Unusual nighttime impulsive foF2 enhancements at low latitudes: Phenomenology and possible explanations

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Advances in Space Research xxx (2013) xxx–xxx www.elsevier.com/locate/asr

Unusual nighttime impulsive foF2 enhancements at low latitudes: Phenomenology and possible explanations L. Perna a, M. Pezzopane b,⇑, E. Zuccheretti b, P.R. Fagundes c, R. de Jesus c, M.A. Cabrera d,e,f, R.G. Ezquer e,f,g a

Dipartimento di Fisica, Universita` di Roma “La Sapienza”, 00185 Rome, Italy b Istituto Nazionale di Geofisica e Vulcanologia, 00143 Rome, Italy c Universidade do Vale do Paraı´ba, 12244-000 Sa˜o Jose´ dos Campos, Brazil d Laboratorio de Telecomunicaciones, DEEC, FACET, Universidad Nacional de Tucuma´n, 4000 Tucuma´n, Argentina e Laboratorio de Iono´sfera, Departamento de Fı´sica, FACET, Universidad Nacional de Tucuma´n, 4000 Tucuma´n, Argentina f CIASUR, Facultad Regional Tucuma´n, Universidad Tecnolo´gica Nacional, 4000 Tucuma´n, Argentina g Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas, 1033 Buenos Aires, Argentina

Abstract This paper is focused on unusual nighttime impulsive electron density enhancements that are rarely observed at low latitudes on a wide region of South America, under quiet and medium/high geomagnetic conditions. The phenomenon under investigation is very peculiar because besides being of brief duration, it is characterized by a pronounced compression of the ionosphere. The phenomenon was studied and analyzed using both the F2 layer critical frequency (foF2) and the virtual height of the base of the F region (h0 F) values recorded at five ionospheric stations widely distributed in space, namely: Jicamarca (12.0°, 76.8°, magnetic latitude 2.0°), Peru; Sao Luis (2.6°, 44.2°, magnetic latitude +6.2°), Cachoeira Paulista (22.4°, 44.6°, magnetic latitude 13.4°), and Sa˜o Jose´ dos Campos (23.2°, 45.9°, magnetic latitude 14.1°), Brazil; Tucuma´n (26.9°, 65.4°, magnetic latitude 16.8°), Argentina. In a more restricted region over Tucuma´n, the phenomenon was also investigated by the total electron content (TEC) maps computed by using measurements from 12 GPS receivers. A detailed analysis of isoheight ionosonde plots suggests that traveling ionospheric disturbances (TIDs) caused by gravity wave (GW) propagation could play a significant role in causing the phenomenon both for quiet and for medium/high geomagnetic activity; in the latter case however a recharging of the fountain effect, due to electric fields penetrating from the magnetosphere, joins the TID propagation and plays an as much significant role in causing impulsive electron density enhancements. Ó 2013 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Equatorial ionosphere; Electron density enhancement; Traveling ionospheric disturbance; Fountain effect; TEC

1. Introduction The ionospheric F2 layer presents a significant day-today variability. The F2 layer characteristics, such as the F2 layer critical frequency (foF2), the electron density max⇑ Corresponding author. Tel.: +39 06 51860525; fax: +39 06 51860397.

E-mail addresses: [email protected] (L. Perna), michael. [email protected] (M. Pezzopane), [email protected] (E. Zuccheretti), [email protected] (P.R. Fagundes), jesus.rodolfo@ hotmail.com (R. de Jesus), [email protected] (M.A. Cabrera), [email protected] (R.G. Ezquer).

imum (NmF2) (foF2 is related to NmF2 as 1.24  1010(foF2)2, where the units of NmF2 and foF2 are electrons/m3 and MHz, respectively) and the height of NmF2 (hmF2) often show significant deviation from long-term values. Many studies have been made to disclose any trends of these F2-layer characteristics as a function of local time, season, and solar/geomagnetic activity (e.g. Liu et al., 2009, 2011, 2012; Liu et al., 2010; Akala et al., 2010; Borries and Hoffmann, 2010; Chen and Liu, 2010; David and Sojka, 2010; Lin et al., 2010; Hall et al., 2011; He et al., 2011; Lee et al., 2011; Burns et al., 2012;

0273-1177/$36.00 Ó 2013 COSPAR. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.asr.2013.05.014

Please cite this article in press as: Perna, L., et al. Unusual nighttime impulsive foF2 enhancements at low latitudes: Phenomenology and possible explanations. J. Adv. Space Res. (2013), http://dx.doi.org/10.1016/j.asr.2013.05.014

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Chen et al., 2012; Meza et al., 2012; Mikhailov et al., 2012; Pavlov, 2012; Pietrella et al., 2012), that are essential for developing empirical ionospheric models (e.g. Bilitza and Reinisch, 2008; Gulyaeva, 2012; Hoque and Jakowski, 2012). Even though usually F2 layer disturbances have been linked to solar and geomagnetic activity variations, there are disturbances, termed “meteorological” by Rishbeth and Mendillo (2001), appearing from the lower part of the atmosphere and hence considered different from those caused by the solar/geomagnetic activity. Mikhailov et al. (2004) suggested the term “Q disturbances” to indicate NmF2 deviations (positive or negative) greater than 40% if all 3 hourly Ap indices were 67 for the previous 24 h. Nighttime values of foF2 and total electron content (TEC) do not always decrease smoothly. With regard to this issue, many statistical studies have been performed on NmF2 and TEC nighttime enhancements at mid and low latitudes (e.g. Mikhailov et al., 2000a; Farelo et al., 2002; Pavlov and Pavlova, 2007; Luan et al., 2008). Recently, Pezzopane et al. (2011) have investigated a very unusual event which they have called “impulsive enhancement”, because a sudden large increase in electron density is immediately followed by an equally rapid recovery phase. The phenomenon occurred on a wide region of South America, below the southern crest of the equatorial anomaly, for low solar/geomagnetic activity. The event was very distinctive, because the impulsive enhancement corresponded to a pronounced compression of the ionosphere. Their analysis showed that the propagation of traveling ionospheric disturbances (TIDs) could be considered as the main mechanism responsible for the observed phenomenology. The event analyzed by Pezzopane et al. (2011) was the only occurred from August 2007, when an Advanced Ionospheric Sounder by Istituto Nazionale di Geofisica e Vulcanologia (AIS-INGV) ionosonde was installed at Tucuma´n (TUC) (26.9°, 65.4°, magnetic latitude 16.8°), Argentina (Pezzopane et al., 2007), to July 2010. This paper is focused on the analysis of all the other similar events occurred from August 2010 to January 2012 both for quiet and disturbed geomagnetic conditions. The aim of this work is on the one hand to confirm for a larger dataset of events what it was already found by Pezzopane et al. (2011) for quiet geomagnetic conditions, and on the other hand to look into the differences between the events occurred for quiet and disturbed geomagnetic conditions. 2. Data sets In order to detect the peculiar foF2 nighttime impulsive enhancements that occurred from August 2010 to January 2012, reference was made to the autoscaling visualization feature of the electronic Space Weather upper atmosphere (eSWua) database (http://www.eswua.ingv.it/) (Romano et al., 2008), simply by checking the daily foF2 plots computed on the basis of the values produced automatically as

