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The occurrence of the mid-latitude ionospheric trough in GPS-TEC measurements
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Advances in Space Research 43 (2009) 1721–1731 www.elsevier.com/locate/asr
The occurrence of the mid-latitude ionospheric trough in GPS-TEC measurements A. Krankowski a,*, I.I. Shagimuratov b, I.I. Ephishov b, A. Krypiak-Gregorczyk a, G. Yakimova b a
Institute of Geodesy, University of Warmia and Mazury in Olsztyn, Oczapowski St.1, 10-957 Olsztyn, Poland b WD IZMIRAN, Prospekt Pabedy 41, 236017 Kaliningrad, Russia Received 19 November 2007; received in revised form 28 May 2008; accepted 28 May 2008
Abstract Simultaneous GPS observations from about 150 stations of European Permanent Network (EPN) have been used for studying dynamics of latitudinal profiles and structure of mid-latitude ionospheric trough (MIT). For the analyses, the TEC maps over Europe were created with high spatial and temporal resolution. The latitudinal profiles were produced from TEC maps with one-hour interval for geographic latitude range from 35N to 75N. The structure of latitudinal profiles relates to the occurrence of the ionospheric trough. The location of the trough depends on season, local time, and both geophysical and geomagnetic conditions. The trough structure in GPS-TEC demonstrates a smooth shape. The trough occurrence as a distinguished structure is more distinct during winter. The relation of TEC in the trough minimum to the equator and polar walls amounted to a factor of 2–4. The diurnal, seasonal as well as storm-time dynamics of the latitudinal profiles and the trough-like structure during different geomagnetic conditions are also presented. Day by day snapshots demonstrate great variability of TEC profiles and MIT, essential changes of the ionosphere structure took place during a storm. Similarly to the F2 region trough, the position of the TEC trough shifts towards lower latitudes during disturbances. In the storm-time, the TEC trough was recognized during day-time at latitudes lower than 70N. This paper presents the statistics describing the dynamics of the trough location. Ó 2008 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Ionosphere; Total Electron Content; Mid-latitude ionospheric trough (MIT); Ionospheric perturbations
1. Introduction The middle-latitude ionospheric trough is the main and dominant scale structure, which is identified in F region of the mid-latitude ionosphere (Muldrew, 1965; Sharp, 1966). The spatial structure of the trough is presented as the latitudinal narrow and longitudinal extended depletion in the electron distribution. The physical mechanisms of the trough formation include complex interconnected physical processes in the mid- and high-latitude ionosphere. A the*
Corresponding author. Fax: +48 895234768. E-mail addresses: [email protected] (A. Krankowski, A. KrypiakGregorczyk), [email protected] (I.I. Shagimuratov, I.I. Ephishov, G. Yakimova).
ory and mechanisms of the trough formation were discussed in Moffett and Quegan (1983) and Rodger et al. (1992). The trough dominates in winter conditions and it is regularly detected in evening and night hours. The occurrence of the trough depends on latitude, longitude as well as the geomagnetic activity. Latitudinal location of the trough can differ essentially in different longitudinal sectors (Deminov et al., 1992; 1986; Karpachev et al., 1996). There are also some typical features in the trough occurrence in southern hemisphere (Mallis and Essex, 1993; Horvath and Essex, 2003). The trough was studied by employing different methods using satellite and ground observations (Tulunay and
0273-1177/$36.00 Ó 2008 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2008.05.014
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Sayers, 1971; Tulunay and Grebowsky, 1978; Tulunay et al., 2003; Grebowsky et al., 1983; Rothkaehl et al., 2000; Whalen, 1989; Werner and Prolss, 1997; Stanislawska and Rothkaehl, 2002). The numerous investigations concerned the occurrence of the trough in F2 region of the ionosphere. The Total Electron Content (TEC) is the key ionospheric parameter for a number of space-ground telecommunication systems, satellite navigation (GPS, GLONASS), orbit determination, ocean altimetry. The ionospheric trough influences HF as well as the transionospheric radio wave propagation. Strong latitudinal gradients were already associated with the trough by Wielgosz et al. (2004). As known, the severe horizontal TEC gradients can hamper the ambiguity resolution and deteriorate the accuracy of the GPS positioning (Wanninger, 1993; Wielgosz et al., 2005). The trough is located equatorward of the auroral oval, where strong phase and amplitude scintillations of transionospheric signals can occur. The scintillation effect can lead to degradation of the navigation system performance. So, the knowledge of the trough occurrence in TEC, its spatial structure and latitudinal location are very important for various practical applications as well as for investigation of the Earth’s ionosphere. The occurrence of the trough in TEC has been studied for southern hemisphere by Mallis and Essex (1993) and for northern hemisphere by Ciraolo and Spalla (1998) and Pryse et al. (1993) using differential carrier-phase method. Recently, using the same technique, the parameters of the TEC trough were presented by Pryse et al. (2006). Rothkaehl et al. (2008) described a new hybrid method for simultaneously diagnostics of the mid-latitude ionospheric trough using GNSS (by UWM technique) and in situ wave measurements – DEMETER. Currently GPS and GLONASS techniques can be effectively used to study the TEC trough. A large and dense network of GPS stations allow regular monitoring of the TEC trough. The GPS measurements were used to study the trough occurrence in south hemisphere by Horvath and Essex (2003). In particular, it was found out that some features of the trough occurrence, its establishment and the existence of day trough is regularly detected in Australian sector. In these analysis TEC measurements of individual GPS satellite passes from three stations were used. It is difficult to recognize the trough in TEC measurements from individual satellite passes because the TEC variations include temporal and spatial (longitudinal and latitudinal) features. A new approach was developed to study the TEC trough based on GPS-derived TEC maps which were created with simultaneous GPS measurements from more than 150 stations of European network. To detect the TEC trough, latitudinal TEC profiles were formed with interval of 1 h at fixed longitude of 20 E. In these studies, the analysis of the structure and dynamics of the TEC trough over Europe for different geophysical conditions over the period covering years 2000–2005 are presented. The observations from more than 15,000 TEC
