magnetic low-latitude region and near magnetic equator based on statistics of two parameters—the foF2to-foF3 ratio and the difference between h′F3 and h′F2. And through statistics of formation patterns of the F3 layer in Sanya and Kwajalein the distribution of formation patterns of the F3 layer at different latitude is given. And its physical mechanism is discussed as well.
2. Data presentation In our work, the magnetic latitude dependence of F3 layer features concerned is highly regarded. Ionograms from seven ground-based ionosonde stations are used which cover regions in magnetic low latitude, near magnetic equator and over magnetic equator. The geographic locations, magnetic dip latitude, temporal interval of ionogram records, and data coverage are listed in Table 1. Geomagnetic information is calculated by
Table 1 Locations of ionosonde stations used in the study. Station
Latitude (deg)
Longitude (deg)
Magnetic dip latitude (deg)
Temporal interval of ionogram records (min)
Data coverage
Sanya Kwajalein Jicamarca Sao Luis Ilorin Fortaleza Vanimo
18.3 9 −12 −2.6 8.5 −3.9 −2.71
109.6 167.2 283.2 315.8 355.5 321.6 141.4
12.6 3.8 0.3 −2 −3.3 −6.6 −11.2
5 7.5 15 10 15 10 5
2010.12–2011.12 2006.9, 2006.12, 2007.1, 2007.6, 2011.1–2011.12 2011.1–2011.12 2006.7, 2010.1 2010.4–2010.10 2007.3, 2007.6, 2011.1–2011.12 2006.7–2007.6
50
J. Zhu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 102 (2013) 48–58
International Geomagnetic Reference Field (IGRF) 2010 (http:// www.ngdc.noaa.gov/IAGA/vmod/igrfhw.html). Measurements at Fortaleza, Kwajalein, Ilorin, Sao Luis and Jicamarca are downloaded from the Lowell Digital Ionogram DataBase (DIDB). At Sanya station, the Canadian Advanced Digital Ionosonde (CADI) was routinely operated from August 2010 and then replaced with a DPS-4D digisonde in April 2011. Measurements at Vanimo were obtained from the Ionospheric Prediction Service (IPS) clean ionogram data available at http://www.ips.gov.au/ World_Data_Centre. Note that the mean F10.7 in 2011 is 101.8 sfu (1 sfu ¼ 10−22 Wm−2Hz−1) higher than 72 sfu in 2006, 65.7 sfu in 2007, and 72 sfu in 2010. Actually, the year 2011 is in the ascending phase of the 24th solar cycle, in which the daily records showed that the F10.7 is low in January with the monthly average 72.7 sfu and gradually increased to maximum monthly value 134.8 sfu in November. The data in 2011 will be used in Section 3.2 where
occurrences of the F3 layer in months with different F10.7 could be compared. In addition, most days in 2006, 2007, 2010 and 2011 were geomagnetically quiet (refer to monthly ratio of days with daily Kp≤2.5 in right panels of Fig. 5). Cases during geomagnetic storms have been removed.
3. Observation and statistics 3.1. Morphological characteristic of ionogram traces with the F3 layer Ionogram records analyzed in this section were obtained in two magnetic low-latitude stations (Sanya, dip latitude 12.61N, and Vanimo, 11.21S) and two magnetic near-equator stations (Fortaleza, 6.61S, Kwajalein, 3.81N). For each station typical ionogram traces of different seasons are illustrated in Fig. 1, from A to D
Fig. 1. Typical ionogram traces with the F3 layer at four stations in each season except spring in (A) Sanya, (B) Vanimo, (C) Fortaleza, and (D) Kwajalein. The blue numbers are dip latitude of stations. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
J. Zhu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 102 (2013) 48–58
representative of Sanya, Vanimo, Fortaleza and Kwajalein, respectively. As Fig. 1 shows, typical ionogram traces in magnetic lowlatitude regions (A and B) are very different from those near magnetic equator (C and D) in shape. For one thing, traces near magnetic equator are morphologically upturned very evidently while those in magnetic low-latitude regions relatively flat. For another, near magnetic equator the cusp between the F2 and F3 layers is near the high-frequency end of the trace while such cusp tends to be close to F1 critical frequency in magnetic low-latitude region. The seasonal variations of those typical traces in each region are also presented in Fig. 1. For stations Sanya and Vanimo in magnetic low-latitude region, traces in local summer are distinguished with more evident cusps between the F2 and F3 layers and with higher formation heights (minimum virtual height 4400 km) of F2 layer stratifications compared with those (weak cusps and formation height 350 km) in local winter and autumn. On the other hand, for stations Fortaleza and Kwajalein near magnetic equator, stratifications of F2 layer in local summer are formed at high altitudes (minimum virtual height 4400 km), and cusps between the F2 and F3 layers are very close to the highfrequency end of traces. Meanwhile, differences between the minimum virtual heights of F3 and F2 layers are of the order of 150–200 km. In local winter, stratifications of F2 layers are formed at lower heights (minimum virtual height 350 km), with cusps between the F2 and F3 layers a little farther away from the highfrequency end and with smaller differences ( 100 km) between the minimum virtual heights of F3 and F2 layers than those in local summer. Typical traces in local autumn are similar to those in local winter. Those features above are consistent with the observation of Batista et al. (2002), and will be investigated in detail by statistics of the foF2-to-foF3 ratio and the difference between h′F3 and h′F2 in Sections 3.1.1 and 3.1.2, respectively. Before making the statistics of the F2 layer stratification, a few words should be stated as regard to the nomenclature of the F2 layer stratification. According to Lynn et al. (2000), the stratification was described in terms of a kink in the F2 profile, which could rise above the peak of the background F2 layer or remain below the peak depending on the latitude of observation. The additional layer was named F3 near the equator and F1.5 at low latitude. This latitudinal effect was suggested as arising from the cusp structure in the profile mapping down the field lines with increasing distance from the magnetic equator. It should be noted that it is not easy to separate F1.5 cases from F3 ones in ionograms. Fig. 2 compares typical developing processes of main F traces with F3 layers near and a little away and far away from magnetic equator in local summer. At the moment of the occurrence of the F3 layer (see the upper panel of Fig. 2), three traces are nearly similar due to the same formation mechanism reported by Balan et al. (1998). However, at the interim of the F3 layer, the trace far away from the magnetic equator turns flat with wider F3 trace because the fieldaligned diffusion of plasma from lower latitude compresses the F2 layer and enhances the peak density of F3 layer. On the other hand, the trace near magnetic equator does not changed obviously. The trace between them develops moderately. As a result, all those transient layers should be named F3 layer because of the same formation mechanism although the trace in the bottom-left corner of Fig. 2 is similar the one with the F1.5 layer. The name of F1.5 has an ancient history and was mainly considered as a possible manifestation of travelling ionospheric disturbances (TIDs), which is associated with the propagation of gravity waves (GWs) in this region (Heisler, 1962). Actually, a number of F1.5 cases at Sanya and Vanimo could most probably caused by the GWs propagating upward as revealed by Fagundes et al. (2007) but at EIA region with coexistence of the F1.5 and F3
51
Fig. 2. Comparing of typical developing processes of main F traces with F3 layers near and a little away and far away from magnetic equator in local summer. (A) Onset of the additional layer and (B) Interim of the additional layer.
layer. This feature is mainly characterized by a higher h′F1.5 comparing with h′F2. Thus the F1.5 layer at low latitude is mainly produced by the two mechanisms. An interesting work is not to separate the F1.5 and F3 layer, but to tell the different evolution feature of the F2 layer stratification caused by the mechanism of Balan et al. (1998) and GWs at Sanya and Vanimo. The topic will be investigated in future work. As regard to the current work, we removed cases with F1.5 layers from the data by requiring the difference between h′F3 and h′F2 more than 20 km in lowmagnetic latitude regions.
3.1.1. Diurnal variation and latitude dependence of the foF2-to-foF3 ratio In order to reveal characteristics of ionogram’s morphology more clearly, the foF2-to-foF3 ratio of ionograms in local summer and winter will be investigated in this section. Ionogram records with an interval of 15 or 20 min during the period when the F3 layers exist are picked out for statistics from continuous ionogram data which covers two separated months in local geomagnetically quiet summer and winter for each station (June 2011 and December 2010 for Sanya, December 2006 and July 2006 for Vanimo, January 2011 and June 2007 for Fortaleza, June 2007 and January 2007 for Kwajalein, Januray 2010 and July 2006 for Sao Luis). The statistical result of the ratio is shown against local time in Fig. 3. The gray circles in the figure are the record points, and the red solid line and the blue dashed line in each panel are respectively the fitting result of the data by the method of 2nd degree polynomial fit and the median value of the data. The dip latitudes of those stations’ locations are labeled in the figure. There are two main features of the foF2-to-foF3 ratio as shown in Fig. 3. First, the median value of the ratio tends to be larger as the location of the observation is closer to the magnetic equator. In summer, the monthly median values of ratios are as low as 70% in Sanya and Vanimo, 90% in Fortaleza and Kwajalein and 490% in Sao Luis according to Fig. 3. That means the cusp between the F2 and F3 layers is closer to the high-frequency end in the ionogram trace as the station’s location more approaching to magnetic equator. This latitude dependence of the trace feature can be also found in local winter as shown in the right panels of the figure. In addition, median values of such ratios in local winter are a little lower than those in local summer at Fortaleza, Kwajalein and Sao Luis stations with decrease of 3–8% and keep nearly constant at Sanya station. Morphologically speaking, near magnetic equator the cusp between the F2 and F3 layers in local summer is generally
52
J. Zhu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 102 (2013) 48–58
Fig. 3. Diurnal variation and latitude dependence of the foF2-to-foF3 ratio against local time in local summer and winter months. The gray circles are record points and the red solid line and the blue dash line in each panel are the fitting line of the data by the method of 2nd degree polynomial fit and the median value of the data respectively. Dip latitudes of stations are marked by red numbers. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
closer to the high-frequency end of the trace than that in local winter. And in magnetic low-latitude region, positions of such cusps relative to main F traces seem to be invariable in both seasons. Those can be illustrated by Fig. 1. Cases in Vanimo are too few to be taken into consideration. Second, the fitting results of the ratios tend to decrease with local time in local summer. That is, the cusp between the F2 and F3 layers is moving away from high-frequency end of the trace as the time progresses diurnally. Such local time variation of the ratio is more obvious as the location of the observation is further away from the magnetic equator. Especially, the ratio in Sao Luis in local summer which is closest to magnetic equator among those five stations nearly keeps constant before 15 LT. However, the local time variation of such ratio is not obvious in local winter in all 5 stations.