output by Autoscala (Pezzopane and Scotto, 2007) from the ionograms recorded at Tucuma´n. The analysis was then based on data recorded from additional four ionosondes: Sao Luis (SL) (2.6°, 44.2°, magnetic latitude +6.2°), Cachoeira Paulista (CP) (22.4°, 44.6°, magnetic latitude 13.4°), Sa˜o Jose´ dos Campos (SJC) (23.2°, 45.9°, magnetic latitude 14.1°), Brazil, and Jicamarca (JIC) (12.0°, 76.8°, magnetic latitude 2.0°), Peru. The ionospheric station at SJC is equipped with a Canadian Advanced Digital Ionosonde (CADI) (MacDougall et al., 1993). The ionospheric stations at JIC, SL and CP are equipped with a Digisonde (Bibl and Reinisch, 1978). JIC, SL, and CP data were downloaded from the Global Ionospheric Radio Observatory web portal (Galkin et al., 2012). Twelve GPS receivers located in South America (see Table 1 for the corresponding locations) were also considered to compute TEC values in a region extending in latitude from 0° to 40° and in longitude from 50° to 80°. The GPS data were obtained from the International Global Navigation Satellite System Service (IGS) database (Dow et al., 2009). The International Geomagnetic Reference Field 11 (Finlay et al., 2010) was used to calculate the geomagnetic latitude from the corresponding geographical coordinates of the ionosonde and the GPS receivers’ stations (http:// wdc.kugi.kyoto-u.ac.jp/igrf/gggm/index.html). The level of geomagnetic activity characterizing the events under study is indicated by the 3 hourly-Kp, AE, and Dst indices, which were downloaded from the World Data Center for Geomagnetism, Kyoto, Japan from the Web site http://wdc.kugi.kyoto-u.ac.jp/. Equatorial zonal electric field data recorded on September and November 2010 by the Communications/Navigation Outage Forecasting System (C/NOFS) satellite, developed by the Air Force Research Laboratory Space Vehicles Directorate, were also taken into account and downloaded from the NASA Space Physics Data Facility section reachable through the site http://cdaweb.gsfc.nasa.gov/istp_public/.

3. Results Fig. 1 shows nine atypical foF2 enhancements that were recorded at Tucuma´n on 8 and 9 September 2010, 2 and 5 November 2010, 19 March 2011, 7 April 2011, 24 June 2011, 26 and 31 January 2012. In particular, all the ionograms recorded for all the month including each event were considered and validated according to the International Union of Radio Science (URSI) standard (Wakai et al., 1987). Once the scaled foF2 values had been obtained, the corresponding monthly mean (green1 curve in Fig. 1) and standard deviation (red and blue curves in Fig. 1 1 For interpretation of color in Fig. 1, the reader is referred to the web version of this article.

Please cite this article in press as: Perna, L., et al. Unusual nighttime impulsive foF2 enhancements at low latitudes: Phenomenology and possible explanations. J. Adv. Space Res. (2013), http://dx.doi.org/10.1016/j.asr.2013.05.014

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Table 1 Coordinates of GPS receivers and ionosondes used in the study. Location

Instrument

Geographical coordinates

Magnetic latitude

Local time

Bogota, Colombia (BOGT) Puerto Ayora, Ecuador (GLPS) Portho Velho, Brazil (POVE) Salvador, Brazil (SAVO) Arequipa, Peru (AREQ) Cachoeira Paulista, Brazil (CHPI) Salta, Argentina (UNSA) Curitiba, Brazil (UFPR) Santiago, Chile (SANT) San Martin, Argentina (BUE1) La Plata, Argentina (LPGS) Concepcion, Chile (CONZ) Sao Luis, Brazil (SL) Jicamarca, Peru (JIC) Cachoeira Paulista, Brazil (CP) Sa˜o Jose´ dos Campos, Brazil (SJC) Tucuma´n, Argentina (TUC)

GPS GPS GPS GPS GPS GPS GPS GPS GPS GPS GPS GPS DPS4 DPS4 DPS4 CADI AIS-INGV

+4.6°, 71.5° +0.7°, 90.3° 8.7°, 63.9° 12.9°, 38.4° 16.5°, 71.5° 22.7°, 45.0° 24.7°, 65.4° 25.4°, 49.2° 33.2°, 70.7° 34.6°, 58.5° 34.9°, 57.9° 36.8°, 73.0° 2.6°, 44.2° 12.0°, 76.8° 22.4°, 44.6° 23.2°, 45.9° 26.9°, 65.4°

+14.6° +10.2° +1.2° 4.5° 6.4° 13.6° 14.6° 16.0° 23.0° 24.7° 25.0° 26.6° +6.2° 2.0° 13.4° 14.1° 16.8°

UT-5 UT-6 UT-4 UT-3 UT-5 UT-3 UT-4 UT-3 UT-5 UT-4 UT-4 UT-5 UT-3 UT-5 UT-3 UT-3 UT-4

represent the [mean ± standard deviation], respectively) were calculated. All the events show a significant and rapid (less than 2 h for all the cases) increase in foF2 beginning at about 05:00 universal time (UT) (local time (LT) = UT  4; LT is calculated as the eastern time of the central meridian of the corresponding time zone), whose peak values are well outside the confidence interval defined by the standard deviation and range from 50% to 85% of the mean value; the only exception is the event occurred the 5 November 2010 presenting a peak which is inside the corresponding confidence interval and whose value is only 18% of the mean value. The phenomenon is extremely impulsive, and the bell-shaped trend of the foF2 plot is very narrow, because after reaching a peak, the foF2 falls to very low values and the decrease phase is even more rapid (about 1 h) than the increase phase. Fig. 2 shows the same as Fig. 1 but for the virtual height of the base of the F region (h0 F); also for this ionospheric characteristic, all the ionograms recorded for all the month including each event were considered and validated. The remarkable feature emerging by comparing Fig. 2 and Fig. 1 is that at Tucuma´n when foF2 presents an impulsive enhancement and reaches its maximum value, the ionosphere is strongly compressed, with the minimum value of h0 F well outside the confidence interval defined by the standard deviation for seven events out of nine. This, as it was already highlighted by Pezzopane et al. (2011), suggests that a secondary fountain effect and/or a downward plasma diffusion from the plasmasphere cannot be considered as the only agents responsible for the event, and that something like a TID might have played a substantial role (Lu et al., 2001). Table 2 shows, respectively, the maximum values of the 3 hourly-Kp, AE, and Dst indices recorded for each of the identified event. According to both the storm classification made by Gonzalez et al. (1994) and the fact that AE values well above 500 nT are necessary to trigger intense sub-

storms, causing serious ionospheric plasma modification at mid and low latitudes (Akasofu, 1970; Pro¨lss, 1993), six of the identified events were considered as occurring for low geomagnetic activity and three of them as occurring for medium/high geomagnetic activity. In order to look for the possible presence of TIDs caused by gravity wave (GW) propagation, the ionogram traces recorded at Tucuma´n from 00:00 to 10:00 UT for all the identified dates were manually digitized, obtaining a sequence of couples of values (N, h0 ) for each ionogram, where N is the electron density and h0 is the virtual height of reflection. Then, inversion from the ionogram trace (N, h0 ) to the profile (N, h), where h is the real height of reflection, was performed using the POLAN inversion technique (Titheridge, 1988). From the profiles (N, h) isoheight curves N(h = 170, 180, 190, 200, 210, 220, 230, 240, 250, 260, 270, 280, 290, 300 km) were obtained and plotted in Fig. 3. This figure shows that for all the considered days maximum N variations occur first at higher altitudes and then at lower altitudes, showing a downward phase shift which is characteristic of GW propagation in the ionospheric F region (Hines, 1960). Using isoheight curves shown in Fig. 3, it is possible to estimate the GW period T; the vertical phase velocity vz is calculated using the peak of two consecutive heights (see Fig. 5 of Pezzopane et al. (2011)), the vertical wavelength kz = vzT is then obtained. The corresponding horizontal wavelength (kh) can be determined using a relationship between kh and kz given by Hines (1960), namely, x2 k2h  ðx2g  x2 Þk2z , where xg is the Brunt–Va¨isa¨la¨ frequency, which was taken as 2p/ 14 min1 (Abdu et al., 1982), and x = 2p/T is the wave angular frequency. Table 3 lists the estimates of the aforementioned wave parameters for all the considered events as inferred from the isoheight plots of Fig. 3. The values of T, vz, and kz are consistent with the analysis performed by Klausner et al. (2009) using the CADI ionosonde installed at SJC. With regard to the horizontal wavelength values,