profiles were utilized over that period. The intention was to obtain experimental data showing the trough locations and on this basis to develop an empirical model of the location for different seasons that depends on time and geomagnetic activity. The results of the current study can be useful to help upgrade the IRI model (Bilitza et al., 1993), since that model currently does not represent the actual trough structure. The results can be also useful to correct for propagation effects in different radio systems. 2. TEC estimation technique GPS observations collected by International GNSS Service (IGS) and European Permanent Network (EPN) were used to create TEC maps. More than 150 stations from Europe were included in the analysis. The dense worldwide GPS network provided high TEC resolution (Hernandez-Pajares et al., 1997). While estimating TEC from GPS observations, the ionosphere was approximated by a spherical shell at a fixed height of 400 km above the Earth’s surface. A geometric factor was used to convert the slanted TEC into a vertical one. To reduce the influence of the horizontal gradients, the lowest elevation angle considered in the GPS data was 20°. High-precision phase measurements were used while processing. The phase ambiguities were removed by fitting the phase measurements to the code data collected along an individual satellite pass. After pre-processing, the phase measurements contained an instrumental bias only (Baran et al., 1997). The biases were determined for each station using GPS measurements of all satellite passes over a given site during a 24-h period. The diurnal variations of TEC over a site and the biases for all satellites were estimated simultaneously. At all stations, before the technique was run, the instrumental biases had been removed in all satellite passes. Using this procedure, an absolute line of sight TEC for all satellite-receiver paths was calculated. In order to obtain the spatial and temporal variation of TEC and to create TEC maps, the measurements were fitted to a spherical harmonic expansion in a geographic latitude and longitude. The spherical harmonic expansion was truncated to the order and degree of 16. The accuracy of TEC maps depends on spatial gaps in TEC data (Mannucci et al., 1998). The large number of GPS stations in Europe and North America provide good coverage for GPS data and enable high-accuracy TEC maps with errors at a level of 0.5–2 TECU. The coverage is a very adequate shell and yields a reasonable surface harmonic fit, providing TEC maps with a spatial resolution of 100–300 km and a temporal resolution of 5 min (Krankowski et al., 2007). 3. Data selection The TEC maps were used to construct the latitudinal profiles, which extended from high to equatorial latitudes. The TEC data were plotted as a function of geographic latitude. The highest latitude of the profiles was restricted to
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the geographic latitude of 75°N. While creating the TEC maps, the GPS measurements along individual satellite passes with elevation angle more than 20° were used. Since GPS satellites have inclination about 55° the highest GPS station used to produce TEC maps was NYAL (78.55°; 11.52°). The profiles were created with resolution of 1° at fixed longitude of 20°E. An automatic procedure was developed to identify the trough signatures. Usually the trough signatures are demonstrated on latitudinal profiles as the minimum values of TEC with well defined equatorial and polar walls. To locate the trough minima we used the criterion of the curvature for TEC function that depends on the geographic latitude. A visual identification was used to control and check the automatic procedure. In the case of several minimums, however, the one located the most equatorward was selected.
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the latitudes lower than 50°N, during the severe summer storm. The trough structure in GPS TEC demonstrates the smooth shape. The width of the TEC trough was evaluated as 5° to 7°. For this purpose two points on equatorward and poleward walls were detected where the latitudinal TEC gradients were 2 times smaller than the maximal gradients on the both walls of the trough. The distance between these points was designated as the trough width. During the equinox, the trough was also detected on quiet days in evening and night hours. On the disturbed day the trough was located at lower latitudes against the quiet day. The figure shows that during storms the trough minima shifted toward middle latitudes. In the evening hours the trough demonstrated clearly sharp equatorial and polar walls. The ratio of the minimum TEC to the TEC on equatorward and poleward edges of the trough exceeds a factor of 1.5.
4. Results and discussion 4.1. TEC maps and trough occurrence The TEC maps created day by day with one-hour interval demonstrate the spatial and temporal distribution of TEC over Europe. As an example, Fig. 1 illustrates dynamics of TEC on December 2nd, 2000 during quiet geomagnetic conditions. The trough was clearly recognized on these maps as a longitude-extended depression of TEC in the latitudinal distribution. The TEC trough appeared after noon on the east part of Europe, after that the trough moved to lower latitudes. The trough demonstrates very well marked polar and equator walls. 4.2. Behavior of latitudinal profiles The latitudinal TEC profiles were constructed from TEC maps at one-hour interval with 1° step in latitude, they are shown in geographical coordinates. It should be noted that subtraction of 2° from the geographic latitudes used here gives the approximate magnetic latitude in European sector (Pryse et al., 2006). Fig. 2 presents a compression of latitudinal profiles for winter, summer and equinox in year 2000 for quiet and stormy days. During winter, the trough-like structure can be recognized in the disturbed as well as quiet days in evening and night hours. The latitudinal location of the trough minima in the disturbed days is lower than during the quiet period. During the disturbances (November 10, 2004), the sharp decrease of TEC, which was associated with the trough, can be seen in day-time after 13 UT. It should be noted that the maximum Kp values occurred early on this day (between 9 and 15 UT). In summer during quiet day, the latitudinal profiles display a monotonous character. During the disturbed day of July P 15, 2004, when Kp indices reached 8–9 after 12 UT ( Kp = 50) the trough-like structures were detected in day-time. In evening hours the trough was recognized at
4.3. Statistical data analysis and the modeling of trough minimum According to the availability of GPS measurements, the latitudinal profiles were derived from TEC maps by plotting the hourly TEC values in relation to geographic latitude. The plots were used to automatically identify the trough and determine the position of the trough minima during different geomagnetic conditions. In order to analyze temporal variations of the location of the trough minima, the corrected geomagnetic latitude (CGL) and local time (LT) were used. Fig. 3a and b shows the location of all trough minima for winter seasons at high (November, 2000) and low (January, 2005) solar activity and also during the equinox – Fig. 3c (March, 2001), respectively. During these months, the day by day trough minima were determined from latitudinal profiles. The single point corresponds to the single day and time (LT), respectively. The step of time interval corresponds to 1 h and one degree of latitude. The scatters of the trough minima are generally related to variations of the geomagnetic activity. In Fig. 3, the cross characterizes the locationP of the trough during the geomagnetic disturbances (when K p 20). During disturbed days, the trough was situated at lower latitudes against the quiet period. At noon, the trough was located at higher latitudes, and in the afternoon its position shifted towards lower latitudes. In the afternoon and night the trough was detected on every day of the analyzed month. During the day-time, the trough was usually recognized during disturbances. The comparison of data of the trough minima for high and low solar activity showed that local time behavior of the minima was very similar. The differences were related to variations of magnetic activity. During the equinox the trough was located at higher latitudes than in winter. The occurrence of the trough in the equinox was regularly detected from 19 LT until 05 LT (next day) and from 17 LT until 07 LT in winter.