3.1.2. Diurnal variation and latitude dependence of the difference between h′F3 and h′F2 To investigate the characteristic of the difference between the minimal virtual height of the F3 layer (h′F3) and the minimal virtual height of the F2 layer (h′F2) in ionograms of different magnetic latitudes and seasons, the same data and procedure as
those in Section 3.1.1 were used. The statistical results are shown in Fig. 4 where the gray circles are record points and all of them are fitted by the 2nd degree polynomial fit shown by a red solid line in each panel. And the blue dashed line presents median value of data for each station. Fig. 4 presents two main features of the difference between h′F3 and h′F2. First, the fitting results in local summer tend to decrease with the progress of local time at all stations except Sao Luis where the fitting line keeps constant while those in local winter are nearly invariable for all stations. Second, the median values of results are as low as 60 km in magnetic low-latitude region (Sanya and Vanimo) while 4100 km near magnetic equator (Fortaleza, Kwajalein and Sao Luis) in local summer. Such feature reveals that traces in magnetic low-latitude regions are morphologically flatter than those near magnetic equator just as Fig. 1 shows. In local winter, median values of results have little latitude variation. And also in local winter, those values in regions near magnetic equator are lower than those in local summer (mainly in Fortaleza and Sao Luis with decreases of 100 km). However, median values in Sanya are almost equivalent in both seasons (Vanimo is excluded because of few cases).
J. Zhu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 102 (2013) 48–58
53
Fig. 4. Same as Fig. 3 but data are differences between h′F3 and h′F2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Such statistical results strongly support the seasonal variation of typical traces in Fig. 1 where typical ionogram traces near magnetic equator in local winter are not so upturned as those in local summer and traces in magnetic low-latitude region keep flat in both local winter and summer. 3.2. Features of occurrence of the F3 layer The occurrence of the F3 layer in this section is defined as the ratio of days with F3 layer in a specific hour in a month to total days of that month. We calculated the F3 occurrence for five ground-based ionosonde stations (From North to South are Sanya, Kwajalein, Jicamarca, Fortaleza and Vanimo) in an entire year and for one station (Ilorin) in 9 months because of data shortage. The duration of data in each station refers to continuous data coverage in Table 1. As Fig. 5 shows, the F3 occurrence rate is highest at stations of magnetic low-latitude region and lowest at the one over magnetic equator. The duration of F3 layer which could be observed from ground is mostly from morning to afternoon at stations in magnetic low-latitude region and near magnetic equator. Such time span is much shorter at the station over magnetic equator. It should be noted that the longitudinal location of the station
could affect the occurrence of the additional layer according to Raghavarao et al. (1977), Zhao et al. (2011b), and Karpachev et al. (2013) although researches before emphasized its latitude dependence (Lynn et al., 2000; Batista et al., 2002; Thampi et al., 2007). On the other hand, the rate is generally high in local summer while keeps low in local winter. And the rates in Sanya and Fortaleza in equinox are high as well. In addition, it is Jicamarca where the F3 occurrence is notable at sunset in local summer and especially in December 2011 as the solar flux (F10.7) increasing from monthly mean 72.7 sfu in January 2011 to the maximum 134.8 sfu in November 2011. For comparison, at Fortaleza the occurrence of the F3 layer in December 2011 is obviously lower than in January and February 2011 which shows that the daytime F3 layer occurrence reduces when solar flux increases. This is in accordance with the past observation (Balan et al., 1998; Batista et al., 2002; and Rama Rao et al., 2005). 3.3. Relationship between fmax and hmax when the F3 layer occurs The development of fmax (the F layer maximum frequency, foF2/ foF3) and hmax (the F layer peak height, hmF2/hmF3) around the onset of the occurrence of the F3 layer presents different patterns
54
J. Zhu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 102 (2013) 48–58
Fig. 5. Left panel: the F3 occurrence rate of each month in six stations. The black number in each panel is dip latitude of the station. Right panel: the rate of days with daily Kp≤2.5 in a month to total days of that month.
(formation patterns of the F3 layer). In order to define these patterns and compare them in different magnetic latitudes and seasons, we choose ground-based ionosonde data from a lowlatitude station Sanya and near-equator station Kwajalein in three separated months (covering local summer, local winter and local autumn respectively) of 2011 and make statistics of cases in which the F3 layer existed for at least one hour that is the characteristic time scale of the F3 layer occurrence according to Rama Rao et al. (2005). There are two formation patterns of the F3 layer during concerned periods. Pattern A is that the increase of fmax is after that of hmax while pattern B is that fmax keeps constant or decreases when hmax is increasing. These two patterns are illustrated in Fig. 6 where traces are presented at 15-min intervals and sequence plots of red traces and areas filled with pink color show durations of F3 layer. The blue solid and the green solid lines in each panel are fmax and hmax against local time respectively. The statistical results are listed in Table 2 where the number in column ‘Doy’ (days of year) presents the day with the F3 layer and
the letter in column ‘Type’ shows the corresponding formation pattern of the additional layer. And for each station in a specific season the rate of two patterns is also given at the bottom of the table. It should be noticed that both two patterns could appear in one day in different cases. According to Table 2, all formation patterns of the F3 layer in summer and autumn of Sanya are pattern A while the numbers of pattern A and B are nearly equivalent in winter with 45% versus 55%. However, Kwajalein has much more pattern B (75%) than A (25%) in summer, and all of cases in winter and in autumn are pattern B although only 4 cases in each season.
4. Discussion In this work, stations concerned in statistics are divided into two groups—one is near magnetic equator and the other is in magnetic low-latitude region—according to magnetic latitudes of their locations. That is reasonable because statistical results in
J. Zhu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 102 (2013) 48–58
55
Fig. 6. Illustrations of two formation patterns (A and B) of F3 layer at Sanya and Kwajalein stations in 2011. Sequence plots of ionogram traces in each panel are shown at 15-min intervals and among which red ones are traces with F3 layer. Red dots on ionogram traces highlight cusps between the F2 and F3 layers, and the pink filled areas indicate durations of F3 layers. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Section 3 reveal that features in one group are very similar but quite different from those in the other group. Such two regions were also used in the work of Uemoto et al. (2011). Diurnal variations and latitude dependence of F3 layer’s morphological features are investigated quantitatively in Section 3.1. foF2-to-foF3 ratios and differences between h′F3 and h′F2 tend to decrease with local time progress in local summer and such trend is much weaker as it is observed at the latitude more approaching magnetic equator. Those diurnal variations are not obvious in local winter. On the other hand, foF2-to-foF3 ratios and differences between h′F3 and h′F2 generally keep low in magnetic lowlatitude region while higher near magnetic equator in local summer. In local winter, this latitude dependence of foF2-to-foF3 ratios is the same as that in local summer but the latitude variation of differences between h′F3 and h′F2 does not exist. Features of the occurrence of the F3 layer are also investigated in Section 3.2 through data from six stations covering regions in magnetic low latitude, near magnetic equator and over magnetic equator in both hemispheres. Fig. 5 shows that the F3 occurrence rate is highest at stations in magnetic low-latitude region and lowest at the one over magnetic equator. Zhao et al. (2011b) obtained the similar results through the observation by the COSMIC/FORMOSAT-3 satellites. Furthermore, durations of the F3 layers are from morning to afternoon at stations in magnetic lowlatitude region and near magnetic equator but much shorter at the one over magnetic equator. Those results agree with the work of
Uemoto et al. (2011). In addition, it should be noted that the occurrence of the sunset F3 layer is notable in Jicamarca in local summer and especially in December 2011 as the solar flux increasing from monthly mean 72.7 sfu in January 2011 to 134.8 sfu in November 2011. Sunset F3 layer in Jicamarca was also reported by Zhao et al. (2011a) who investigated detailed features of sunset F3 layer in the low-latitude ionosphere. As mentioned in the introduction, the F3 layer is formed by a combination of upward E B drift and the neutral wind suggested by Balan et al. (1998). And then, Uemoto et al. (2007, 2011) improved such mechanism by claiming that the field-aligned diffusion of plasma acts to make the F3 layer prominent in the magnetic low-latitude region corresponding to the electron density-enhanced region associated with the equatorial anomaly and decrease the peak density of the F3 layer near magnetic equator. Based on statistical results in Section 3.3 where two formation patterns (A and B) of the F3 layer are investigated, we present in Fig. 7 an ideal equinoctial (without the summer-towinter neutral wind) and a solstitial (with the summer-to-winter neutral wind) distributions of those patterns. Pattern A indicates that the F3 peak density increases with time progress as the peak moving upward around the onset of the F3 layer’s occurrence while pattern B represents that the F3 peak density keeps nearly constant or decreases with time progress as the peak moving upward around the time when the F3 layer forms. According to Fig. 7a, it seems that because of faster uplift of F2 peak near
56
J. Zhu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 102 (2013) 48–58
Table 2 Statistical results of formation patterns of F3 layer in 2011 at Sanya and Kwajalein. Summer (June)
Winter (Jan.)
Autumn (Sept.)