Please cite this article in press as: Perna, L., et al. Unusual nighttime impulsive foF2 enhancements at low latitudes: Phenomenology and possible explanations. J. Adv. Space Res. (2013), http://dx.doi.org/10.1016/j.asr.2013.05.014

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Fig. 1. The foF2 plots obtained at Tucuma´n on 8 and 9 September 2010, 2 and 5 November 2010, 19 March 2011, 7 April 2011, 24 June 2011, 26 and 31 January 2012.

Please cite this article in press as: Perna, L., et al. Unusual nighttime impulsive foF2 enhancements at low latitudes: Phenomenology and possible explanations. J. Adv. Space Res. (2013), http://dx.doi.org/10.1016/j.asr.2013.05.014

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Fig. 2. Same as Fig. 1 for h0 F.

and according to Leitinger and Rieger (2005), some can be considered as characteristic of medium-scale TIDs (MSTIDs), some as characteristic of large-scale TIDs (LSTIDs).

In order to investigate the spatial extent of the unusual nighttime impulsive foF2 enhancements detected at Tucuma´n, Figs. 4 and 5 show the foF2 and the h0 F plots

Please cite this article in press as: Perna, L., et al. Unusual nighttime impulsive foF2 enhancements at low latitudes: Phenomenology and possible explanations. J. Adv. Space Res. (2013), http://dx.doi.org/10.1016/j.asr.2013.05.014

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Table 2 Three hourly-Kp, AE, and Dst maximum values characterizing the nine dates under study. Date

Kpmax

AEmax [nT]

Dstmax [nT]

Magnetic Activity

8–9 September 2010 2–5 November 2010 19 March 2011 7 April 2011 24 June 2011 26 January 2012 31 January 2012

3

210

35

Low

2

350

12

Low

3 6 4 5 3

420 1176 750 550 400

20 60 30 73 20

Low Medium–high Medium Medium Low

obtained at different ionospheric stations for two out of the nine identified events, on 8 September 2010 and 7 April 2011, for low and medium–high geomagnetic activity respectively. Also the values recorded at JIC, SL, SJC, and CP were manually scaled according to the URSI standard and, in particular, for SJC the corresponding monthly mean and standard deviation were also calculated. Unfortunately, on 8 September 2010 at SJC from 02:00 to 06:00 UT the ionograms were characterized by spread F phenomena and it was impossible to obtain a value for foF2. In Fig. 4d, a significant and rapid (less than 2 h) increase in foF2 beginning at about 05:00 UT (01:00 LT) is evident at TUC, the foF2 peak reaching a value about 60% larger than the mean value. The phenomenon is really impulsive, and the bell-shaped development of the foF2 plot is very narrow, because after reaching a peak, the foF2 falls to very low values and the decrease phase is even more rapid (about 1 h) than the increase phase. Even though less pronounced, Fig. 4b shows that the same phenomenon also occurs at CP (LT = UT-3). Moreover, given that the foF2 plot obtained at SL does not exhibit any impulsive enhancement, Fig. 4 also highlights how the event was confined around the southern anomaly crest. Fig. 4e–h also show that when foF2 undergoes impulsive enhancement at TUC, the ionosphere, besides TUC, is strongly compressed also at CP and SJC (LT = UT-3). Again, at SL the h0 F trend does not exhibit any significant feature. Fig. 4 is representative and presents the same characteristics of all the other cases occurred for low and medium geomagnetic activity (figures not shown here). Concerning the case recorded on 7 April 2011, occurred for medium–high geomagnetic activity, again (see Fig. 5d), a significant and rapid (less than 2 h) increase in foF2 beginning at about 05:00 UT (01:00 LT) is evident at TUC, the foF2 peak reaching a value about 55% larger than the mean value. Also in this case the phenomenon is really impulsive, and the increase phase is followed by a decrease phase even more rapid (about 1 h). Even though less pronounced, an electron density increase occurs also at CP and SJC (see Fig. 5b and c). For this date an electron density increase is observed also at the magnetic equator, specifically at JIC (see Fig. 5a); hence, contrarily to what

observed on 8 September 2010, Fig. 5 highlights that the event is in this case not confined around the southern anomaly crest. Moreover, Fig. 5e–h show that when foF2 undergoes an enhancement at TUC, SJC, and slightly at CP, the ionosphere is compressed, while at JIC this compression is not perceivable. Electron density profiles obtained at Tucuma´n on 8 September 2010 at 04:25, 06:15, and 07:25 UT, and on 7 April 2011 at 04:55, 06:50, and 08:10 UT using the POLAN inversion technique are shown in Fig. 6. This figure clearly illustrates how the impulsive enhancement of foF2 is associated with a compression of the ionospheric layers. In order to establish deeper analysis of the behavior of the ionosphere from the geomagnetic equator toward the southern crest of the anomaly, Figs. 7 and 8 show the time sequence of two-dimensional maps of vertical TEC (vTEC) over South America obtained on 8 September 2010 and 7 April 2011 from 04:30 to 06:50 UT with a time resolution of 10 min, over an area extending from 0° to 40° in latitude and from 50° to 80° in longitude. This region was considered because within it the GPS receivers, with data downloadable from IGS, are sufficiently dense to guarantee a good quality of the two-dimensional computation of vTEC. In order to generate these figures, twelve GPS receivers (see Table 1) were included. The slant TEC (sTEC) values, affected by an offset XArc, constant for each “phase-connected” arc of data (Mannucci et al., 1998), were obtained for each receiver from the frequency differenced phase delay S (where S = sTEC + XArc) computed from the RINEX files. In order to avoid problems with the leveling procedure, instead of leveling the slants to differential pseudoranges, offsets were directly estimated, receiver by receiver, using a multiday thin shell (400 km) solution (Ciraolo et al., 2007). After estimating the offsets, the resulting sTEC values (sTEC = S  XArc) were then converted to vTEC values at the ionospheric pierce point using the well-known mapping function vTEC = sTECcosv, where v is the angle formed by the satellitereceiver line of sight and the normal at the thin shell. These vTEC values are then used to generate the two-dimensional maps shown in Figs. 7 and 8 applying a polynomial Kriging interpolation (third order in latitude and longitude displacement). In Figs. 7 and 8 it is worth noting that some receivers listed in Table 1 are outside the region considered but their inclusion is useful to ensure a reliable interpolation over the area under investigation. Fig. 7 shows that on 8 September 2010 the electron density is always larger around the magnetic equator, while Fig. 8 shows that on 7 April 2011 from 04:30 to 05:10 UT the electron density is larger around the magnetic equator, but that after from 05:20 to 06:50 UT a reversal is observed. In fine, Fig. 9 shows the zonal electric field recorded on 8 September 2010 by the C/NOFS satellite, over an area extending from +1.8° to 12.3° in latitude, and from 40° to 90° in longitude, in an altitude range from 757 to 831 km, from 00:00 to 05:00 LT averaging on 10 min

Please cite this article in press as: Perna, L., et al. Unusual nighttime impulsive foF2 enhancements at low latitudes: Phenomenology and possible explanations. J. Adv. Space Res. (2013), http://dx.doi.org/10.1016/j.asr.2013.05.014

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Fig. 3. Electron density variations for the real height range 170–300 km computed for all the nine dates investigated in this study from 00:00 to 10:00 UT. Oblique lines highlight the downward phase shift typical of gravity wave propagation.