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Fig. 1. TEC distribution over Europe with 1-h resolution on December 2, 2000.
In Fig. 3, the median of the trough minima location is also presented (grey line). For estimating of the median
P the days with K p 20 (quiet condition) were selected. In addition, this figure represents time variations of the
A. Krankowski et al. / Advances in Space Research 43 (2009) 1721–1731
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Fig. 2. (a) Dynamics of latitudinal profiles (in geographic coordinates) on July 8, 2000 (quiet day – dashed line) and July 15, 2000 (disturbed day – solid line). (b) Dynamics of latitudinal TEC profiles (in geographic coordinates) on September 14, 2000 (quiet day – dashed line) and September 17, 2000 (disturbed day – solid line). (c) Dynamics of latitudinal TEC profiles (in geographic coordinates) on November 1, 2000 (quiet day – dashed line) and November 10, 2000 (disturbed day – solid line).
trough minima obtained using model presented by Werner and Prolss (1997), which was constructed on the base of DE2 satellite data for the high of 300–400 km (dotted line). The dashed line presents the location of the trough minima using the model based on ionosonde measurements (Ben’kova et al., 1993): U ¼ 66 2:4 K p 0:5 t þ 0:03 t2 ;
and Prolss (1997) was used. The model represents a Fourier series expansion of degree 2, which was fitted to the average curve using a least squares adjustment: 4p 4p sin cos LT þ a LT UðLTÞ ¼ b2 2 24 24 2p 2p LT þ a1 cos LT þ a0 þ b1 sin 24 24
where t is time (LT) calculated from midnight with sign ‘‘minus” before it and ‘‘plus” after . The TEC data were used to model the local time variation of the trough. An approximation analogous to Werner
The coefficients of the approximation are presented in Table 1. Comparison of our model to the model of Werner and Prolss (1997) as well as the model of Ben’kova
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et al. (1993) shows general agreement of the local time dependency. At the same time in winter the TEC trough was located at lower latitudes than the trough in the electron density in F2 region obtained from Werner and Ben’kova models. In the equinox the offset is less pronounced. 4.4. Effect of geomagnetic activity on the trough location As shown in Figs. 2 and 3, the trough position essentially depends on the geomagnetic activity. Fig. 4 demonstrates the location of the trough minima depending on Kp index in January 2005. With increasing geomagnetic activity the trough shifted towards low latitudes. When Kp index amounted to 7–8 the trough location was lower
than 50°. In first approximation the trough minima linearly depends on Kp index: U ¼ U0 þ b K p ; where the b coefficient slightly depends on time. It is clear that the dependence of the trough location is not linear. Fig. 4 presents scatter plots, thus it is well known that the trough position may not only depend on the instantaneous level of magnetic activity but also on its previous history, e.g., the ‘‘memory effect” (Werner and Prolss, 1997). The solid line demonstrates the linear approximation of the trough position. In order to compare our results with Tulunay and Grebowsky, 1978 the statistics of the experimental data for night location of the TEC trough in L coordinates depend-
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ing on 3h_Kp indices (dots) and its empirical linear approximation (solid line) for January 2005 are both presented in Fig. 5. The results presented by Tulunay and Grebowsky, 1978 were obtained from electron density (Ne) measured at height of about 550 km (Ariel 4) (dashed line). The trough location at 550 km height more strongly depends on 3h_Kp indices than in the case of the TEC trough. The latitudinal location of the TEC trough is different against the one at the defined height, hence this is an evidence that spatial structure of the trough occurrence depends on the height as it has been already shown by Pryse et al. (1993). Detailed response of dynamics of the trough minima to the magnetic activity can be seen from behavior of the latitudinal profiles. Fig. 6 demonstrates variations of latitudinal profiles for November 21–30, 2004. As it is shown in this figure the trough location changes with the variability
of Kp index. The trough followed changes of the geomagnetic activity and these variations did not have linear character either. 5. Conclusions and summary The analyses of the TEC trough appearance in European sector was carried out on the base of GPS-TEC measurements for the years of 2000–2005 for different geophysical conditions. The presented study investigates an essential aspect of the trough morphology in TEC. Our research has led us to the following: – The model of the position of the trough minima, based on large data set, was developed using the TEC maps with the high temporary and spatial resolution.
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Corrected Geomagnetic Latitude (CGL), [degree]
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LT (hours) Fig. 3. (a) Day by day position of the trough minimum for November 2000 (in CGL and LT). Dots – daily values, crosses – disturbed days with RK p 20, grey solid line – median with RK p 20, thin black solid line – presented model, dotted line – Werner and Prolss (1997) model, dashed line – Ben’kova et al. (1993) model. (b) Day by day position of the trough minimum for January 2005 (in CGL and LT). Dots – daily values, crosses – disturbed days with RK p 20, grey solid line – median with RK p 20, thin black solid line – presented model, dotted line – Werner and Prolss (1997) model, dashed line – Ben’kova et al. (1993) model. (c) Day by day position of the trough minimum for March 2001 (in CGL and LT). Dots – daily values, crosses – disturbed days with RK p 20, grey solid line – median with RK p 20, thin black solid line – presented model, dotted line – Werner and Prolss (1997) model, dashed line – Ben’kova et al. (1993) model.