Sanya
Sanya
Sanya
Kwajalein
Kwajalein
Kwajalein
Doy Type Doy Type Doy Type Doy Type Doy Type Doy Type 152 153 154 155 156 157 158 159 160 161 162 164 165 166 167 168 169 170 171 172 174 175 176 177 179 180 181 A 100 B 0 Type Rate
A A A A A A A A A A A A A A A A A A A A A A A A A A A
(%)
152 154 155 156 158 159 160 161 162 163 164 166 167 169 170 171 172 173 176 177 178 179 180 181
B B B B A B B A A A A B B A B B B B B B B B B B
25 75 Rate (%)
7 8 9 10 12 13 15 16 19 20 21 22 23 24 25 26 27 28 29 30
B A B A A B B B B B A, B B B A, B A A A A B A
45 55 Rate (%)
10 25 26 27
B B B B
0 100 Rate (%)
244 245 246 247 249 251 252 254 255 256 257 258 259 261 262 263 264 265 266 267 268 269 272
A A A A A A A A A A A A A A A A A A A A A A A
100 0 Rate (%)
245 248 254 264
B B B B
Fig. 7. The distribution of formation patterns of F3 layer (A) without summer-towinter neutral wind and (B) with summer-to-winter neutral wind when F3 layer occurs. Areas colored by pink and blue represent regions near magnetic equator and in magnetic low latitude respectively. Gray dashed lines in each panel are magnetic field lines (a sketch). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
0 100 Rate (%)
magnetic equator due to upward E B drift the plasma there diffuses to a higher-latitude region with moving upward (Uemoto et al., 2011). Such diffusion can cause that the F3 layer near magnetic equator, which forms later when the normal F2 layer develops at lower altitude through the photochemical and dynamical processes if the F3 layer reaches the altitude separated sufficiently from the F2 layer (Balan et al., 1998), develops with decreasing or nearly constant peak density as time progress near magnetic equator. However, in magnetic low-latitude region the F3 peak density tends to increasing with time progress when it forms because of the positive field-aligned diffusion effect (Uemoto et al., 2011). In Fig. 7b, the summer-to-winter neutral wind is taken into consideration in a solstice and it resists (enhances) the plasma diffusion to higher latitude in the windward (leeward). As a result, pattern A could be observed near magnetic equator in summer hemisphere and pattern B in magnetic low-latitude region in winter hemisphere compared with the ideal distribution in the equinox. In order to further investigate the relationship between the occurrence of the F3 layer in Sanya and the electron densityenhanced region, we have plotted the TEC contour map in the Southeast Asian region along longitude 1101E as a function of latitude and local time. One can refer to Zhao et al. (2009) to obtain the details of the method for deriving TEC map. The left panel of Fig. 8 shows the mean TEC contour map for selected day on January 7th, 9th, 21st, and 24th in 2011 when pattern B develops most at Sanya. It is shown that EIA was intensified during these days while the location of Sanya was at the northern crest’s inner edge near to the magnetic equator. The middle and right panels of Fig. 8 show the situations for March and July where pattern A is dominated. The locations of the crests for both two
months were more equatorward compared to the January situation. The position of Sanya was right situated in the density enhance region of northern EIA. Because the crest of the plasma density moves to magnetic lower-latitude region with increasing altitude, the crest of NmF2 should be a little farther away from magnetic equator than the crest of TEC does. Fig. 9 illustrates a sketch map of latitude distribution of NmF2 at two different moments in the forenoon of winter, equinox and summer. The later latitude distribution of NmF2 becomes broader with larger value of crests than that of current moment because of enhanced plasma fountain effect related to the stronger zonal electric field in the forenoon. On account of the ion drag effect of the summer-to-winter neutral wind, both two crests of NmF2 move a little to the winter hemisphere in solstice seasons. As a result, Sanya locates at northern crest’s inner edge near the equator in winter and NmF2 over it decreases with time progress. On the other hand, in summer and equinox seasons, the location of Sanya is under the crest of NmF2 and consequently the peak density over it increases with time progress. It should be noted that the effect of stations’ longitudes on the result above is ignored because it is not the main factor of the variation of the additional layer’s formation. The detail about longitude effect on the formation patterns of F3 layers could be investigated in future study.
5. Summary Our current work concerns morphological features of ionogram traces with F3 layer, the occurrence and the formation pattern of the F3 layer in magnetic low-latitude region and near magnetic equator. Statistical results show two morphological parameters of traces with F3 layers have the latitude dependence, diurnal variation, and seasonal variation. All two parameters tend to decrease with local time progress and their values mostly keep low in magnetic low-latitude region in local summer. However, near magnetic equator, their diurnal variations are much weaker. On the other hand, all variations above became weak or even disappeared in local winter. In addition, we obtained the distribution of formation patterns of the F3 layer which is caused by plasma diffusion and the transequatorial neutral wind. Here we have shown that even in magnetic low-latitude region, the occurrence of the F3 layer is not always accompanied by the density-enhanced region proposed by Uemoto et al. (2011) as the location of the EIA crest is affected by the intensity of the zonal electric field and transequatorial
J. Zhu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 102 (2013) 48–58
57
Fig. 8. Mean TEC contour map in January (7th, 9th, 21st, and 24th), March, and July 2011. The horizontal black dashed line marks the location of Sanya (geographic latitude 18.21N).
Acknowledgements This research was supported by the National Natural Science Foundation of China (41174138, 41131066), the National Important Basic Research Project of China (2011CB811405). Special thanks should be given to the Ionospheric Prediction Service of Australia for providing data in Australia and to the Digisonde Global Ionospheric Observatory (GIRO).
References
Fig. 9. The sketch map of latitude distribution of NmF2 at two different moments in the forenoon of winter, equinox and summer. The red and the black dashed lines mark the location of Sanya (geographic latitude 18.21N) and the magnetic equator respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
meridional wind. The F3 layer feature at a single site is quite variable and should be understood associated with the entire motion of the EIA region.
Balan, N., Bailey, G.J., 1995. Equatorial plasma fountain and its effects: possibility of an additional layer. Journal of Geophysical Research 100, 21421–21432, http: //dx.doi.org/10.1029/95JA01555. Balan, N., Bailey, G.J., Abdu, M.A., Oyama, K.I., Richards, P.G., Macdougall, J., Batista, I.S., 1997. Equatorial plasma fountain and its effects over three locations: evidence for an additional layer, the F3 layer. Journal of Geophysical Research 102, 2047–2056, http: //dx.doi.org/10.1029/95JA02639. Balan, N., Batista, I.S., Abdu, M.A., MacDougall, J., Bailey, G.J., 1998. Physical mechanism and statistics of occurrence of an additional layer in the equatorial ionosphere. Journal of Geophysical Research 103, 29169–29181, http://dx.doi. org/10.1029/98JA02823. Balan, N., Batista, I.S., Abdu, M.A., Bailey, G.J., Watanabe, S., MacDougall, J., Sobral, J.H.A., 2000. Variability of an additional layer in the equatorial ionosphere over Fortaleza. Journal of Geophysical Research 105 (A5), 10,603–10,613, http://dx.doi.org/10.1029/ 1999JA000020. Balan, N., Thampi, S.V., Lynn, K., Otsuka, Y., Alleyne, H., Watanabe, S., Abdu, M.A., Fejer, B.G., 2008. F3 layer during penetration electric field. Journal of Geophysical Research 113, A00A07, http://dx.doi.org/10.1029/2008JA013206. Balan, N., Yamamoto, M., Sreeja, V., Batista, I.S., Lynn, K.J.W., Abdu, M.A., Ravindran, S., Kikuchi, T., Otsuka, Y., Shokawa, K., Alex, S., 2011. A statistical study of the response of the dayside equatorial F2 layer to the main phase of intense geomagnetic storms as an indicator of penetration electric field. Journal of Geophysical Research 116, A03323, http://dx.doi.org/10.1029/2010JA016001. Batista, I.S., Abdu, M.A., McDougall, J., Souza, J.R., 2002. Long term trends in the frequency of occurrence of the F3 layer over Fortaleza. Journal of Atmospheric and Solar-Terrestrial Physics 64, 1409–1412. Chaitanya, P.P., Patra, A.K., Rao, S.V.B., 2012. Contrasting features of the F3 layer during high and low solar activity conditions observed from Indian low latitude. Indian Journal of Radio & Space Physics 41, 121–129. Depuev, V.H., Pulinets, S.A., 2001. Intercosmos-19 observations of an additional topside ionization layer: the F3-layer. Advances in Space Research 27 (6–7), 1289–1292, http://dx.doi.org/10.1016/S0273-1177(01)00205. Fagundes, P.R., Klausner, V., Sahai, Y., Pillat, V.G., Becker-Guedes, F., Bertoni, F.C.P., Bolzan, M.J.A., Abalde, J.R., 2007. Observations of daytime F2-layer stratification under the southern crest of the equatorial ionization anomaly region. Journal of Geophysical Research 112, A04302, http://dx.doi.org/10.1029/2006JA011888. Fagundes, P.R., Klausner, V., Bittencourt, J.A., Sahai, Y., Abalde, J.R., 2011. Seasonal and solar cycle dependence of F3-layer near the southern crest of the equatorial ionospheric anomaly. Advances in Space Research 48 (3), 472–477.
58
J. Zhu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 102 (2013) 48–58
Heisler, L.H., 1962. The anomalous ionospheric stratification F1.5. Journal of Atmospheric and Solar-Terrestrial Physics 24, 483–489. Hsiao, C.C., Liu, J.Y., Tsunoda, R.T., Fukao, S., Saroso, S., Nozaki, K., Badillo, V.L., Berkey, F.T., Chen, S.W., Yamamoto, M., 2001. Evidence for the geographic control of additional layer formation in the low latitude ionosphere. Advances in Space Research 27, 1293–1297. Huang, C., 1974. A certain behavior of the ionospheric F2 region at low latitudes. Radio Science 9 (5), 519–532, http://dx.doi.org/10.1029/RS009i005p00519. Huang, C., 1975. The travelling bifurcation of the equatorial F2 layer. Radio Science 10 (5), 507–516, http://dx.doi.org/10.1029/RS010i005p00507. Jenkins, B., Bailey, G.J., Abdu, M.A., Batista, I.S., Balan, N., 1997. Observations and model calculations of an additional layer in the topside ionosphere above Fortaleza, Brazil. Annales of Geophysics 15, 753–759, http://dx.doi.org/10.1007/ s00585-997-0753-3. Karpachev, A.T., Klimenko, M.V., Klimenko, V.V., Zhbankov, G.A., Telegin, V.A., 2012. Latitudinal structure of the equatorial F3 layer based on the Intercosmos-19 topside sounding data. Journal of Atmospheric and Solar-Terrestrial Physics 77, 186–193, http://dx.doi.org/10.1016/j.jastp.2011.12.018. Karpachev, A.T., Klimenko, M.V., Klimenko, V.V., Kuleshova, 2013. Statistical study of the F3 layer characteristics retrieved from Intercosmos-19 satellite data. Journal of Atmospheric and Solar-Terrestrial Physics, http://dx.doi.org/10.1016/j. jastp.2013.01.010. Klimenko, M.V., Klimenko, V.V., Ratovsky, K.G., Goncharenko, L.P., Sahai, Y., Fagundes, P.R., Jesus, R., Abreu, A.J., Vesnin, A.M., 2011. Numerical modeling of ionospheric effects in the middle- and low-latitude F region during geomagnetic storm sequence of 9–14 September 2005. Radio Science 46, RS0D03, http://dx.doi.org/10.1029/2010RS004590. Klimenko, M.V., Klimenko, V.V., 2012. Disturbance dynamo, prompt penetration electric field and overshielding in the Earth’s ionosphere during geomagnetic storm. Journal of Atmospheric and Solar-Terrestrial Physics 90–91, 146–155, http://dx.doi.org/10.1016/j.jastp.2012.02.018. Klimenko, Zhao, B., Karpachev, A.T., Klimenko, V.V., 2012a. Stratification of the LowLatitude and Near-Equatorial F2 Layer, Topside Ionization Ledge, and F3 Layer: What We Know about This? A Review, International Journal of Geophysics, doi:10.1155/2012/938057. Klimenko, M.V., Klimenko, V.V., Karpachev, A.T., 2012b. Formation mechanism of additional layers above regular F2 layer in the near-equatorial ionosphere during quiet period. Journal of Atmospheric and Solar-Terrestrial Physics 10.1016/j.jastp.2012.02.011. Lin, C.H., Richmond, A.D., Bailey, G.J., Liu, J.Y., Lu, G., Heelis, R.A., 2009a. Neutral wind effect in producing a storm time ionospheric additional layer in the equatorial ionization anomaly region. Journal of Geophysical Research 114, A09306, http://dx.doi.org/10.1029/2009JA014050. Lin, C.H., Richmond, A.D., Liu, J.Y., Bailey, G.J., Reinisch, B.W., 2009b. Theoretical study of new plasma structures in the low-latitude ionosphere during a major magnetic storm. Journal of Geophysical Research 114, A05303, http://dx.doi. org/10.1029/2008JA013951. Lockwood, G.E.K., Nelms, G.L., 1964. Topside sounder observations of the equatorial anomaly in the 751W longitude zone. Journal of Atmospheric and Terrestrial Physics 26, 569–580. Lynn, K.J.W., Harris, T.J., Sjarifudin, M., 2000. Stratification of the F2 layer observed in Southeast Asia. Journal of Geophysical Research 105, 27,147–27,156, http://dx. doi.org/10.1029/2000JA900056. Paznukhov, V.V., Reinisch, B.W., Song, P., Huang, X., Bullett, T.W., Veliz, O., 2007. Formation of an F3 layer in the equatorial ionosphere: a result from strong IMF changes. Journal of Atmospheric and Solar-Terrestrial Physics 69, 1292–1304.