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Fig. 4. The foF2 and h0 F plots obtained at Sao Luis (SL), Cachoeira Paulista (CP), Sa˜o Jose´ dos Campos (SJC), and Tucuma´n (TUC) on 8 September 2010. Green ellipses highlight the occurrence time of the electron density enhancement maximum at CP and TUC. The vertical gray line visible both on the foF2 and the h0 F plots highlights the occurrence time of the electron density enhancement maximum recorded at TUC. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

bins. According to this figure, the zonal electric field does not exhibit any significant feature. 4. Discussion Using ground based measurements in the South American region, it was shown that the equatorial ionosphere suffers significant and atypical nighttime modifications showing the following main features: (1) anomalous and impulsive post-midnight enhancement of foF2 recorded at some ionospheric stations widely distributed in space; (2) the enhancement is associated with a strong compression of the ionosphere; (3) for quiet and medium geomagnetic conditions the enhancement is recorded only by those sta-

tions located around the southern crest of the equatorial anomaly, and vTEC maps show a preponderance of electrons in the region of the magnetic equator; (4) for medium–high geomagnetic conditions the enhancement is recorded also by those stations located at the magnetic equator, and vTEC maps show a reversal, that is the vTEC values near the magnetic equator diminish while at the southern crest increase, indicating a recharging of the fountain effect. In general, the mechanisms responsible for the post-midnight enhancements are still somewhat unclear. Several authors (Bailey et al., 1991; Su et al., 1994; Mikhailov et al., 2000a, 2000b; Richards et al., 2000; Farelo et al., 2002; Pavlov and Pavlova, 2005; Nicolls et al., 2006) agreed

Please cite this article in press as: Perna, L., et al. Unusual nighttime impulsive foF2 enhancements at low latitudes: Phenomenology and possible explanations. J. Adv. Space Res. (2013), http://dx.doi.org/10.1016/j.asr.2013.05.014

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Fig. 5. The foF2 and h0 F plots plots obtained at Jicamarca (JIC), Cachoeira Paulista (CP), Sa˜o Jose´ dos Campos (SJC), and Tucuma´n (TUC) on 7 April 2011. Green ellipses highlight the occurrence time of the electron density enhancement maximum at each station. The vertical gray line visible both on the foF2 and the h0 F plots highlights the occurrence time of the electron density enhancement maximum recorded at TUC. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Table 3 Period (T), vertical phase velocity (vz), vertical wavelength (kz), and horizontal wavelength (kh) characterizing the TIDs observed during the nine dates under study and estimated from the isoheight curves shown in Fig. 3. Date

T [minutes]

vz [m/s]

kz [km]

kh [km]

8 September 2010 9 September 2010 2 November 2010 5 November 2010 19 March 2011 7 April 2011 24 June 2011 26 January 2012 31 January 2012

100 200 90 90 130 110 90 130 100

17 17 17 33 17 17 33 33 11

100 200 90 180 130 110 180 260 67

707 2850 571 1143 1200 857 1143 2400 471

on the explanation that the observed post-midnight NmF2 enhancements were due either to the equatorward thermospheric winds, along with a nighttime plasmaspheric flux into the F region, or to a recharging of the fountain effect, both mechanisms being characterized by an uplifting of the F region. Hence, these explanations cannot be considered satisfactory for the events dealt with in this paper, because the impulsive enhancements of foF2 illustrated in Fig. 1 are associated with a definite compression of the F region as shown by Figs. 2 and 6. With regard to this, it is worth noting that the plasmaspheric O+ ion flux itself has a significant effect on NmF2 but has a very little effect on hmF2. Pezzopane et al. (2011) pointed out that the features of these impulsive electron density enhancements are clearly

Please cite this article in press as: Perna, L., et al. Unusual nighttime impulsive foF2 enhancements at low latitudes: Phenomenology and possible explanations. J. Adv. Space Res. (2013), http://dx.doi.org/10.1016/j.asr.2013.05.014

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Fig. 6. Electron density profiles obtained at Tucuma´n on 8 September 2010 at 04:25, 06:15, and 07:25 UT, and on 7 April 2011 at 04:55, 06:50, and 08:10 UT, showing the compression and expansion phases of the F region. Red profiles point out the association between the foF2 impulsive enhancement and the compression phase. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 7. Two-dimensional maps of vertical total electron content (vTEC) on 8 September 2010 from 04:30 to 06:50 UT, with a time resolution of 10 min, over an area extending in latitude from 0° to 40° and in longitude from 50° to 80°. Black solid circles in each plot represent the location of GPS receivers. Blue rhombuses represent the location of the ionospheric stations of JIC and TUC. Color scale is in TEC unit (1016 electrons/m2). Coordinates are geographic. The continuous and dashed black curves represent the magnetic equator and the geomagnetic latitude of 15° respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Please cite this article in press as: Perna, L., et al. Unusual nighttime impulsive foF2 enhancements at low latitudes: Phenomenology and possible explanations. J. Adv. Space Res. (2013), http://dx.doi.org/10.1016/j.asr.2013.05.014

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Fig. 8. Same as Fig. 7 on 7 April 2011.

Fig. 9. Zonal electric field (negative values indicate westward direction) recorded by the Communications/Navigation Outage Forecasting System satellite on 8 September 2010 over an area extending from +1.8° to 12.3° in latitude and from 40° to 90° in longitude, in an altitude range from 757 to 831 km, from 00:00 to 05:00 LT averaging on 10 min bins.

different if compared with other similar events (Balan and Rao, 1987; Su et al., 1994; Farelo et al., 2002; Nicolls et al., 2006; Liu et al., 2008; Zhao et al., 2008), occurred

both for quiet and disturbed geomagnetic conditions, especially with regard to the short duration of the phenomenon and the anticorrelation between foF2 and h0 F. On the other

Please cite this article in press as: Perna, L., et al. Unusual nighttime impulsive foF2 enhancements at low latitudes: Phenomenology and possible explanations. J. Adv. Space Res. (2013), http://dx.doi.org/10.1016/j.asr.2013.05.014