– The TEC trough was regularly detected in winter and equinox during evening and night-time. The trough was observed also in day-time during geomagnetic disturbances. In summer the trough appears during the disturbed days only.
– The occurrence of the trough in the equinox was regularly detected from 19 LT until 05 LT and from 17 LT until 07 LT in winter. During night the trough was located near 55N geomagnetic latitudes in winter and near 57–58N in equinox.
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– The latitudinal size of the bottom of the trough was usually more than 5–7 degrees. – The ratio of TEC values in the trough minima to the polar/equatorward walls amount a factor of 1.5–4.0.
– The TEC trough probed by GPS demonstrated lower latitude location (about 3–5 degree) against the trough, which have been observed in F region electron density data.
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Fig. 4. Location of the trough minimum in dependence on Kp index between 0 and 6 UT (a) and 15–24 UT (b) in January 2005.
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January 2005,
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Latitude Table 1 Approximation of the coefficients. Coefficient
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a0 a1 b1 a2 b2
65.05 14.50 0.78 7.63 0.13
62.72 3.39 2.38 1.93 0.67
65.28 11.15 2.90 4.17 3.54
– GPS technique is very suitable to monitor the TEC trough on regular basis. – The study show that the trough in TEC is the regular phenomena, which should be taken into account in the ionospheric models for adequate evaluating of the propagation errors due to the ionosphere in navigation and radiocommunication.
Acknowledgement These studies have been supported by the Grant 4T12E 011 30 of the Polish Ministry of Science and Higher Education. References Baran, L.W., Shagimuratov, I.I., Tepenitzina, N.J. The use of GPS for ionospheric studies. Artif. Satellites 32 (1), 49–60, 1997. Ben’kova, N.P., Kozlov, E.F., Kochevanova, N.A., et al. Structure and Dynamics of the Subauroral Ionosphere. Nauka, Moscow, 144 pp., 1993. Bilitza, D., Rawer, K., Bossy, L., Gulayeva, T. International reference ionosphere – past, present, future. Adv. Space Res. 13 (3), 3–23, 1993. Ciraolo, L., Spalla, P. Preliminary study of the latitudinal dependence of TEC. Adv. Space Res. 22 (6), 807–810, 1998.
Fig. 6. Day by day dynamics of the trough position for November 21–30, 2004. Grey solid line presents the movement of the trough in dependence on Kp index.
Deminov, M.G., Karpachev, A.T. A longitudinal effect in the configuration of the main ionospheric trough. 1 – Location of the trough. Geomagnet. Aeronomy 26, 63–68 (in Russian), 1986. Deminov, M.G., Karpachev, A.T., Afonin, V.V. Longitudinal variations in the MIT location under nighttime conditions based on Cosmos-900 and Intercosmos-19 satellite data. Geomagnet. Aeronomy 32 (2), 75– 78 (in Russian), 1992. Grebowsky, J.M., Taylor, M., Lindsay, G. Location and source of ionospheric high-latitude trough. Planet. Space Sci. 31, 99–105, 1983. Hernandez-Pajares, M., Juan, J.M., Sanz, J. High resolution TEC monitoring method using permanent ground GPS receivers. Geophys. Res. Lett. 24 (13), 1643–1646, 1997. Horvath, I., Essex, E.A. The southern-hemisphere mid-latitude day-time and night-time trough at low-sunspot numbers. J. Atmos. Solar Terrest. Phys. 65, 917–940, 2003. Karpachev, A.T., Deminov, M.G., Afonin, V.V. Model of the midlatitude ionospheric trough on the base of Cosmos-900 and Intercosmos-19 satellites data. Adv. Space Res. 18 (6), 221–230, 1996. Krankowski, A., Shagimuratov, I.I., Baran, L.W., Yakimova, G. The structure of the mid- and high-latitude ionosphere during the November 2004 storm event obtained from GPS observations. Acta Geophys. 55 (4), 490–508, 2007. Mallis, M., Essex, E.A. Diurnal and seasonal variability of the southernhemisphere main ionospheric trough from differential-phase measurements. J. Atmos. Terres. Phys. 55, 1021–1037, 1993. Mannucci, A.J., Wilson, B.D., Yuan, D.N., Ho, C.H., Lindqwister, U.J., Runge, T.F. A global mapping technique for GPS-derived ionospheric total electron content measurements. Radio Sci. 33 (3), 565–582, 1998. Moffett, R.J., Quegan, S. The mid-latitude trough in the electron concentration of the ionospheric F-layer: a review of observations and modeling. J. Atmos. Terrest. Phys. 45, 315–343, 1983.
A. Krankowski et al. / Advances in Space Research 43 (2009) 1721–1731 Muldrew, D.B. F-layer ionization trough deduced from Alouette data. J. Geophys. Res. 70, 2635–2650, 1965. Pryse, S.E., Kersley, L., Rice, D.L., Russell, C.D., Walker, I.K. Tomographic imaging of the ionospheric mid-latitude trough. Ann. Geophys. 11, 144–149, 1993. Pryse, S.E., Kersley, L., Malan, D., Bishop, G.J. Parameterization of the main ionospheric trough in the European sector. Radio Sci. 41, doi:10.1029/2005RS003364, 2006. Rodger, A.S., Moffett, R.J., Quegan, S. The role of ion drift in the formation of ionization troughs in the mid- and high-latitude ionosphere – a review. J. Atmos. Terrest. Phys. 54 (1), 1–30, 1992. Rothkaehl, H., Stanisławska, I., Leitinger, R., Tulunay, Y. Application of the trough phenomena for telecommunication purposes. Phys. Chem. Earth 25 (4), 315–318, 2000. Rothkaehl, H., Krankowski, A., Stanislawska, I., Błe˛cki, J., Parrot, M., Berthelier, J.-J., Lebreton, J.-P. Wave and plasma measurements and GPS diagnostics of the main ionospheric trough as a hybrid method used for Space Weather purposes. Ann. Geophys. 26, 295– 304, 2008. Sharp, G.W. Midlatitude trough in the night ionosphere. J. Geophys. Res. 71, 1345–1356, 1966. Stanislawska, I., Rothkaehl, H. PLES model in the plasmapause diagnostics. Adv. Space Res. 29 (6), 833–838, 2002.