Raghavarao, R., Sivaraman, M.R., 1974. Ionization ledges in the equatorial ionosphere. Nature 249, 331–332, http://dx.doi.org/10.1038/249331a0. Raghavarao, R., Sharma, P., Jain, A.R., 1977. Ionization ledges and the counterelectrojets in the equatorial ionosphere. Space Research 17, 417–421. Rama Rao, P.V.S., Niranjan, K., Prasad, D.S.V.V.D., Brahmanandam, P.S., Gopikrishna, S., 2005. Features of additional stratification in ionospheric F2 layer observed for half a solar cycle over Indian low latitudes. Journal of Geophysical Research (110), A04307, http://dx.doi.org/10.1029/2004JA010646. Ratcliffe, J.A., 1951. Some regularities in the F2 region of the ionosphere. Journal of Geophysical Research 110, 56,487–56,507. Sayers, J., Rothwell, P., Wager, J.H., 1963. Field aligned strata in the ionization above the ionospheric F2 layer. Nature 198, 230–233, http://dx.doi.org/10.1038/ 198230a0. Sen, H.Y., 1949. Stratification of the F2 layer of the ionosphere over Singapore. Journal of Geophysical Research 54, 363–366. Sharma, P., Raghavarao, R., 1989. Simultaneous occurrence of ionization ledge and counter electrojet in the equatorial ionosphere: Observational evidence and its implications. Canadian Journal of Physics 67, 166–172. Skinner, N.J., Brown, R.A., Wright, R.W., 1954. Multiple stratification of the F-layer at Ibadan. Journal of Atmospheric and Terrestrial Physics 5, 92–100. Sreeja, V., Ravindran, S., Pant, T.K., 2010. Features of the F3 layer occurrence over the equatorial location of Trivandrum. Annales of Geophysics 28, 1741–1747, http://dx.doi.org/10.5194/angeo-28-1741-2010. Thampi, S.V., Balan, N., Ravindran, S., Pant, T.K., Devasia, C.V., Sreelatha, P., Sridharan, R., Bailey, G.J., 2007. An additional layer in the low-latitude ionosphere in Indian longitudes: total electron content observations and modeling. Journal of Geophysical Research 112, A06301, http://dx.doi.org/10.1029/ 2006JA011974. Uemoto, J., Ono, T., Kumamoto, A., Iizima, M., 2006. Statistical analysis of the ionization ledge in the equatorial ionosphere observed from topside sounder satellites. Journal of Atmospheric and Solar-Terrestrial Physics 68 (12), 1340–1351, http://dx.doi.org/10.1016/j.jastp.2006.05.015. Uemoto, J., Ono, T., Maruyama, T., Saito, S., Iizima, M., Kumamoto, A., 2007. Magnetic conjugate observation of the F3 layer using the SEALION ionosonde network. Geophysical Research Letters 34, L02110, http://dx.doi.org/10.1029/ 2006GL028783. Uemoto, J., Maruyama, T., Ono, T., Saito, S., Iizima, M., Kumamoto, A., 2011. Observations and model calculations of the F3 layer in the Southeast Asian equatorial ionosphere. Journal of Geophysical Research 116, A03311, http://dx. doi.org/10.1029/2010JA016086. Zain, A.F.M., Abdullah, S., Homam, M.J., Seman, F.C., Abdullah, M., Ho, Y.H., 2008. Observations of the F3-layer at equatorial region during 2005. Journal of Atmospheric and Solar-Terrestrial Physics 70, 918–925, http://dx.doi.org/ 10.1016/j.jastp.2007.12.002. Zhao, B., Wan, W., Liu, L., 2005. Responses of equatorial anomaly to the October– November 2003 superstorms. Annales of Geophysics 23 (3), 693–706. Zhao, B., Wan, W., Liu, L., Ren, Z., 2009. Characteristics of the ionospheric total electron content of the equatorial ionization anomaly in the Asia/Australian region during 1996–2004. Annales of Geophysics 27, 3861–3873. Zhao, B., Wan, W., Reinisch, B., Yue, X., Le, H., Liu, J., Xiong, B., 2011a. Features of the F3 layer in the low-latitude ionosphere at sunset. Journal of Geophysical Research 116, A01313, http://dx.doi.org/10.1029/2010JA016111. Zhao, B., Wan, W., Yue, X., Liu, L., Ren, Z., He, M., Liu, J., 2011b. Global characteristics of occurrence of an additional layer in the ionosphere observed by COSMIC/ FORMOSAT-3. Geophysical Research Letters 38, L02101, http://dx.doi.org/ 10.1029/2010GL045744.
Contents lists available at ScienceDirect
Journal of Atmospheric and Solar-Terrestrial Physics journal homepage: www.elsevier.com/locate/jastp
An investigation of the formation patterns of the ionospheric F3 layer in low and equatorial latitudes Jie Zhu a,b,c, Biqiang Zhao a,b,n, Weixing Wan a,b, Baiqi Ning a,b a
Key Laboratory of Ionospheric Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China Beijing National Observatory of Space Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China c University of Chinese Academy of Sciences, Beijing, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 20 October 2012 Received in revised form 21 March 2013 Accepted 22 April 2013 Available online 18 May 2013
Ionogram traces with the F3 layer in different latitude do not always seem similar. In our work, we tend to describe morphological features of traces with the F3 layer in magnetic low-latitude region and near magnetic equator through the quantitative investigation of the diurnal variation and latitude dependence of two morphologically characteristic parameters – the foF2-to-foF3 ratio and the difference between h′F3 and h′F2 – in geomagnetically quiet period. The distribution of two formation patterns (pattern A and pattern B are defined with increasing F3 peak density and with nearly constant or decreasing F3 peak density respectively as the peak moving upward around the onset of the F3 layer’s occurrence) of the F3 layer is also investigated based on statistics of formation patterns of the F3 layer in Sanya and Kwajalein in 2011. The ideal equinoctial distribution (without the summer-to-winter neutral wind) of those patterns is symmetrical about magnetic equator with pattern A in magnetic low-latitude region and pattern B near magnetic equator. When taking the summer-to-winter neutral wind which resists (enhances) the plasma diffusion to higher latitude in the windward (leeward) into consideration in a solstice, pattern A could be observed near magnetic equator in summer hemisphere and pattern B in magnetic low-latitude region in winter hemisphere compared with the ideal distribution in the equinox. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Ionosphere Low latitude F3 layer
1. Introduction The F2 layer stratification has received much attention since the additional layer of F region was first observed in the middle of 20 century distinguished as a ‘spur’ structure near the highfrequency end of the virtual height–frequency ionogram records from ground-based ionosondes (e.g., Sen, 1949; Ratcliffe, 1951; Skinner et al., 1954; Huang, 1974, 1975). This new layer above regular F2 layer was called ‘G layer’ at first (Balan and Bailey, 1995), and then renamed to ‘F3 layer’ because of the same chemical composition as that of the F region (Balan et al., 1997). During several decades, the stratification of the F2 layer at low and equatorial latitude has been investigated extensively (e.g., Jenkins et al., 1997; Balan et al., 1998, 2000; Lynn et al., 2000; Hsiao et al., 2001; Batista et al., 2002; Rama Rao et al., 2005; Zain et al., 2008; Uemoto et al., 2007, 2011; Sreeja et al., 2010; Chaitanya et al., 2012). Generally, the features of the F3 layer under geomagnetically quiet conditions are as follows [Klimenko
n Corresponding author at: No. 19, Beitucheng Western Road Chaoyang District, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China. Tel.: +86 10 82998310; fax: +86 10 82998332. E-mail addresses: [email protected] (J. Zhu), [email protected] (B. Zhao).