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hand, Lu et al. (2001) illustrated that this anticorrelation can be explained in terms of the positive/negative vertical shear, due to upward GW propagation, affecting the velocity of the meridional wind. In fact, in these conditions, the F layer can be compressed/expanded, causing the observed anticorrelation in the middle to low-latitude regions (see Fig. 5 of Lu et al. (2001)). Fig. 3 confirms what suggested by Lu et al. (2001), and highlights that GW propagation might actually play a significant role in causing impulsive enhancements characterized by very large amplitude and very short duration, like the ones investigated in this work. TIDs are generally considered as plasma manifestations of GWs propagating in the ionosphere (Hines, 1960). In particular, the horizontal wavelengths and periods illustrated in Table 3 are consistent with those of MSTIDs/ LSTIDs. Two-dimensional mapping techniques using GPS receiver networks demonstrated to be a very useful tool for investigating TEC latitudinal distributions (Valladares et al., 2001; Valladares and Chau, 2012), and propagation of MSTIDs (Kotake et al., 2007; Tsugawa et al., 2007), specifically in the North America, where the density of GPS receivers is very high (more than 1400). Fig. 7 shows the TEC latitudinal distribution obtained on 8 September 2010. In this case, the increase of electron density around the magnetic equator is due to a large reverse fountain which, according to the mechanism proposed by Balan and Bailey (1995), causes an increase of TEC at the magnetic equator and a weakening of TEC with increasing latitude. Fig. 7, as Fig. 4 for foF2 and h0 F, is representative and presents the same characteristics of all the other cases occurred for low and medium geomagnetic activity (figures not shown here). Fig. 8 shows instead the TEC latitudinal distribution obtained on 7 April 2011. In this case, from 05:20 to 06:50 UT it is possible to note an increase of TEC near the southern crest of the equatorial anomaly, indicating a recharging of the fountain effect. This recharge can be due to either a zonal electric field reversal or a decreasing westward electric field (Nicolls et al., 2006; Liu et al., 2008), that are generally caused by penetrating interplanetary electric fields (IEFs) of magnetospheric origin. In fact, the equatorial ionization anomaly, especially for disturbed conditions, is strongly affected by a magnetosphere/ionosphere coupling which causes the appearance of these IEFs (Fejer and Scherliess, 1995; Zhao et al., 2008). Hence, while for quiet geomagnetic conditions the equatorial zonal electric field is univocally attributable to the ionospheric dynamo action, for disturbed geomagnetic conditions interplanetary electric fields of magnetospheric origin can penetrate into the low latitude ionosphere for several hours (Huang et al., 2007), adding (subtracting) themselves to (from) the usual dynamo electric field. Furthermore, a careful examination of Fig. 4 reveals that the phenomenon was recorded at CP about 1 h prior to TUC. This time delay could be justified by the different effect that the fountain effect can generate at different latitudes. Anyhow, the fact that the foF2 plot obtained at SL

does not show any enhancement (see Fig. 4a), the fact that the zonal electric field recorded by C/NOFS does not present any significant change (see Fig. 9) and the TEC latitudinal distribution obtained on 8 September 2010 (see Fig. 7) suggest that the event is confined around the southern anomaly crest. Again, it is important to point out once more that an increase of electron density associated with a compression of the ionosphere implies that TIDs should be taken into account to explain the phenomenon. According to this and to the evidence given by Makela et al. (2010) that MSTIDs can propagate also equatorward of the equatorial anomalies, the red straight lines in Fig. 10 illustrates the wavefront of a MSTID propagating in the northwest direction, that is coherent with the fact that the foF2 enhancement is observed before at CP and approximately 1 h later at TUC, assuming for the MSTID a horizontal phase velocity of about 118 m/s, which is consistent with the T and the kh values previously found (see Table 3), and a distance between TUC and CP along the propagation direction of about 400 km. Concerning the event occurred on 7 April 2011, a careful examination of Fig. 5 shows that, unlike the event recorded on 8 September 2010, an impulsive increase of foF2 was recorded also at the magnetic equator, at JIC, at 05:45 UT, about one hour prior to the enhancement recorded at TUC. Moreover, the increase recorded at JIC is associated with no ionospheric compression, this being a further evidence (besides that given by Fig. 8) that in this case a recharging of the fountain effect played an important job in causing the phenomenon under study. Unfortunately, for this date zonal electric field data by C/NOFS were not available to have a further confirmation of this. Again, Fig. 5 shows that an impulsive increase of foF2 occurred also at CP and at SJC, at 05:00 UT and at 05:25 UT respectively. The times of occurrence of these two peaks (45 and 20 min before than the one observed at JIC) indicate that they are not correlated with the electron density increase recorded at JIC. Moreover, these two peaks are associated with a compression of the ionosphere implying that also in this case TIDs should be taken into account to explain the phenomenon. Similarly to what it was just previously done for the event occurred on 8 September 2010, the green straight lines in Fig. 10 illustrates the wavefront of a MSTID propagating in the northwest direction. This is coherent with the fact that the foF2 enhancement is observed before at SJC and approximately 1 h and 20 min later at TUC, assuming for the MSTID a horizontal phase velocity of about 129 m/s, which is consistent with the T and the kh values previously found (see Table 3), and a distance between TUC and SJC along the propagation direction of about 600 km. The same scenario after all accounts pretty well also for the delays of 25 min between CP and SJC and 1 h and 45 min between CP and TUC. Finally, the amplification of the phenomenon recorded at TUC, as seen by the foF2 plots of Figs. 4 and 5, for quiet and moderate geomagnetic conditions, could be attributed

Please cite this article in press as: Perna, L., et al. Unusual nighttime impulsive foF2 enhancements at low latitudes: Phenomenology and possible explanations. J. Adv. Space Res. (2013), http://dx.doi.org/10.1016/j.asr.2013.05.014

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Fig. 10. Wavefront of a medium-scale traveling ionospheric disturbance propagating northwestward in the Southern Hemisphere at time t1 and time t2 > t1 on 8 September 2010 (in red) and 7 April 2011 (in green). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

to the different behavior generally shown by different ionospheric longitudinal sectors (e.g. Hoang et al., 2010; Pezzopane et al., 2013), for instance due to the diverse magnetic declination angle, as it was shown by Abdu et al. (1981) and Batista et al. (1986). For medium–high geomagnetic conditions, this amplification could also be due to the fact that the longitudinally dependent E  B upward drift

progressively adds up to the northwestward MSTID caused by GW propagation. 5. Conclusions This work focused on nine unusual nighttime electron density “impulsive enhancements” occurred at low

Please cite this article in press as: Perna, L., et al. Unusual nighttime impulsive foF2 enhancements at low latitudes: Phenomenology and possible explanations. J. Adv. Space Res. (2013), http://dx.doi.org/10.1016/j.asr.2013.05.014