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Tulunay, Y.K., Grebowsky, J.M. The noon and midnight midlatitude trough as seen by ARIEL-4. J. Atmos. Terrest. Phys. 40, 845–855, 1978. Tulunay, Y.K., Sayers, J. Characteristics of mid-latitude trough as determined by electron density experiment on ARIEL 3. J. Atmos. Terrest. Phys. 33, 1737–1761, 1971. Tulunay, Y., Stanisawska, I., Rothkaehl, H. Revisiting the Ariel trough work for HF telecommunication purposes. Cosmic Res. 41 (4), 319–331, 2003. Wanninger, L. The occurrence of ionospheric disturbances above Japan and their effects on GPS positioning. Proceedings of the 8th International Symposium on Recent Crust Movements (CRCM 93), 6–11. Kobe, Japan, pp. 175–179, 1993. Werner, S., Prolss, G.W. The position of the ionospheric trough as a function of local time and magnetic activity. Adv. Space Res. 20 (9), 1717–1722, 1997. Whalen, J.A. The daytime F layer trough and its relation to ionosphericmagnetospheric convection. J. Geophys. Res. 94, 17169–17184, 1989. Wielgosz, P., Baran, L.W., Shagimuratov, I.I., Aleshnikova, M.V. Latitudinal variations of TEC over Europe obtained from GPS observations. Ann. Geophys. 22, 405–415, 2004. Wielgosz, P., Kashani, I., Grejner-Brzezinska, D.A. Analysis of longrange network RTK during severe ionospheric storm. J. Geodesy 79 (9), 524–531, 2005.
Advances in Space Research 43 (2009) 1721–1731 www.elsevier.com/locate/asr
The occurrence of the mid-latitude ionospheric trough in GPS-TEC measurements A. Krankowski a,*, I.I. Shagimuratov b, I.I. Ephishov b, A. Krypiak-Gregorczyk a, G. Yakimova b a
Institute of Geodesy, University of Warmia and Mazury in Olsztyn, Oczapowski St.1, 10-957 Olsztyn, Poland b WD IZMIRAN, Prospekt Pabedy 41, 236017 Kaliningrad, Russia Received 19 November 2007; received in revised form 28 May 2008; accepted 28 May 2008
Abstract Simultaneous GPS observations from about 150 stations of European Permanent Network (EPN) have been used for studying dynamics of latitudinal profiles and structure of mid-latitude ionospheric trough (MIT). For the analyses, the TEC maps over Europe were created with high spatial and temporal resolution. The latitudinal profiles were produced from TEC maps with one-hour interval for geographic latitude range from 35N to 75N. The structure of latitudinal profiles relates to the occurrence of the ionospheric trough. The location of the trough depends on season, local time, and both geophysical and geomagnetic conditions. The trough structure in GPS-TEC demonstrates a smooth shape. The trough occurrence as a distinguished structure is more distinct during winter. The relation of TEC in the trough minimum to the equator and polar walls amounted to a factor of 2–4. The diurnal, seasonal as well as storm-time dynamics of the latitudinal profiles and the trough-like structure during different geomagnetic conditions are also presented. Day by day snapshots demonstrate great variability of TEC profiles and MIT, essential changes of the ionosphere structure took place during a storm. Similarly to the F2 region trough, the position of the TEC trough shifts towards lower latitudes during disturbances. In the storm-time, the TEC trough was recognized during day-time at latitudes lower than 70N. This paper presents the statistics describing the dynamics of the trough location. Ó 2008 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Ionosphere; Total Electron Content; Mid-latitude ionospheric trough (MIT); Ionospheric perturbations
1. Introduction The middle-latitude ionospheric trough is the main and dominant scale structure, which is identified in F region of the mid-latitude ionosphere (Muldrew, 1965; Sharp, 1966). The spatial structure of the trough is presented as the latitudinal narrow and longitudinal extended depletion in the electron distribution. The physical mechanisms of the trough formation include complex interconnected physical processes in the mid- and high-latitude ionosphere. A the*
Corresponding author. Fax: +48 895234768. E-mail addresses: [email protected] (A. Krankowski, A. KrypiakGregorczyk), [email protected] (I.I. Shagimuratov, I.I. Ephishov, G. Yakimova).