1364-6826/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jastp.2013.04.015
et al., 2012a]: (1) the F3 layer can occur during daytime (0800– 1700 LT) within about 7 101 magnetic latitudes, (2) the occurrence of the F3 layer can be frequent on the summer side of the geomagnetic equator at solar minimum, (3) the F3 layer is observed to be dependent on the magnetic field inclination with lower occurrence at the equator and higher far off the equator. Besides the daytime F3 layer, Zhao et al. (2011a) investigated characteristics of the sunset F3 layer using a solar cycle of ionosonde data (1995–2010) from the magnetic equatorial station Jicamarca and evidence shows that the local time distribution of the occurrence of the F3 layer can extend to the postsunset time (18:00–21:00 LT). Unlike the daytime F3 layer, the occurrence of the sunset F3 layer clearly increases with increasing solar activity. The F3 layer not only occurs during quiescent ionosphere periods but also during magnetic storm periods. Recent observations show the existence of storm time F3 layer occurring at the magnetic equator in response to a strong penetration electric field (e.g., Zhao et al., 2005; Paznukhov et al., 2007; Balan et al., 2008, 2011; Lin et al., 2009a, 2009b; Sreeja et al., 2010; Klimenko et al., 2011), or to a non-uniform in height zonal electric field generated by the disturbance dynamo electric field (Klimenko and Klimenko, 2012). Lynn et al. (2000) discussed the problem of the nomenclature to describe F2 stratification. They proposed that the additional
J. Zhu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 102 (2013) 48–58
layer be referred to as F3 or F1.5 depending on whether the transitory layer moved above or stayed below the F2 layer peak which maintained continuity with the prestratification and poststratification F2 layer. The stratified layer is commonly seen as an F3 layer near magnetic equator but as an F1.5 layer away from magnetic equator. Besides the traditional ground ionosonde measurement, new techniques have been used to investigate the F3 layer characteristics. For example, the position of the appearance of the F3 layer is also derived through the single station total electron content (TEC) measurements using radio beacon transmissions from Low Earth Orbiting Satellites (LEOS) (Thampi et al., 2007). They have found a hump structure in the latitude variation of vertical TEC centered at magnetic latitude 7–81N at longitude 801E. Zhao et al. (2011b) infer the global F3 layer structure directly from COSMIC/FORMOSAT-3 radio occultation (RO) data. Statistical results show an accurate magnetic latitude dependence of the occurrence of the additional layer and reveal that the highest occurrence of F3 layer appears at dip latitude 781 during summer months—a few degrees inside the equatorial ionization anomaly (EIA). The spatial distribution of stratification of the F2 layer can also be observed by the satellite equipment. It is identified as the ‘topside ledge’ on the topside electron density profiles recorded from in-situ and the topside sounding ionograms (e.g., Sayers et al., 1963; Lockwood and Nelms, 1964; Raghavarao and Sivaraman, 1974; Sharma and Raghavarao, 1989). Unlike the F3 layer being observed mainly during the daytime, the topside ledge at both the daytime and night was observed by Depuev and Pulinets (2001) using Intercosmos-19 (IK-19) satellite ionograms. Uemoto et al. (2006) reveals that the ionization ledge is observable in almost all local time sectors except the period from 03 to 08LT sector based on several hundred topside ionograms from ISIS and Ohzora satellites. Recently, Karpachev et al. (2012) demonstrated that the topside ledge is most often observed in the afternoon and evening hours, less often at night and rarely in the morning based on large dataset (about 3600 passes across the equator) of the IK19 topside sounding data. Uemoto et al. (2006) examined the possible link between the topside ledge and the F3 layer as regard to the seasonal and the local time dependences of those two structures. The coexistence of the F3 layer and topside ledge, and the statistical maximum occurrence of the topside ledge observed 1–2 h delay with respect to the F3 layer, suggest these two phenomena have close relationship. However, the seasonal dependence of the occurrence probability of the ionization ledge shows contradict manner to the F3 layer. Meanwhile, Karpachev et al. (2013) reveals the latitudinal variation of topside ledge from the IK-19 topside sounding data and showed that the distribution differs from that latitude distribution observed from the ground observation and COSMIC radio occultation result of Zhao et al. (2011b). They suggested that the discrepancy could result from the fact that the bottomside measurement cannot observe the F3 layer over magnetic equator because of smaller foF3 than foF2 and much higher hmF3 than
49
hmF2, but can do in regions away from the magnetic equator due to increasing foF3 and decreasing hmF3. That raises a question: is the F3 layer in regions away from the magnetic equator formed by the transportation of the topside ledge from lower latitude along magnetic line or by local photochemical and dynamical processes? To answer this question, simultaneous ground-based and satellites observations are needed. According to Balan et al. (1998), the physical mechanism of F3 layer formation is the combined effect of the upward E B drift and neutral wind. The F2 peak is transported by upward E B drift to much higher altitude and could be prevented from diffusion along the magnetic field line if the neutral wind is equatorward. Then the F3 layer forms while the normal F2 layer develops at lower altitude through the usual photochemical and dynamical processes (Balan et al., 1998). Uemoto et al. (2007, 2011) improved the mechanism of Balan et al. (1998) by claiming that the fieldaligned diffusion of plasma acts to make the F3 layer prominent in the magnetic latitude far off the magnetic equator region (more than 71). Fagundes et al. (2007) reported for the first time the daytime F2 layer stratification over an equatorial anomaly crest location. This type of F2 layer stratification seems to be associated with a possible manifestation of middle scale travelling ionospheric disturbances (MSTIDs) due to the propagation of atmospheric gravity waves (AGWs) in the middle latitudes. Later, Fagundes et al. (2011) discussed the occurrence of the F3 layer as a function of solar cycle and season near the EIA southern crest in Brazil. Klimenko et al. [2012a,b] demonstrated that the F3 layer is formed as a result of the nonuniformity of vertical E B plasma drifts in height at the geomagnetic equator. So the mechanism of the F2 layer stratification in the low latitudes is complicated, and we do not know which is dominant that well explained the observed phenomenon of different latitude. Thus a detailed analysis is needed from a comprehensive study based on both the observation and modeling [Klimenko et al. 2012a]. In this paper, we tend to describe morphological features of traces with the F3 layer in magnetic low-latitude region and near magnetic equator based on statistics of two parameters—the foF2to-foF3 ratio and the difference between h′F3 and h′F2. And through statistics of formation patterns of the F3 layer in Sanya and Kwajalein the distribution of formation patterns of the F3 layer at different latitude is given. And its physical mechanism is discussed as well.
2. Data presentation In our work, the magnetic latitude dependence of F3 layer features concerned is highly regarded. Ionograms from seven ground-based ionosonde stations are used which cover regions in magnetic low latitude, near magnetic equator and over magnetic equator. The geographic locations, magnetic dip latitude, temporal interval of ionogram records, and data coverage are listed in Table 1. Geomagnetic information is calculated by
Table 1 Locations of ionosonde stations used in the study. Station
Latitude (deg)
Longitude (deg)
Magnetic dip latitude (deg)
Temporal interval of ionogram records (min)
Data coverage
Sanya Kwajalein Jicamarca Sao Luis Ilorin Fortaleza Vanimo
18.3 9 −12 −2.6 8.5 −3.9 −2.71
109.6 167.2 283.2 315.8 355.5 321.6 141.4
12.6 3.8 0.3 −2 −3.3 −6.6 −11.2
5 7.5 15 10 15 10 5
2010.12–2011.12 2006.9, 2006.12, 2007.1, 2007.6, 2011.1–2011.12 2011.1–2011.12 2006.7, 2010.1 2010.4–2010.10 2007.3, 2007.6, 2011.1–2011.12 2006.7–2007.6
50
J. Zhu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 102 (2013) 48–58
International Geomagnetic Reference Field (IGRF) 2010 (http:// www.ngdc.noaa.gov/IAGA/vmod/igrfhw.html). Measurements at Fortaleza, Kwajalein, Ilorin, Sao Luis and Jicamarca are downloaded from the Lowell Digital Ionogram DataBase (DIDB). At Sanya station, the Canadian Advanced Digital Ionosonde (CADI) was routinely operated from August 2010 and then replaced with a DPS-4D digisonde in April 2011. Measurements at Vanimo were obtained from the Ionospheric Prediction Service (IPS) clean ionogram data available at http://www.ips.gov.au/ World_Data_Centre. Note that the mean F10.7 in 2011 is 101.8 sfu (1 sfu ¼ 10−22 Wm−2Hz−1) higher than 72 sfu in 2006, 65.7 sfu in 2007, and 72 sfu in 2010. Actually, the year 2011 is in the ascending phase of the 24th solar cycle, in which the daily records showed that the F10.7 is low in January with the monthly average 72.7 sfu and gradually increased to maximum monthly value 134.8 sfu in November. The data in 2011 will be used in Section 3.2 where
occurrences of the F3 layer in months with different F10.7 could be compared. In addition, most days in 2006, 2007, 2010 and 2011 were geomagnetically quiet (refer to monthly ratio of days with daily Kp≤2.5 in right panels of Fig. 5). Cases during geomagnetic storms have been removed.
3. Observation and statistics 3.1. Morphological characteristic of ionogram traces with the F3 layer Ionogram records analyzed in this section were obtained in two magnetic low-latitude stations (Sanya, dip latitude 12.61N, and Vanimo, 11.21S) and two magnetic near-equator stations (Fortaleza, 6.61S, Kwajalein, 3.81N). For each station typical ionogram traces of different seasons are illustrated in Fig. 1, from A to D
Fig. 1. Typical ionogram traces with the F3 layer at four stations in each season except spring in (A) Sanya, (B) Vanimo, (C) Fortaleza, and (D) Kwajalein. The blue numbers are dip latitude of stations. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
J. Zhu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 102 (2013) 48–58
representative of Sanya, Vanimo, Fortaleza and Kwajalein, respectively. As Fig. 1 shows, typical ionogram traces in magnetic lowlatitude regions (A and B) are very different from those near magnetic equator (C and D) in shape. For one thing, traces near magnetic equator are morphologically upturned very evidently while those in magnetic low-latitude regions relatively flat. For another, near magnetic equator the cusp between the F2 and F3 layers is near the high-frequency end of the trace while such cusp tends to be close to F1 critical frequency in magnetic low-latitude region. The seasonal variations of those typical traces in each region are also presented in Fig. 1. For stations Sanya and Vanimo in magnetic low-latitude region, traces in local summer are distinguished with more evident cusps between the F2 and F3 layers and with higher formation heights (minimum virtual height 4400 km) of F2 layer stratifications compared with those (weak cusps and formation height 350 km) in local winter and autumn. On the other hand, for stations Fortaleza and Kwajalein near magnetic equator, stratifications of F2 layer in local summer are formed at high altitudes (minimum virtual height 4400 km), and cusps between the F2 and F3 layers are very close to the highfrequency end of traces. Meanwhile, differences between the minimum virtual heights of F3 and F2 layers are of the order of 150–200 km. In local winter, stratifications of F2 layers are formed at lower heights (minimum virtual height 350 km), with cusps between the F2 and F3 layers a little farther away from the highfrequency end and with smaller differences ( 100 km) between the minimum virtual heights of F3 and F2 layers than those in local summer. Typical traces in local autumn are similar to those in local winter. Those features above are consistent with the observation of Batista et al. (2002), and will be investigated in detail by statistics of the foF2-to-foF3 ratio and the difference between h′F3 and h′F2 in Sections 3.1.1 and 3.1.2, respectively. Before making the statistics of the F2 layer stratification, a few words should be stated as regard to the nomenclature of the F2 layer stratification. According to Lynn et al. (2000), the stratification was described in terms of a kink in the F2 profile, which could rise above the peak of the background F2 layer or remain below the peak depending on the latitude of observation. The additional layer was named F3 near the equator and F1.5 at low latitude. This latitudinal effect was suggested as arising from the cusp structure in the profile mapping down the field lines with increasing distance from the magnetic equator. It should be noted that it is not easy to separate F1.5 cases from F3 ones in ionograms. Fig. 2 compares typical developing processes of main F traces with F3 layers near and a little away and far away from magnetic equator in local summer. At the moment of the occurrence of the F3 layer (see the upper panel of Fig. 2), three traces are nearly similar due to the same formation mechanism reported by Balan et al. (1998). However, at the interim of the F3 layer, the trace far away from the magnetic equator turns flat with wider F3 trace because the fieldaligned diffusion of plasma from lower latitude compresses the F2 layer and enhances the peak density of F3 layer. On the other hand, the trace near magnetic equator does not changed obviously. The trace between them develops moderately. As a result, all those transient layers should be named F3 layer because of the same formation mechanism although the trace in the bottom-left corner of Fig. 2 is similar the one with the F1.5 layer. The name of F1.5 has an ancient history and was mainly considered as a possible manifestation of travelling ionospheric disturbances (TIDs), which is associated with the propagation of gravity waves (GWs) in this region (Heisler, 1962). Actually, a number of F1.5 cases at Sanya and Vanimo could most probably caused by the GWs propagating upward as revealed by Fagundes et al. (2007) but at EIA region with coexistence of the F1.5 and F3
51
Fig. 2. Comparing of typical developing processes of main F traces with F3 layers near and a little away and far away from magnetic equator in local summer. (A) Onset of the additional layer and (B) Interim of the additional layer.