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latitudes over a wide region of South America for quiet and medium/high geomagnetic conditions from August 2010 to January 2012. The striking feature that makes all these events peculiar is that the narrow bell-shaped electron density increases were simultaneously tagged by a compression of the ionosphere, which is contrary to the more common mechanism that associates postmidnight enhancements with an uplift of the F region (e.g. Sastri, 1998; Farelo et al., 2002; Nicolls et al., 2006). According to what it has been already found by Pezzopane et al. (2011), the eight events occurred for quiet and medium geomagnetic conditions can be only ascribable to northwestward LSTIDs/MSTIDs caused by GW propagation. For these events, both the foF2 plots and the vTEC maps did not suggest any additional mechanism responsible for the phenomenon under investigation. On the other hand, also the event occurred on 7 April 2011 for medium– high geomagnetic conditions appeared to be mainly triggered by northwestward LSTIDs/MSTIDs caused by GW propagation. Therefore, an important result reached by our study is that, independently of the geomagnetic activity, GW propagation plays without doubt a significant role in causing these anomalous postmidnight electron density increases characterizing the low latitude ionosphere. The GWs characterizing the three events recorded for medium and medium–high geomagnetic conditions can be of auroral origin, this explanation being supported by the work of many authors who observed a correlation between nighttime GWs and high values of the magnetic activity index Kp (Kp > 3) (e.g. MacDougall et al., 2009). In these cases the magnetospheric energy input caused by auroral precipitation and Joule heating generate GWs/ TIDs which propagate from the auroral zone to lower latitudes. The mechanism triggering GWs characterizing the other six events recorded for quiet geomagnetic conditions can be found in the transition of solar terminator (Somsikov, 1995). The fast increase/decrease of solar flux at sunrise/sunset can act as a source of atmospheric irregularities and generate GWs in the F region (Somsikov and Ganguly, 1995), although the evening terminator transition excites less regular and weaker GW effects (Altadill et al., 2004). Concerning the event occurred on 7 April 2011 for medium–high geomagnetic activity, both the foF2 plots and the vTEC maps indicated a nocturnal recharging of the fountain effect, probably caused by penetrating IEFs, which adds up to the northwestward MSTID caused by GW propagation. Seeing as the events recorded on 24 June 2011 (AE = 750 nT, Kp = 4) and 26 January 2012 (AE = 550 nT, Kp = 5) were both characterized by geomagnetic indices lower than those characterizing the 7 April 2011 (AE = 1176 nT, Kp = 6), our study suggests that values of AE > 1000 nT and Kp > 5 are necessary to trigger a nocturnal recharging of the fountain effect contributing to the anomalous electron density enhancements investigated by our work. It is intention of the authors to con-

tinue this kind of study to verify the goodness of this claim and the correspondent thresholds. References Abdu, M.A., Bittencourt, J.A., Batista, I.S. Magnetic declination control of the equatorial F region dynamo field development and spread-F. J. Geophys. Res. 86, 11443–11446, 1981. Abdu, M.A., Batista, I.S., Kantor, I.J., Sobral, J.H.A. Gravity wave induced ionization layers in the night Fregion over Cachoeira Paulista (22°S, 45°W). J. Atmos. Terr. Phys. 44, 759–767, http://dx.doi.org/ 10.1016/0021-9169(82)90004-6, 1982. Akala, A.O., Adeloye, A.B., Somoye, E.O. Ionospheric foF2 variability over the Southeast Asian sector. J. Geophys. Res. 115, A09329, http:// dx.doi.org/10.1029/2010JA015250, 2010. Akasofu, S.I. Diagnostics of magnetosphere using geomagnetic, auroral and airglow phenomena. Ann. Geophys. 26, 443–457, 1970. Altadill, D., Apostolov, E.M., Boska, J., Sauli, P. Planetary and gravity wave signatures in the F region ionosphere with impact to radio propagation predictions and variability. Ann. Geophys. 47, 1109–1119, 2004. Bailey, G.J., Sellek, R., Balan, N. The effect of interhemispheric coupling on nighttime enhancements in ionospheric total electron content during winter at solar minimum. Ann. Geophys. 9, 734–747, 1991. Balan, N., Rao, P.B. Latitudinal variations of nighttime enhancements in total electron content. J. Geophys. Res. 92 (A4), 3436–3440, http:// dx.doi.org/10.1029/JA092iA04p03436, 1987. Balan, N., Bailey, G.J. Equatorial plasma fountain and its effects: possibility of an additional layer. J. Geophys. Res. 100 (A11), 21,421–21,432, http://dx.doi.org/10.1029/95JA01555, 1995. Batista, I.S., Abdu, M.A., Bittencourt, J.A. Equatorial F-region vertical plasma drifts: seasonal and longitudinal asymmetries in the American sector. J. Geophys. Res. 91, 12055–12064, 1986. Bibl, K., Reinisch, B.W. The universal digital ionosonde. Radio Sci. 13, 519–530, http://dx.doi.org/10.1029/RS013i003p00519, 1978. Bilitza, D., Reinisch, B.W. International reference ionosphere 2007: improvements and new parameters. Adv. Space Res. 42 (4), 599–609, http://dx.doi.org/10.1016/j.asr.2007.07.048, 2008. Borries, C., Hoffmann, P. Characteristics of F2-layer planetary wave-type oscillations in northern middle and high latitudes during 2002 to 2008. J. Geophys. Res. 115, A00G10, http://dx.doi.org/10.1029/ 2010JA015456, 2010. Burns, A.G., Solomon, S.C., Wang, W., Qian, L., Zhang, Y., Paxton, L.J. Daytime climatology of ionospheric NmF2 and hmF2 from COSMIC data. J. Geophys. Res. 117, A09315, http://dx.doi.org/10.1029/ 2012JA017529, 2012. Chen, Y., Liu, L. Further study on the solar activity variation of daytime NmF2. J. Geophys. Res. 115, A12337, http://dx.doi.org/10.1029/ 2010JA015847, 2010. Chen, Y., Liu, L., Wan, W., Ren, Z. Equinoctial asymmetry in solar activity variations of NmF2 and TEC. Ann. Geophys. 30, 613–622, http://dx.doi.org/10.5194/angeo-30-613-2012, 2012. Ciraolo, L., Azpilicueta, F., Brunini, C., Meza, A., Radicella, S.M. Calibration errors on experimental slant total electron content (TEC) determined with GPS. J. Geod. 81, 111–120, http://dx.doi.org/ 10.1007/s00190-006-0093-1, 2007. David, M., Sojka, J.J. Single-day dayside enhancements over Europe: a survey of a half-century of ionosonde data. J. Geophys. Res. 115, A12311, http://dx.doi.org/10.1029/2010JA015711, 2010. Dow, J.M., Neilan, R.E., Rizos, C. The International GNSS Service in a changing landscape of Global Navigation Satellite Systems. J. Geod. 83, 191–198, http://dx.doi.org/10.1007/s00190-008-0300-3, 2009. Farelo, A.F., Herraiz, M., Mikhailov, A.V. Global morphology of nighttime NmF2 enhancements. Ann. Geophys. 20, 1795–1806, http:// dx.doi.org/10.5194/angeo-20-1795-2002, 2002. Fejer, B.G., Scherliess, L. Time dependent response of equatorial ionospheric electric field to magnetospheric disturbances. Geophys. Res. Lett. 22, 851–854, 1995.