ory and mechanisms of the trough formation were discussed in Moffett and Quegan (1983) and Rodger et al. (1992). The trough dominates in winter conditions and it is regularly detected in evening and night hours. The occurrence of the trough depends on latitude, longitude as well as the geomagnetic activity. Latitudinal location of the trough can differ essentially in different longitudinal sectors (Deminov et al., 1992; 1986; Karpachev et al., 1996). There are also some typical features in the trough occurrence in southern hemisphere (Mallis and Essex, 1993; Horvath and Essex, 2003). The trough was studied by employing different methods using satellite and ground observations (Tulunay and
0273-1177/$36.00 Ó 2008 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2008.05.014
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Sayers, 1971; Tulunay and Grebowsky, 1978; Tulunay et al., 2003; Grebowsky et al., 1983; Rothkaehl et al., 2000; Whalen, 1989; Werner and Prolss, 1997; Stanislawska and Rothkaehl, 2002). The numerous investigations concerned the occurrence of the trough in F2 region of the ionosphere. The Total Electron Content (TEC) is the key ionospheric parameter for a number of space-ground telecommunication systems, satellite navigation (GPS, GLONASS), orbit determination, ocean altimetry. The ionospheric trough influences HF as well as the transionospheric radio wave propagation. Strong latitudinal gradients were already associated with the trough by Wielgosz et al. (2004). As known, the severe horizontal TEC gradients can hamper the ambiguity resolution and deteriorate the accuracy of the GPS positioning (Wanninger, 1993; Wielgosz et al., 2005). The trough is located equatorward of the auroral oval, where strong phase and amplitude scintillations of transionospheric signals can occur. The scintillation effect can lead to degradation of the navigation system performance. So, the knowledge of the trough occurrence in TEC, its spatial structure and latitudinal location are very important for various practical applications as well as for investigation of the Earth’s ionosphere. The occurrence of the trough in TEC has been studied for southern hemisphere by Mallis and Essex (1993) and for northern hemisphere by Ciraolo and Spalla (1998) and Pryse et al. (1993) using differential carrier-phase method. Recently, using the same technique, the parameters of the TEC trough were presented by Pryse et al. (2006). Rothkaehl et al. (2008) described a new hybrid method for simultaneously diagnostics of the mid-latitude ionospheric trough using GNSS (by UWM technique) and in situ wave measurements – DEMETER. Currently GPS and GLONASS techniques can be effectively used to study the TEC trough. A large and dense network of GPS stations allow regular monitoring of the TEC trough. The GPS measurements were used to study the trough occurrence in south hemisphere by Horvath and Essex (2003). In particular, it was found out that some features of the trough occurrence, its establishment and the existence of day trough is regularly detected in Australian sector. In these analysis TEC measurements of individual GPS satellite passes from three stations were used. It is difficult to recognize the trough in TEC measurements from individual satellite passes because the TEC variations include temporal and spatial (longitudinal and latitudinal) features. A new approach was developed to study the TEC trough based on GPS-derived TEC maps which were created with simultaneous GPS measurements from more than 150 stations of European network. To detect the TEC trough, latitudinal TEC profiles were formed with interval of 1 h at fixed longitude of 20 E. In these studies, the analysis of the structure and dynamics of the TEC trough over Europe for different geophysical conditions over the period covering years 2000–2005 are presented. The observations from more than 15,000 TEC
profiles were utilized over that period. The intention was to obtain experimental data showing the trough locations and on this basis to develop an empirical model of the location for different seasons that depends on time and geomagnetic activity. The results of the current study can be useful to help upgrade the IRI model (Bilitza et al., 1993), since that model currently does not represent the actual trough structure. The results can be also useful to correct for propagation effects in different radio systems. 2. TEC estimation technique GPS observations collected by International GNSS Service (IGS) and European Permanent Network (EPN) were used to create TEC maps. More than 150 stations from Europe were included in the analysis. The dense worldwide GPS network provided high TEC resolution (Hernandez-Pajares et al., 1997). While estimating TEC from GPS observations, the ionosphere was approximated by a spherical shell at a fixed height of 400 km above the Earth’s surface. A geometric factor was used to convert the slanted TEC into a vertical one. To reduce the influence of the horizontal gradients, the lowest elevation angle considered in the GPS data was 20°. High-precision phase measurements were used while processing. The phase ambiguities were removed by fitting the phase measurements to the code data collected along an individual satellite pass. After pre-processing, the phase measurements contained an instrumental bias only (Baran et al., 1997). The biases were determined for each station using GPS measurements of all satellite passes over a given site during a 24-h period. The diurnal variations of TEC over a site and the biases for all satellites were estimated simultaneously. At all stations, before the technique was run, the instrumental biases had been removed in all satellite passes. Using this procedure, an absolute line of sight TEC for all satellite-receiver paths was calculated. In order to obtain the spatial and temporal variation of TEC and to create TEC maps, the measurements were fitted to a spherical harmonic expansion in a geographic latitude and longitude. The spherical harmonic expansion was truncated to the order and degree of 16. The accuracy of TEC maps depends on spatial gaps in TEC data (Mannucci et al., 1998). The large number of GPS stations in Europe and North America provide good coverage for GPS data and enable high-accuracy TEC maps with errors at a level of 0.5–2 TECU. The coverage is a very adequate shell and yields a reasonable surface harmonic fit, providing TEC maps with a spatial resolution of 100–300 km and a temporal resolution of 5 min (Krankowski et al., 2007). 3. Data selection The TEC maps were used to construct the latitudinal profiles, which extended from high to equatorial latitudes. The TEC data were plotted as a function of geographic latitude. The highest latitude of the profiles was restricted to
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the geographic latitude of 75°N. While creating the TEC maps, the GPS measurements along individual satellite passes with elevation angle more than 20° were used. Since GPS satellites have inclination about 55° the highest GPS station used to produce TEC maps was NYAL (78.55°; 11.52°). The profiles were created with resolution of 1° at fixed longitude of 20°E. An automatic procedure was developed to identify the trough signatures. Usually the trough signatures are demonstrated on latitudinal profiles as the minimum values of TEC with well defined equatorial and polar walls. To locate the trough minima we used the criterion of the curvature for TEC function that depends on the geographic latitude. A visual identification was used to control and check the automatic procedure. In the case of several minimums, however, the one located the most equatorward was selected.
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the latitudes lower than 50°N, during the severe summer storm. The trough structure in GPS TEC demonstrates the smooth shape. The width of the TEC trough was evaluated as 5° to 7°. For this purpose two points on equatorward and poleward walls were detected where the latitudinal TEC gradients were 2 times smaller than the maximal gradients on the both walls of the trough. The distance between these points was designated as the trough width. During the equinox, the trough was also detected on quiet days in evening and night hours. On the disturbed day the trough was located at lower latitudes against the quiet day. The figure shows that during storms the trough minima shifted toward middle latitudes. In the evening hours the trough demonstrated clearly sharp equatorial and polar walls. The ratio of the minimum TEC to the TEC on equatorward and poleward edges of the trough exceeds a factor of 1.5.