layer. This feature is mainly characterized by a higher h′F1.5 comparing with h′F2. Thus the F1.5 layer at low latitude is mainly produced by the two mechanisms. An interesting work is not to separate the F1.5 and F3 layer, but to tell the different evolution feature of the F2 layer stratification caused by the mechanism of Balan et al. (1998) and GWs at Sanya and Vanimo. The topic will be investigated in future work. As regard to the current work, we removed cases with F1.5 layers from the data by requiring the difference between h′F3 and h′F2 more than 20 km in lowmagnetic latitude regions.
3.1.1. Diurnal variation and latitude dependence of the foF2-to-foF3 ratio In order to reveal characteristics of ionogram’s morphology more clearly, the foF2-to-foF3 ratio of ionograms in local summer and winter will be investigated in this section. Ionogram records with an interval of 15 or 20 min during the period when the F3 layers exist are picked out for statistics from continuous ionogram data which covers two separated months in local geomagnetically quiet summer and winter for each station (June 2011 and December 2010 for Sanya, December 2006 and July 2006 for Vanimo, January 2011 and June 2007 for Fortaleza, June 2007 and January 2007 for Kwajalein, Januray 2010 and July 2006 for Sao Luis). The statistical result of the ratio is shown against local time in Fig. 3. The gray circles in the figure are the record points, and the red solid line and the blue dashed line in each panel are respectively the fitting result of the data by the method of 2nd degree polynomial fit and the median value of the data. The dip latitudes of those stations’ locations are labeled in the figure. There are two main features of the foF2-to-foF3 ratio as shown in Fig. 3. First, the median value of the ratio tends to be larger as the location of the observation is closer to the magnetic equator. In summer, the monthly median values of ratios are as low as 70% in Sanya and Vanimo, 90% in Fortaleza and Kwajalein and 490% in Sao Luis according to Fig. 3. That means the cusp between the F2 and F3 layers is closer to the high-frequency end in the ionogram trace as the station’s location more approaching to magnetic equator. This latitude dependence of the trace feature can be also found in local winter as shown in the right panels of the figure. In addition, median values of such ratios in local winter are a little lower than those in local summer at Fortaleza, Kwajalein and Sao Luis stations with decrease of 3–8% and keep nearly constant at Sanya station. Morphologically speaking, near magnetic equator the cusp between the F2 and F3 layers in local summer is generally
52
J. Zhu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 102 (2013) 48–58
Fig. 3. Diurnal variation and latitude dependence of the foF2-to-foF3 ratio against local time in local summer and winter months. The gray circles are record points and the red solid line and the blue dash line in each panel are the fitting line of the data by the method of 2nd degree polynomial fit and the median value of the data respectively. Dip latitudes of stations are marked by red numbers. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
closer to the high-frequency end of the trace than that in local winter. And in magnetic low-latitude region, positions of such cusps relative to main F traces seem to be invariable in both seasons. Those can be illustrated by Fig. 1. Cases in Vanimo are too few to be taken into consideration. Second, the fitting results of the ratios tend to decrease with local time in local summer. That is, the cusp between the F2 and F3 layers is moving away from high-frequency end of the trace as the time progresses diurnally. Such local time variation of the ratio is more obvious as the location of the observation is further away from the magnetic equator. Especially, the ratio in Sao Luis in local summer which is closest to magnetic equator among those five stations nearly keeps constant before 15 LT. However, the local time variation of such ratio is not obvious in local winter in all 5 stations.
3.1.2. Diurnal variation and latitude dependence of the difference between h′F3 and h′F2 To investigate the characteristic of the difference between the minimal virtual height of the F3 layer (h′F3) and the minimal virtual height of the F2 layer (h′F2) in ionograms of different magnetic latitudes and seasons, the same data and procedure as
those in Section 3.1.1 were used. The statistical results are shown in Fig. 4 where the gray circles are record points and all of them are fitted by the 2nd degree polynomial fit shown by a red solid line in each panel. And the blue dashed line presents median value of data for each station. Fig. 4 presents two main features of the difference between h′F3 and h′F2. First, the fitting results in local summer tend to decrease with the progress of local time at all stations except Sao Luis where the fitting line keeps constant while those in local winter are nearly invariable for all stations. Second, the median values of results are as low as 60 km in magnetic low-latitude region (Sanya and Vanimo) while 4100 km near magnetic equator (Fortaleza, Kwajalein and Sao Luis) in local summer. Such feature reveals that traces in magnetic low-latitude regions are morphologically flatter than those near magnetic equator just as Fig. 1 shows. In local winter, median values of results have little latitude variation. And also in local winter, those values in regions near magnetic equator are lower than those in local summer (mainly in Fortaleza and Sao Luis with decreases of 100 km). However, median values in Sanya are almost equivalent in both seasons (Vanimo is excluded because of few cases).
J. Zhu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 102 (2013) 48–58
53
Fig. 4. Same as Fig. 3 but data are differences between h′F3 and h′F2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Such statistical results strongly support the seasonal variation of typical traces in Fig. 1 where typical ionogram traces near magnetic equator in local winter are not so upturned as those in local summer and traces in magnetic low-latitude region keep flat in both local winter and summer. 3.2. Features of occurrence of the F3 layer The occurrence of the F3 layer in this section is defined as the ratio of days with F3 layer in a specific hour in a month to total days of that month. We calculated the F3 occurrence for five ground-based ionosonde stations (From North to South are Sanya, Kwajalein, Jicamarca, Fortaleza and Vanimo) in an entire year and for one station (Ilorin) in 9 months because of data shortage. The duration of data in each station refers to continuous data coverage in Table 1. As Fig. 5 shows, the F3 occurrence rate is highest at stations of magnetic low-latitude region and lowest at the one over magnetic equator. The duration of F3 layer which could be observed from ground is mostly from morning to afternoon at stations in magnetic low-latitude region and near magnetic equator. Such time span is much shorter at the station over magnetic equator. It should be noted that the longitudinal location of the station
could affect the occurrence of the additional layer according to Raghavarao et al. (1977), Zhao et al. (2011b), and Karpachev et al. (2013) although researches before emphasized its latitude dependence (Lynn et al., 2000; Batista et al., 2002; Thampi et al., 2007). On the other hand, the rate is generally high in local summer while keeps low in local winter. And the rates in Sanya and Fortaleza in equinox are high as well. In addition, it is Jicamarca where the F3 occurrence is notable at sunset in local summer and especially in December 2011 as the solar flux (F10.7) increasing from monthly mean 72.7 sfu in January 2011 to the maximum 134.8 sfu in November 2011. For comparison, at Fortaleza the occurrence of the F3 layer in December 2011 is obviously lower than in January and February 2011 which shows that the daytime F3 layer occurrence reduces when solar flux increases. This is in accordance with the past observation (Balan et al., 1998; Batista et al., 2002; and Rama Rao et al., 2005). 3.3. Relationship between fmax and hmax when the F3 layer occurs The development of fmax (the F layer maximum frequency, foF2/ foF3) and hmax (the F layer peak height, hmF2/hmF3) around the onset of the occurrence of the F3 layer presents different patterns
54
J. Zhu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 102 (2013) 48–58
Fig. 5. Left panel: the F3 occurrence rate of each month in six stations. The black number in each panel is dip latitude of the station. Right panel: the rate of days with daily Kp≤2.5 in a month to total days of that month.
(formation patterns of the F3 layer). In order to define these patterns and compare them in different magnetic latitudes and seasons, we choose ground-based ionosonde data from a lowlatitude station Sanya and near-equator station Kwajalein in three separated months (covering local summer, local winter and local autumn respectively) of 2011 and make statistics of cases in which the F3 layer existed for at least one hour that is the characteristic time scale of the F3 layer occurrence according to Rama Rao et al. (2005). There are two formation patterns of the F3 layer during concerned periods. Pattern A is that the increase of fmax is after that of hmax while pattern B is that fmax keeps constant or decreases when hmax is increasing. These two patterns are illustrated in Fig. 6 where traces are presented at 15-min intervals and sequence plots of red traces and areas filled with pink color show durations of F3 layer. The blue solid and the green solid lines in each panel are fmax and hmax against local time respectively. The statistical results are listed in Table 2 where the number in column ‘Doy’ (days of year) presents the day with the F3 layer and
the letter in column ‘Type’ shows the corresponding formation pattern of the additional layer. And for each station in a specific season the rate of two patterns is also given at the bottom of the table. It should be noticed that both two patterns could appear in one day in different cases. According to Table 2, all formation patterns of the F3 layer in summer and autumn of Sanya are pattern A while the numbers of pattern A and B are nearly equivalent in winter with 45% versus 55%. However, Kwajalein has much more pattern B (75%) than A (25%) in summer, and all of cases in winter and in autumn are pattern B although only 4 cases in each season.