Please cite this article in press as: Perna, L., et al. Unusual nighttime impulsive foF2 enhancements at low latitudes: Phenomenology and possible explanations. J. Adv. Space Res. (2013), http://dx.doi.org/10.1016/j.asr.2013.05.014

L. Perna et al. / Advances in Space Research xxx (2013) xxx–xxx Finlay, C.C. et al. International geomagnetic reference field: the eleventh generation. Geophys. J. Int. 183, 1216–1230, http://dx.doi.org/ 10.1111/j.1365-246X.2010.04804.x, 2010. Galkin, I.A., Reinisch, B.W., Huang, X., Bilitza, D. Assimilation of GIRO data into a real-time IRI. Radio Sci. 47, RS0L07, http:// dx.doi.org/10.1029/2011RS004952, 2012. Gonzalez, W.D., Joselyn, J.A., Kamide, Y., Krochl, W., Rostoker, G., Tsurutani, B.T., Vasyliunas, V.M. What is a geomagnetic storm?. J. Geophys. Res. 99, 5771–5792, http://dx.doi.org/10.1029/93JA02867, 1994. Gulyaeva, T.L. Empirical model of ionospheric storm effects on the F2 layer peak height associated with changes of peak electron density. J. Geophys. Res. 117, A02302, http://dx.doi.org/10.1029/2011JA017158, 2012. Hall, C.M., Rypdal, K., Rypdal, M. The E region at 69°N, 19°E: Trends, significances, and detectability. J. Geophys. Res. 116, A05309, http:// dx.doi.org/10.1029/2011JA016431, 2011. He, M., Liu, L., Wan, W., Zhao, B. A study on the nighttime midlatitude ionospheric trough. J. Geophys. Res. 116, A05315, http://dx.doi.org/ 10.1029/2010JA016252, 2011. Hines, C.O. Internal atmospheric gravity waves at ionospheric heights. Can. J. Phys. 38, 1441–1481, http://dx.doi.org/10.1139/p60-150, 1960. Hoang, T.L., Abdu, M.A., MacDougall, J., Batista, I.S. Longitudinal differences in the equatorial spread F characteristics between Vietnam and Brazil. Adv. Space Res. 45, 351–360, http://dx.doi.org/10.1016/ j.asr.2009.08019, 2010. Hoque, M.M., Jakowski, N. A new global model for the ionospheric F2 peak height for radio wave propagation. Ann. Geophys. 30, 797–809, http://dx.doi.org/10.5194/angeo-30-797-2012, 2012. Huang, C.-S., Sazykin, S., Chau, J.L., Maruyama, N., Kelley, M.C. Penetration electric fields: efficiency and characteristic time scale. J. Atmos. Sol. Terr. Phys. 69, 1135–1146, http://dx.doi.org/10.1016/ j.jastp.2006.08.016, 2007. Klausner, V., Fagundes, P.R., Sahai, Y., Wrasse, C.M., Pillat, V.G., Becker-Guedes, F. Observations of GW/TID oscillations in the F2 layer at low latitude during high and low solar activity, geomagnetic quiet and disturbed periods. J. Geophys. Res. 114, A02313, http:// dx.doi.org/10.1029/2008JA013448, 2009. Kotake, N., Otsuka, Y., Ogawa, T., Tsugawa, T., Saito, A. Statistical study of medium-scale travelling ionospheric disturbances observed with the GPS networks in Southern California. Earth Planets Space 59, 95–102, 2007. Lee, W.K., Kil, H., Kwak, Y.-S., Wu, Q., Cho, S., Park, J.U. The winter anomaly in the middle-latitude F region during the solar minimum period observed by the constellation observing system for meteorology, ionosphere, and climate. J. Geophys. Res. 116, A02302, http:// dx.doi.org/10.1029/2010JA015815, 2011. Leitinger, R., Rieger, M. The TID model for modulation of large scale electron density models. Ann. Geophys. Italy 48 (3), 515–523, 2005. Lin, C.H., Liu, C.H., Liu, J.Y., Chen, C.H., Burns, A.G., Wang, W. Midlatitude summer nighttime anomaly of the ionospheric electron density observed by FORMOSAT-3/COSMIC. J. Geophys. Res. 115, A03308, http://dx.doi.org/10.1029/2009JA014084, 2010. Liu, L., Wan, W., Zhang, M.-L., Zhao, B., Ning, B. Prestorm enhancements in NmF2 and total electron content at low latitudes. J. Geophys. Res. 113, A02311, http://dx.doi.org/10.1029/2007JA012832, 2008. Liu, L., Zhao, B., Wan, W., Ning, B., Zhang, M.-L., He, M. Seasonal variations of the ionospheric electron densities retrieved from constellation observing system for meteorology, ionosphere, and climate mission radio occultation measurements. J. Geophys. Res. 114, A02302, http://dx.doi.org/10.1029/2008JA013819, 2009. Liu, G., Immel, T.J., England, S.L., Kumar, K.K., Ramkumar, G. Temporal modulations of the longitudinal structure in F2 peak height in the equatorial ionosphere as observed by COSMIC. J. Geophys. Res. 115, A04303, http://dx.doi.org/10.1029/2009JA014829, 2010. Liu, L., Chen, Y., Le, H., Kurkin, V.I., Polekh, N.M., Lee, C.-C. The ionosphere under extremely prolonged low solar activity. J. Geophys. Res. 116, A04320, http://dx.doi.org/10.1029/2010JA016296, 2011.

15

Liu, L., Yang, J., Le, H., Chen, Y., Wan, W., Lee, C.-C. Comparative study of the equatorial ionosphere over Jicamarca during recent two solar minima. J. Geophys. Res. 117, A01315, http://dx.doi.org/ 10.1029/2011JA017215, 2012. Lu, G., Richmond, A.D., Roble, R.G., Emery, B.A. Coexistence of ionospheric positive and negative storm phase under northern winter conditions: a case study. J. Geophys. Res. 106 (A11), 24,493–24,504, http://dx.doi.org/10.1029/2001JA000003, 2001. Luan, X., Wang, W., Burns, A., Solomon, S.C., Lei, J. Midlatitude nighttime enhancement in F region electron density from global COSMIC measurements under solar minimum winter condition. J. Geophys. Res. 113, A09319, http://dx.doi.org/10.1029/2008JA013063, 2008. MacDougall, J.W., Grant, I.F., Shen, X. The Canadian advanced digital ionosonde: design and results, in: Paper presented at 24th General Assembly of the International Union of Radio Science, Kyoto, Japan, 1993. MacDougall, J.W., Li, G., Jayachandran, P.T. Traveling ionospheric disturbances near London. Canada J. Atmos. Sol. Terr. Phys. 71, 2077–2084, http://dx.doi.org/10.1016/j.jastp.2009.09.016, 2009. Makela, J.J., Miller, E.S., Talaat, E.R. Nighttime medium-scale traveling ionospheric disturbances at low geomagnetic latitudes. Geophys. Res. Lett. 37, L24104, http://dx.doi.org/10.1029/2010GL045922, 2010. Mannucci, A., Wilson, B., Yuan, D., Ho, C., Lindqwister, U., Runge, T. A global technique for GPS-derived ionospheric total electron content measurements. Radio Sci. 33, 565–582, http://dx.doi.org/10.1029/ 97RS02707, 1998. Meza, A., Natali, M.P., Ferna´ndez, L.I. Analysis of the winter and semiannual ionospheric anomalies in 1999–2009 based on GPS global international GNSS service maps. J. Geophys. Res. 117, A01319, http://dx.doi.org/10.1029/2011JA016882, 2012. Mikhailov, A.V., Fo¨rster, M., Leschinskaya, T.Y. On the mechanism of the post-midnight winter NmF2 enhancements: dependence on solar activity. Ann. Geophys. 18, 1422–1434, http://dx.doi.org/10.1007/ s00585-000-1422-y, 2000a. Mikhailov, A.V., Leschinkaya, T.Y., Fo¨rster, M. Morphology of NmF2 nighttime increases in the Eurasian sector. Ann. Geophys. 18, 618–628, http://dx.doi.org/10.1007/s00585-000-0618-5, 2000b. Mikhailov, A.V., Depueva, A.K., Leschinskaya, T.Y. Morphology of quiet time F2-layer disturbances: high to lower latitudes. Int. J. Geomagn. Aeronaut. 5, GI1006, http://dx.doi.org/10.1029/2003GI000058, 2004. Mikhailov, A.V., Perrone, L., Smirnova, N.V. Two types of positive disturbances in the daytime mid-latitude F2-layer: morphology and formation mechanisms. J. Atmos. Sol. Terr. Phys. 81–82, 59–75, http://dx.doi.org/10.1016/j.jastp.2012.04.003, 2012. Nicolls, M.J., Kelley, M.C., Vlasov, M.N., Sahai, Y., Chau, J.L., Hysell, D.L., Fagundes, P.R., Becker-Guedes, F., Lima, W.L.C. Observations and modeling of post-midnight uplifts near the magnetic equator. Ann. Geophys. 24, 1317–1331, http://dx.doi.org/10.5194/angeo-24-13172006, 2006. Pavlov, A.V., Pavlova, N.M. Mechanism of the post-midnight winter night-time enhancements in NmF2 over Millstone Hill during 14–17 January 1986. J. Atmos. Sol. Terr. Phys. 67 (4), 381–395, http:// dx.doi.org/10.1016/j.jastp.2004.11.004, 2005. Pavlov, A.V., Pavlova, N.M. Anomalous nighttime peaks in diurnal variations of NmF2 close to the geomagnetic equator: a statistical study. J. Atmos. Sol. Terr. Phys. 69, 1871–1883, http://dx.doi.org/ 10.1016/j.jastp.2007.07.003, 2007. Pavlov, A.V. The low and middle latitude semi-annual anomaly in NmF2 near noon: a statistical study. Adv. Space Res. 49, 922-936, http:// dx.doi.org/10.1016/j.asr.2011.12.024, 2012. Pezzopane, M., Scotto, C. The automatic scaling of critical frequency foF2 and MUF(3000)F2: a comparison between Autoscala and ARTIST 4.5 on Rome data. Radio Sci. 42, RS4003, http://dx.doi.org/10.1029/ 2006RS003581, 2007. Pezzopane, M., Zuccheretti, E., Bianchi, C., Scotto, C., Zolesi, B., Cabrera, M.A., Ezquer, R.G. The new ionospheric station of Tucuma´n: first results. Ann. Geophys. Italy 50 (3), 483–492, 2007.