4. Results and discussion 4.1. TEC maps and trough occurrence The TEC maps created day by day with one-hour interval demonstrate the spatial and temporal distribution of TEC over Europe. As an example, Fig. 1 illustrates dynamics of TEC on December 2nd, 2000 during quiet geomagnetic conditions. The trough was clearly recognized on these maps as a longitude-extended depression of TEC in the latitudinal distribution. The TEC trough appeared after noon on the east part of Europe, after that the trough moved to lower latitudes. The trough demonstrates very well marked polar and equator walls. 4.2. Behavior of latitudinal profiles The latitudinal TEC profiles were constructed from TEC maps at one-hour interval with 1° step in latitude, they are shown in geographical coordinates. It should be noted that subtraction of 2° from the geographic latitudes used here gives the approximate magnetic latitude in European sector (Pryse et al., 2006). Fig. 2 presents a compression of latitudinal profiles for winter, summer and equinox in year 2000 for quiet and stormy days. During winter, the trough-like structure can be recognized in the disturbed as well as quiet days in evening and night hours. The latitudinal location of the trough minima in the disturbed days is lower than during the quiet period. During the disturbances (November 10, 2004), the sharp decrease of TEC, which was associated with the trough, can be seen in day-time after 13 UT. It should be noted that the maximum Kp values occurred early on this day (between 9 and 15 UT). In summer during quiet day, the latitudinal profiles display a monotonous character. During the disturbed day of July P 15, 2004, when Kp indices reached 8–9 after 12 UT ( Kp = 50) the trough-like structures were detected in day-time. In evening hours the trough was recognized at
4.3. Statistical data analysis and the modeling of trough minimum According to the availability of GPS measurements, the latitudinal profiles were derived from TEC maps by plotting the hourly TEC values in relation to geographic latitude. The plots were used to automatically identify the trough and determine the position of the trough minima during different geomagnetic conditions. In order to analyze temporal variations of the location of the trough minima, the corrected geomagnetic latitude (CGL) and local time (LT) were used. Fig. 3a and b shows the location of all trough minima for winter seasons at high (November, 2000) and low (January, 2005) solar activity and also during the equinox – Fig. 3c (March, 2001), respectively. During these months, the day by day trough minima were determined from latitudinal profiles. The single point corresponds to the single day and time (LT), respectively. The step of time interval corresponds to 1 h and one degree of latitude. The scatters of the trough minima are generally related to variations of the geomagnetic activity. In Fig. 3, the cross characterizes the locationP of the trough during the geomagnetic disturbances (when K p 20). During disturbed days, the trough was situated at lower latitudes against the quiet period. At noon, the trough was located at higher latitudes, and in the afternoon its position shifted towards lower latitudes. In the afternoon and night the trough was detected on every day of the analyzed month. During the day-time, the trough was usually recognized during disturbances. The comparison of data of the trough minima for high and low solar activity showed that local time behavior of the minima was very similar. The differences were related to variations of magnetic activity. During the equinox the trough was located at higher latitudes than in winter. The occurrence of the trough in the equinox was regularly detected from 19 LT until 05 LT (next day) and from 17 LT until 07 LT in winter.
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In Fig. 3, the median of the trough minima location is also presented (grey line). For estimating of the median
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trough minima obtained using model presented by Werner and Prolss (1997), which was constructed on the base of DE2 satellite data for the high of 300–400 km (dotted line). The dashed line presents the location of the trough minima using the model based on ionosonde measurements (Ben’kova et al., 1993): U ¼ 66 2:4 K p 0:5 t þ 0:03 t2 ;
and Prolss (1997) was used. The model represents a Fourier series expansion of degree 2, which was fitted to the average curve using a least squares adjustment: 4p 4p sin cos LT þ a LT UðLTÞ ¼ b2 2 24 24 2p 2p LT þ a1 cos LT þ a0 þ b1 sin 24 24
where t is time (LT) calculated from midnight with sign ‘‘minus” before it and ‘‘plus” after . The TEC data were used to model the local time variation of the trough. An approximation analogous to Werner
The coefficients of the approximation are presented in Table 1. Comparison of our model to the model of Werner and Prolss (1997) as well as the model of Ben’kova
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et al. (1993) shows general agreement of the local time dependency. At the same time in winter the TEC trough was located at lower latitudes than the trough in the electron density in F2 region obtained from Werner and Ben’kova models. In the equinox the offset is less pronounced. 4.4. Effect of geomagnetic activity on the trough location As shown in Figs. 2 and 3, the trough position essentially depends on the geomagnetic activity. Fig. 4 demonstrates the location of the trough minima depending on Kp index in January 2005. With increasing geomagnetic activity the trough shifted towards low latitudes. When Kp index amounted to 7–8 the trough location was lower
than 50°. In first approximation the trough minima linearly depends on Kp index: U ¼ U0 þ b K p ; where the b coefficient slightly depends on time. It is clear that the dependence of the trough location is not linear. Fig. 4 presents scatter plots, thus it is well known that the trough position may not only depend on the instantaneous level of magnetic activity but also on its previous history, e.g., the ‘‘memory effect” (Werner and Prolss, 1997). The solid line demonstrates the linear approximation of the trough position. In order to compare our results with Tulunay and Grebowsky, 1978 the statistics of the experimental data for night location of the TEC trough in L coordinates depend-
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Fig. 2 (continued)
ing on 3h_Kp indices (dots) and its empirical linear approximation (solid line) for January 2005 are both presented in Fig. 5. The results presented by Tulunay and Grebowsky, 1978 were obtained from electron density (Ne) measured at height of about 550 km (Ariel 4) (dashed line). The trough location at 550 km height more strongly depends on 3h_Kp indices than in the case of the TEC trough. The latitudinal location of the TEC trough is different against the one at the defined height, hence this is an evidence that spatial structure of the trough occurrence depends on the height as it has been already shown by Pryse et al. (1993). Detailed response of dynamics of the trough minima to the magnetic activity can be seen from behavior of the latitudinal profiles. Fig. 6 demonstrates variations of latitudinal profiles for November 21–30, 2004. As it is shown in this figure the trough location changes with the variability
of Kp index. The trough followed changes of the geomagnetic activity and these variations did not have linear character either. 5. Conclusions and summary The analyses of the TEC trough appearance in European sector was carried out on the base of GPS-TEC measurements for the years of 2000–2005 for different geophysical conditions. The presented study investigates an essential aspect of the trough morphology in TEC. Our research has led us to the following: – The model of the position of the trough minima, based on large data set, was developed using the TEC maps with the high temporary and spatial resolution.