4. Discussion In this work, stations concerned in statistics are divided into two groups—one is near magnetic equator and the other is in magnetic low-latitude region—according to magnetic latitudes of their locations. That is reasonable because statistical results in
J. Zhu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 102 (2013) 48–58
55
Fig. 6. Illustrations of two formation patterns (A and B) of F3 layer at Sanya and Kwajalein stations in 2011. Sequence plots of ionogram traces in each panel are shown at 15-min intervals and among which red ones are traces with F3 layer. Red dots on ionogram traces highlight cusps between the F2 and F3 layers, and the pink filled areas indicate durations of F3 layers. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Section 3 reveal that features in one group are very similar but quite different from those in the other group. Such two regions were also used in the work of Uemoto et al. (2011). Diurnal variations and latitude dependence of F3 layer’s morphological features are investigated quantitatively in Section 3.1. foF2-to-foF3 ratios and differences between h′F3 and h′F2 tend to decrease with local time progress in local summer and such trend is much weaker as it is observed at the latitude more approaching magnetic equator. Those diurnal variations are not obvious in local winter. On the other hand, foF2-to-foF3 ratios and differences between h′F3 and h′F2 generally keep low in magnetic lowlatitude region while higher near magnetic equator in local summer. In local winter, this latitude dependence of foF2-to-foF3 ratios is the same as that in local summer but the latitude variation of differences between h′F3 and h′F2 does not exist. Features of the occurrence of the F3 layer are also investigated in Section 3.2 through data from six stations covering regions in magnetic low latitude, near magnetic equator and over magnetic equator in both hemispheres. Fig. 5 shows that the F3 occurrence rate is highest at stations in magnetic low-latitude region and lowest at the one over magnetic equator. Zhao et al. (2011b) obtained the similar results through the observation by the COSMIC/FORMOSAT-3 satellites. Furthermore, durations of the F3 layers are from morning to afternoon at stations in magnetic lowlatitude region and near magnetic equator but much shorter at the one over magnetic equator. Those results agree with the work of
Uemoto et al. (2011). In addition, it should be noted that the occurrence of the sunset F3 layer is notable in Jicamarca in local summer and especially in December 2011 as the solar flux increasing from monthly mean 72.7 sfu in January 2011 to 134.8 sfu in November 2011. Sunset F3 layer in Jicamarca was also reported by Zhao et al. (2011a) who investigated detailed features of sunset F3 layer in the low-latitude ionosphere. As mentioned in the introduction, the F3 layer is formed by a combination of upward E B drift and the neutral wind suggested by Balan et al. (1998). And then, Uemoto et al. (2007, 2011) improved such mechanism by claiming that the field-aligned diffusion of plasma acts to make the F3 layer prominent in the magnetic low-latitude region corresponding to the electron density-enhanced region associated with the equatorial anomaly and decrease the peak density of the F3 layer near magnetic equator. Based on statistical results in Section 3.3 where two formation patterns (A and B) of the F3 layer are investigated, we present in Fig. 7 an ideal equinoctial (without the summer-towinter neutral wind) and a solstitial (with the summer-to-winter neutral wind) distributions of those patterns. Pattern A indicates that the F3 peak density increases with time progress as the peak moving upward around the onset of the F3 layer’s occurrence while pattern B represents that the F3 peak density keeps nearly constant or decreases with time progress as the peak moving upward around the time when the F3 layer forms. According to Fig. 7a, it seems that because of faster uplift of F2 peak near
56
J. Zhu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 102 (2013) 48–58
Table 2 Statistical results of formation patterns of F3 layer in 2011 at Sanya and Kwajalein. Summer (June)
Winter (Jan.)
Autumn (Sept.)
Sanya
Sanya
Sanya
Kwajalein
Kwajalein
Kwajalein
Doy Type Doy Type Doy Type Doy Type Doy Type Doy Type 152 153 154 155 156 157 158 159 160 161 162 164 165 166 167 168 169 170 171 172 174 175 176 177 179 180 181 A 100 B 0 Type Rate
A A A A A A A A A A A A A A A A A A A A A A A A A A A
(%)
152 154 155 156 158 159 160 161 162 163 164 166 167 169 170 171 172 173 176 177 178 179 180 181
B B B B A B B A A A A B B A B B B B B B B B B B
25 75 Rate (%)
7 8 9 10 12 13 15 16 19 20 21 22 23 24 25 26 27 28 29 30
B A B A A B B B B B A, B B B A, B A A A A B A
45 55 Rate (%)
10 25 26 27
B B B B
0 100 Rate (%)
244 245 246 247 249 251 252 254 255 256 257 258 259 261 262 263 264 265 266 267 268 269 272
A A A A A A A A A A A A A A A A A A A A A A A
100 0 Rate (%)
245 248 254 264
B B B B
Fig. 7. The distribution of formation patterns of F3 layer (A) without summer-towinter neutral wind and (B) with summer-to-winter neutral wind when F3 layer occurs. Areas colored by pink and blue represent regions near magnetic equator and in magnetic low latitude respectively. Gray dashed lines in each panel are magnetic field lines (a sketch). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
0 100 Rate (%)
magnetic equator due to upward E B drift the plasma there diffuses to a higher-latitude region with moving upward (Uemoto et al., 2011). Such diffusion can cause that the F3 layer near magnetic equator, which forms later when the normal F2 layer develops at lower altitude through the photochemical and dynamical processes if the F3 layer reaches the altitude separated sufficiently from the F2 layer (Balan et al., 1998), develops with decreasing or nearly constant peak density as time progress near magnetic equator. However, in magnetic low-latitude region the F3 peak density tends to increasing with time progress when it forms because of the positive field-aligned diffusion effect (Uemoto et al., 2011). In Fig. 7b, the summer-to-winter neutral wind is taken into consideration in a solstice and it resists (enhances) the plasma diffusion to higher latitude in the windward (leeward). As a result, pattern A could be observed near magnetic equator in summer hemisphere and pattern B in magnetic low-latitude region in winter hemisphere compared with the ideal distribution in the equinox. In order to further investigate the relationship between the occurrence of the F3 layer in Sanya and the electron densityenhanced region, we have plotted the TEC contour map in the Southeast Asian region along longitude 1101E as a function of latitude and local time. One can refer to Zhao et al. (2009) to obtain the details of the method for deriving TEC map. The left panel of Fig. 8 shows the mean TEC contour map for selected day on January 7th, 9th, 21st, and 24th in 2011 when pattern B develops most at Sanya. It is shown that EIA was intensified during these days while the location of Sanya was at the northern crest’s inner edge near to the magnetic equator. The middle and right panels of Fig. 8 show the situations for March and July where pattern A is dominated. The locations of the crests for both two
months were more equatorward compared to the January situation. The position of Sanya was right situated in the density enhance region of northern EIA. Because the crest of the plasma density moves to magnetic lower-latitude region with increasing altitude, the crest of NmF2 should be a little farther away from magnetic equator than the crest of TEC does. Fig. 9 illustrates a sketch map of latitude distribution of NmF2 at two different moments in the forenoon of winter, equinox and summer. The later latitude distribution of NmF2 becomes broader with larger value of crests than that of current moment because of enhanced plasma fountain effect related to the stronger zonal electric field in the forenoon. On account of the ion drag effect of the summer-to-winter neutral wind, both two crests of NmF2 move a little to the winter hemisphere in solstice seasons. As a result, Sanya locates at northern crest’s inner edge near the equator in winter and NmF2 over it decreases with time progress. On the other hand, in summer and equinox seasons, the location of Sanya is under the crest of NmF2 and consequently the peak density over it increases with time progress. It should be noted that the effect of stations’ longitudes on the result above is ignored because it is not the main factor of the variation of the additional layer’s formation. The detail about longitude effect on the formation patterns of F3 layers could be investigated in future study.
5. Summary Our current work concerns morphological features of ionogram traces with F3 layer, the occurrence and the formation pattern of the F3 layer in magnetic low-latitude region and near magnetic equator. Statistical results show two morphological parameters of traces with F3 layers have the latitude dependence, diurnal variation, and seasonal variation. All two parameters tend to decrease with local time progress and their values mostly keep low in magnetic low-latitude region in local summer. However, near magnetic equator, their diurnal variations are much weaker. On the other hand, all variations above became weak or even disappeared in local winter. In addition, we obtained the distribution of formation patterns of the F3 layer which is caused by plasma diffusion and the transequatorial neutral wind. Here we have shown that even in magnetic low-latitude region, the occurrence of the F3 layer is not always accompanied by the density-enhanced region proposed by Uemoto et al. (2011) as the location of the EIA crest is affected by the intensity of the zonal electric field and transequatorial
J. Zhu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 102 (2013) 48–58
57
Fig. 8. Mean TEC contour map in January (7th, 9th, 21st, and 24th), March, and July 2011. The horizontal black dashed line marks the location of Sanya (geographic latitude 18.21N).
Acknowledgements This research was supported by the National Natural Science Foundation of China (41174138, 41131066), the National Important Basic Research Project of China (2011CB811405). Special thanks should be given to the Ionospheric Prediction Service of Australia for providing data in Australia and to the Digisonde Global Ionospheric Observatory (GIRO).
References
Fig. 9. The sketch map of latitude distribution of NmF2 at two different moments in the forenoon of winter, equinox and summer. The red and the black dashed lines mark the location of Sanya (geographic latitude 18.21N) and the magnetic equator respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
meridional wind. The F3 layer feature at a single site is quite variable and should be understood associated with the entire motion of the EIA region.
Balan, N., Bailey, G.J., 1995. Equatorial plasma fountain and its effects: possibility of an additional layer. Journal of Geophysical Research 100, 21421–21432, http: //dx.doi.org/10.1029/95JA01555. Balan, N., Bailey, G.J., Abdu, M.A., Oyama, K.I., Richards, P.G., Macdougall, J., Batista, I.S., 1997. Equatorial plasma fountain and its effects over three locations: evidence for an additional layer, the F3 layer. Journal of Geophysical Research 102, 2047–2056, http: //dx.doi.org/10.1029/95JA02639. Balan, N., Batista, I.S., Abdu, M.A., MacDougall, J., Bailey, G.J., 1998. Physical mechanism and statistics of occurrence of an additional layer in the equatorial ionosphere. Journal of Geophysical Research 103, 29169–29181, http://dx.doi. org/10.1029/98JA02823. Balan, N., Batista, I.S., Abdu, M.A., Bailey, G.J., Watanabe, S., MacDougall, J., Sobral, J.H.A., 2000. Variability of an additional layer in the equatorial ionosphere over Fortaleza. Journal of Geophysical Research 105 (A5), 10,603–10,613, http://dx.doi.org/10.1029/ 1999JA000020. Balan, N., Thampi, S.V., Lynn, K., Otsuka, Y., Alleyne, H., Watanabe, S., Abdu, M.A., Fejer, B.G., 2008. F3 layer during penetration electric field. Journal of Geophysical Research 113, A00A07, http://dx.doi.org/10.1029/2008JA013206. Balan, N., Yamamoto, M., Sreeja, V., Batista, I.S., Lynn, K.J.W., Abdu, M.A., Ravindran, S., Kikuchi, T., Otsuka, Y., Shokawa, K., Alex, S., 2011. A statistical study of the response of the dayside equatorial F2 layer to the main phase of intense geomagnetic storms as an indicator of penetration electric field. Journal of Geophysical Research 116, A03323, http://dx.doi.org/10.1029/2010JA016001. Batista, I.S., Abdu, M.A., McDougall, J., Souza, J.R., 2002. Long term trends in the frequency of occurrence of the F3 layer over Fortaleza. Journal of Atmospheric and Solar-Terrestrial Physics 64, 1409–1412. Chaitanya, P.P., Patra, A.K., Rao, S.V.B., 2012. Contrasting features of the F3 layer during high and low solar activity conditions observed from Indian low latitude. Indian Journal of Radio & Space Physics 41, 121–129. Depuev, V.H., Pulinets, S.A., 2001. Intercosmos-19 observations of an additional topside ionization layer: the F3-layer. Advances in Space Research 27 (6–7), 1289–1292, http://dx.doi.org/10.1016/S0273-1177(01)00205. Fagundes, P.R., Klausner, V., Sahai, Y., Pillat, V.G., Becker-Guedes, F., Bertoni, F.C.P., Bolzan, M.J.A., Abalde, J.R., 2007. Observations of daytime F2-layer stratification under the southern crest of the equatorial ionization anomaly region. Journal of Geophysical Research 112, A04302, http://dx.doi.org/10.1029/2006JA011888. Fagundes, P.R., Klausner, V., Bittencourt, J.A., Sahai, Y., Abalde, J.R., 2011. Seasonal and solar cycle dependence of F3-layer near the southern crest of the equatorial ionospheric anomaly. Advances in Space Research 48 (3), 472–477.