Please cite this article in press as: Perna, L., et al. Unusual nighttime impulsive foF2 enhancements at low latitudes: Phenomenology and possible explanations. J. Adv. Space Res. (2013), http://dx.doi.org/10.1016/j.asr.2013.05.014

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L. Perna et al. / Advances in Space Research xxx (2013) xxx–xxx

Pezzopane, M., Fagundes, P.R., Ciraolo, L., Correia, E., Cabrera, M.A., Ezquer, R.G. Unusual nighttime impulsive foF2 enhancement below the southern anomaly crest under geomagnetically quiet conditions. J. Geophys. Res. 116, A12314, http://dx.doi.org/10.1029/2011JA016593, 2011. Pezzopane, M., Zuccheretti, E., Abadi, P., de Abreu, A.J., de Jesus, R., Fagundes, P.R., Supnithi, P., Rungraengwajiake, S., Nagatsuma, T., Tsugawa, T., Cabrera, M.A., Ezquer, R.G. Low-latitude equinoctial spread-F occurrence at different longitude sectors under low solar activity. Ann. Geophys. 31, 153–162, http://dx.doi.org/10.5194/angeo31-153-2013, 2013. Pietrella, M., Pezzopane, M., Scotto, C. Variability of foF2 over Rome and Gibilmanna during three solar cycles (1976–2000). J. Geophys. Res. 117, A05316, http://dx.doi.org/10.1029/2011JA017462, 2012. Pro¨lss, G.W. Common origin of positive ionospheric storms at middle latitudes and the geomagnetic activity effect at low latitudes. J. Geophys. Res. 98, 5981–5991, http://dx.doi.org/10.1029/92JA02777, 1993. Richards, P.G. et al. On the relative importance of convection and temperature to the behavior of the ionosphere in North America during January 6–12, 1997. J. Geophys. Res. 105 (A6), 12,763–12,776, http://dx.doi.org/10.1029/1999JA000253, 2000. Rishbeth, H., Mendillo, M. Patterns of F2-layer variability. J. Atmos. Terr. Phys. 63, 1661–1680, http://dx.doi.org/10.1016/S13646826(01)00036-0, 2001. Romano, V., Pau, S., Pezzopane, M., Zuccheretti, E., Zolesi, B., De Franceschi, G., Locatelli, S. The electronic space weather upper atmosphere (eSWua) project at INGV: advancements and state of the art. Ann. Geophys. 26, 345–351, http://dx.doi.org/10.5194/angeo-26345-2008, 2008. Sastri, J.H. On the development of abnormally large postsunset upward drift of equatorial F region under quiet geomagnetic conditions. J. Geophys. Res. 103 (A3), 3983–3991, http://dx.doi.org/10.1029/ 97JA02649, 1998.

Somsikov, V.M. On the mechanism for the formation of atmospheric irregularities in the solar terminator region. J. Atmos. Terr. Phys. 57, 75–83, http://dx.doi.org/10.1016/0021-9169(93)E0017-4, 1995. Somsikov, V.M., Ganguly, B. On the formation of atmospheric inhomogeneities in the solar terminator region. J. Atmos. Terr. Phys. 57, 1513– 1523, http://dx.doi.org/10.1016/0021-9169(95)00014-S, 1995. Su, Y.Z., Bailey, G.J., Balan, N. Night-time enhancements in TEC at equatorial anomaly latitudes. J. Atmos. Sol. Terr. Phys. 56, 1619–1628, http://dx.doi.org/10.1016/0021-9169(94)90091-4, 1994. Titheridge, J.E. The real height analysis of ionograms: a generalized formulation. Radio Sci. 23 (5), 831–849, http://dx.doi.org/10.1029/ RS023i005p00831, 1988. Tsugawa, T., Otsuka, T., Coster, A.J., Saito, A. Medium-scale traveling ionospheric disturbances detected with dense and wide TEC maps over North America. Geophys. Res. Lett. 34, L22101, http://dx.doi.org/ 10.1029/2007GL031663, 2007. Valladares, C.E., Basu, S., Groves, K., Hagan, M.P., Hysell, D., Mazzella Jr., A.J., Sheehan, R.E. Measurement of the latitudinal distributions of total electron content during equatorial spread F events. J. Geophys. Res. 106 (A12), 29,133–29,152, http://dx.doi.org/10.1029/ 2000JA000426, 2001. Valladares, C.E., Chau, J.L. The low-latitude ionosphere sensor network: initial results. Radio Sci. 47, RS0L17, http://dx.doi.org/10.1029/ 2011RS004978, 2012. Wakai, N., Ohyama, H., Koizumi, T. Manual of ionogram Scaling, 3rd version, report, Radio Research Laboratory, Ministry of Posts and Telecommunication, Tokyo, 1987. Zhao, B., Wan, W., Liu, L., Igarashi, K., Nakamura, M., Paxton, L.J., Su, S.-Y., Li, G., Ren, Z. Anomalous enhancement of ionospheric electron content in the Asian–Australian region during a geomagnetically quiet day. J. Geophys. Res. 113, A11302, http://dx.doi.org/10.1029/ 2007JA012987, 2008.

Please cite this article in press as: Perna, L., et al. Unusual nighttime impulsive foF2 enhancements at low latitudes: Phenomenology and possible explanations. J. Adv. Space Res. (2013), http://dx.doi.org/10.1016/j.asr.2013.05.014