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Corrected Geomagnetic Latitude (CGL), [degree]
a
November 2000 75
70
65
60
55
50
45 12
18
0
6
12
6
12
LT (hours)
Corrected Geomagnetic Latitude (CGL), [degree]
b
January 2005 75
70
65
60
55
50
45
12
18
0
LT (hours) Fig. 3. (a) Day by day position of the trough minimum for November 2000 (in CGL and LT). Dots – daily values, crosses – disturbed days with RK p 20, grey solid line – median with RK p 20, thin black solid line – presented model, dotted line – Werner and Prolss (1997) model, dashed line – Ben’kova et al. (1993) model. (b) Day by day position of the trough minimum for January 2005 (in CGL and LT). Dots – daily values, crosses – disturbed days with RK p 20, grey solid line – median with RK p 20, thin black solid line – presented model, dotted line – Werner and Prolss (1997) model, dashed line – Ben’kova et al. (1993) model. (c) Day by day position of the trough minimum for March 2001 (in CGL and LT). Dots – daily values, crosses – disturbed days with RK p 20, grey solid line – median with RK p 20, thin black solid line – presented model, dotted line – Werner and Prolss (1997) model, dashed line – Ben’kova et al. (1993) model.
– The TEC trough was regularly detected in winter and equinox during evening and night-time. The trough was observed also in day-time during geomagnetic disturbances. In summer the trough appears during the disturbed days only.
– The occurrence of the trough in the equinox was regularly detected from 19 LT until 05 LT and from 17 LT until 07 LT in winter. During night the trough was located near 55N geomagnetic latitudes in winter and near 57–58N in equinox.
A. Krankowski et al. / Advances in Space Research 43 (2009) 1721–1731
c
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March 2001
Corrected Geomagnetic Latitude (CGL), [degree]
75
70
65
60
55
50
45 12
18
0
6
12
LT (hours) Fig. 3 (continued)
– The latitudinal size of the bottom of the trough was usually more than 5–7 degrees. – The ratio of TEC values in the trough minima to the polar/equatorward walls amount a factor of 1.5–4.0.
– The TEC trough probed by GPS demonstrated lower latitude location (about 3–5 degree) against the trough, which have been observed in F region electron density data.
January 2005
Corrected Geomagnetic Latitude (CGL), [degree]
b)
a)
75
70
75
70
Φ = 64,3 -1,2Kp Φ = 62,8 -1,6Kp
65
65
Φ = 59,7 -1,2Kp 60
60
55
55
50
50
Φ = 58,7 -1,0Kp
Φ = 59,8 -1,3Kp 45
45 0
3
6
9
3
6
9
3
6
9
3
6
9
3
6
9
3
6
9
Kp index
0
3
6
15
18
21
24
UT (hours)
Fig. 4. Location of the trough minimum in dependence on Kp index between 0 and 6 UT (a) and 15–24 UT (b) in January 2005.
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January 2005,
00 UT - 03 UT
9 8
8
L=3.66 - 0.19 Kp
7
7
6
6
5
5
Σ Kp 25
2004-11-30
25 20
22 19 21
15
4
4
3
3 L=4.85- 0.37 Kp
2
2
1
TECU
3h - Kp index
25
9
27 17 12
10 15
1 25
0
0
2
3
4
5
6
7
2004-11-21 5
8
L coordinate Fig. 5. Location of the trough minimum in dependence on Kp index between 0 and 3 UT in January 2005, black solid line – presented model, dashed line – Tulunay model.
0
40
50
60
70
Latitude Table 1 Approximation of the coefficients. Coefficient
November 2000
March 2001
January 2005
a0 a1 b1 a2 b2
65.05 14.50 0.78 7.63 0.13
62.72 3.39 2.38 1.93 0.67
65.28 11.15 2.90 4.17 3.54
– GPS technique is very suitable to monitor the TEC trough on regular basis. – The study show that the trough in TEC is the regular phenomena, which should be taken into account in the ionospheric models for adequate evaluating of the propagation errors due to the ionosphere in navigation and radiocommunication.
Acknowledgement These studies have been supported by the Grant 4T12E 011 30 of the Polish Ministry of Science and Higher Education. References Baran, L.W., Shagimuratov, I.I., Tepenitzina, N.J. The use of GPS for ionospheric studies. Artif. Satellites 32 (1), 49–60, 1997. Ben’kova, N.P., Kozlov, E.F., Kochevanova, N.A., et al. Structure and Dynamics of the Subauroral Ionosphere. Nauka, Moscow, 144 pp., 1993. Bilitza, D., Rawer, K., Bossy, L., Gulayeva, T. International reference ionosphere – past, present, future. Adv. Space Res. 13 (3), 3–23, 1993. Ciraolo, L., Spalla, P. Preliminary study of the latitudinal dependence of TEC. Adv. Space Res. 22 (6), 807–810, 1998.
Fig. 6. Day by day dynamics of the trough position for November 21–30, 2004. Grey solid line presents the movement of the trough in dependence on Kp index.
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