58
J. Zhu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 102 (2013) 48–58
Heisler, L.H., 1962. The anomalous ionospheric stratification F1.5. Journal of Atmospheric and Solar-Terrestrial Physics 24, 483–489. Hsiao, C.C., Liu, J.Y., Tsunoda, R.T., Fukao, S., Saroso, S., Nozaki, K., Badillo, V.L., Berkey, F.T., Chen, S.W., Yamamoto, M., 2001. Evidence for the geographic control of additional layer formation in the low latitude ionosphere. Advances in Space Research 27, 1293–1297. Huang, C., 1974. A certain behavior of the ionospheric F2 region at low latitudes. Radio Science 9 (5), 519–532, http://dx.doi.org/10.1029/RS009i005p00519. Huang, C., 1975. The travelling bifurcation of the equatorial F2 layer. Radio Science 10 (5), 507–516, http://dx.doi.org/10.1029/RS010i005p00507. Jenkins, B., Bailey, G.J., Abdu, M.A., Batista, I.S., Balan, N., 1997. Observations and model calculations of an additional layer in the topside ionosphere above Fortaleza, Brazil. Annales of Geophysics 15, 753–759, http://dx.doi.org/10.1007/ s00585-997-0753-3. Karpachev, A.T., Klimenko, M.V., Klimenko, V.V., Zhbankov, G.A., Telegin, V.A., 2012. Latitudinal structure of the equatorial F3 layer based on the Intercosmos-19 topside sounding data. Journal of Atmospheric and Solar-Terrestrial Physics 77, 186–193, http://dx.doi.org/10.1016/j.jastp.2011.12.018. Karpachev, A.T., Klimenko, M.V., Klimenko, V.V., Kuleshova, 2013. Statistical study of the F3 layer characteristics retrieved from Intercosmos-19 satellite data. Journal of Atmospheric and Solar-Terrestrial Physics, http://dx.doi.org/10.1016/j. jastp.2013.01.010. Klimenko, M.V., Klimenko, V.V., Ratovsky, K.G., Goncharenko, L.P., Sahai, Y., Fagundes, P.R., Jesus, R., Abreu, A.J., Vesnin, A.M., 2011. Numerical modeling of ionospheric effects in the middle- and low-latitude F region during geomagnetic storm sequence of 9–14 September 2005. Radio Science 46, RS0D03, http://dx.doi.org/10.1029/2010RS004590. Klimenko, M.V., Klimenko, V.V., 2012. Disturbance dynamo, prompt penetration electric field and overshielding in the Earth’s ionosphere during geomagnetic storm. Journal of Atmospheric and Solar-Terrestrial Physics 90–91, 146–155, http://dx.doi.org/10.1016/j.jastp.2012.02.018. Klimenko, Zhao, B., Karpachev, A.T., Klimenko, V.V., 2012a. Stratification of the LowLatitude and Near-Equatorial F2 Layer, Topside Ionization Ledge, and F3 Layer: What We Know about This? A Review, International Journal of Geophysics, doi:10.1155/2012/938057. Klimenko, M.V., Klimenko, V.V., Karpachev, A.T., 2012b. Formation mechanism of additional layers above regular F2 layer in the near-equatorial ionosphere during quiet period. Journal of Atmospheric and Solar-Terrestrial Physics 10.1016/j.jastp.2012.02.011. Lin, C.H., Richmond, A.D., Bailey, G.J., Liu, J.Y., Lu, G., Heelis, R.A., 2009a. Neutral wind effect in producing a storm time ionospheric additional layer in the equatorial ionization anomaly region. Journal of Geophysical Research 114, A09306, http://dx.doi.org/10.1029/2009JA014050. Lin, C.H., Richmond, A.D., Liu, J.Y., Bailey, G.J., Reinisch, B.W., 2009b. Theoretical study of new plasma structures in the low-latitude ionosphere during a major magnetic storm. Journal of Geophysical Research 114, A05303, http://dx.doi. org/10.1029/2008JA013951. Lockwood, G.E.K., Nelms, G.L., 1964. Topside sounder observations of the equatorial anomaly in the 751W longitude zone. Journal of Atmospheric and Terrestrial Physics 26, 569–580. Lynn, K.J.W., Harris, T.J., Sjarifudin, M., 2000. Stratification of the F2 layer observed in Southeast Asia. Journal of Geophysical Research 105, 27,147–27,156, http://dx. doi.org/10.1029/2000JA900056. Paznukhov, V.V., Reinisch, B.W., Song, P., Huang, X., Bullett, T.W., Veliz, O., 2007. Formation of an F3 layer in the equatorial ionosphere: a result from strong IMF changes. Journal of Atmospheric and Solar-Terrestrial Physics 69, 1292–1304.
Raghavarao, R., Sivaraman, M.R., 1974. Ionization ledges in the equatorial ionosphere. Nature 249, 331–332, http://dx.doi.org/10.1038/249331a0. Raghavarao, R., Sharma, P., Jain, A.R., 1977. Ionization ledges and the counterelectrojets in the equatorial ionosphere. Space Research 17, 417–421. Rama Rao, P.V.S., Niranjan, K., Prasad, D.S.V.V.D., Brahmanandam, P.S., Gopikrishna, S., 2005. Features of additional stratification in ionospheric F2 layer observed for half a solar cycle over Indian low latitudes. Journal of Geophysical Research (110), A04307, http://dx.doi.org/10.1029/2004JA010646. Ratcliffe, J.A., 1951. Some regularities in the F2 region of the ionosphere. Journal of Geophysical Research 110, 56,487–56,507. Sayers, J., Rothwell, P., Wager, J.H., 1963. Field aligned strata in the ionization above the ionospheric F2 layer. Nature 198, 230–233, http://dx.doi.org/10.1038/ 198230a0. Sen, H.Y., 1949. Stratification of the F2 layer of the ionosphere over Singapore. Journal of Geophysical Research 54, 363–366. Sharma, P., Raghavarao, R., 1989. Simultaneous occurrence of ionization ledge and counter electrojet in the equatorial ionosphere: Observational evidence and its implications. Canadian Journal of Physics 67, 166–172. Skinner, N.J., Brown, R.A., Wright, R.W., 1954. Multiple stratification of the F-layer at Ibadan. Journal of Atmospheric and Terrestrial Physics 5, 92–100. Sreeja, V., Ravindran, S., Pant, T.K., 2010. Features of the F3 layer occurrence over the equatorial location of Trivandrum. Annales of Geophysics 28, 1741–1747, http://dx.doi.org/10.5194/angeo-28-1741-2010. Thampi, S.V., Balan, N., Ravindran, S., Pant, T.K., Devasia, C.V., Sreelatha, P., Sridharan, R., Bailey, G.J., 2007. An additional layer in the low-latitude ionosphere in Indian longitudes: total electron content observations and modeling. Journal of Geophysical Research 112, A06301, http://dx.doi.org/10.1029/ 2006JA011974. Uemoto, J., Ono, T., Kumamoto, A., Iizima, M., 2006. Statistical analysis of the ionization ledge in the equatorial ionosphere observed from topside sounder satellites. Journal of Atmospheric and Solar-Terrestrial Physics 68 (12), 1340–1351, http://dx.doi.org/10.1016/j.jastp.2006.05.015. Uemoto, J., Ono, T., Maruyama, T., Saito, S., Iizima, M., Kumamoto, A., 2007. Magnetic conjugate observation of the F3 layer using the SEALION ionosonde network. Geophysical Research Letters 34, L02110, http://dx.doi.org/10.1029/ 2006GL028783. Uemoto, J., Maruyama, T., Ono, T., Saito, S., Iizima, M., Kumamoto, A., 2011. Observations and model calculations of the F3 layer in the Southeast Asian equatorial ionosphere. Journal of Geophysical Research 116, A03311, http://dx. doi.org/10.1029/2010JA016086. Zain, A.F.M., Abdullah, S., Homam, M.J., Seman, F.C., Abdullah, M., Ho, Y.H., 2008. Observations of the F3-layer at equatorial region during 2005. Journal of Atmospheric and Solar-Terrestrial Physics 70, 918–925, http://dx.doi.org/ 10.1016/j.jastp.2007.12.002. Zhao, B., Wan, W., Liu, L., 2005. Responses of equatorial anomaly to the October– November 2003 superstorms. Annales of Geophysics 23 (3), 693–706. Zhao, B., Wan, W., Liu, L., Ren, Z., 2009. Characteristics of the ionospheric total electron content of the equatorial ionization anomaly in the Asia/Australian region during 1996–2004. Annales of Geophysics 27, 3861–3873. Zhao, B., Wan, W., Reinisch, B., Yue, X., Le, H., Liu, J., Xiong, B., 2011a. Features of the F3 layer in the low-latitude ionosphere at sunset. Journal of Geophysical Research 116, A01313, http://dx.doi.org/10.1029/2010JA016111. Zhao, B., Wan, W., Yue, X., Liu, L., Ren, Z., He, M., Liu, J., 2011b. Global characteristics of occurrence of an additional layer in the ionosphere observed by COSMIC/ FORMOSAT-3. Geophysical Research Letters 38, L02101, http://dx.doi.org/ 10.1029/2010GL045744.