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SCAR Report 23

Storm-Time Structure and Dynamics of the Ionosphere obtained from GPS observations

I.I. Shagimuratov (1), A. Krankowski (2), L.W. Baran (2), J. Cisak (3), G. Yakimova (1)

(1) WD Izmiran, Kaliningrad, Russia
(2) Institute of Geodesy, Warmia and Mazury University in Olsztyn, Poland, kand@uwm.edu.pl;
(3) Institute of Geodesy and Cartography, Warsaw, Poland

Abstract.

The analyses of the structure of the ionosphere during the greatest storm in recent years, which took place on 31 March 2001 obtained by multi-stations technique using GPS observations of IGS and EPN network are presented. Storm-time changes of the ionosphere were analyzed via the TEC maps for American and European regions. The response of the ionosphere in the Antarctic area eliminates temporal TEC variations obtained over an individual station. High spatial resolution of TEC maps was realized using GPS observation from 80-100 European and American stations. The TEC maps were produced in latitudinal range of 40-75∞ with 15 min interval. Time-depended features of the ionospheric storm were identified using the differential TEC maps based on the deviation of TEC during the storm in comparison to a quiet period. The response of TEC to geomagnetic storm consists of effects of both enhancement and depletion (positive and negative disturbances). A short-duration positive effect on the first stage of the storm was observed over Europe on subauroral ionosphere probably due to the auroral particle precipitation. The enhancement of TEC exceeds 150% compared to the quiet time. The negative effect took place during daytime on the first day of storm and lasted till next night.

In the American sector the effect was more pronounced than over Europe. The essential changes of the ionosphere are observed on subauroral latitude, which we attribute to the occurrence of ionospheric trough and it developed during the storm. Maximal latitudinal gradients which occurred at the equatorial or polar walls of the trough depend on geophysical conditions. Over America the spatial distribution of TEC demonstrate the large scale structures, which probably can be associated with perturbations of the neutral winds. The strong storm effect took place over the Antarctic and Arctic regions also. During 31 March day time depression of TEC exceeded 200% daytime level of TEC in comparison to night TEC. The diurnal variations TEC over a high latitude station are essentially modified. In the auroral region in magnetic storm period the ionospheric different scale irregularities developed, which caused increasing intensity phase fluctuations of GPS signals. On the whole the results demonstrate complex storm patterns as a function of geophysical conditions, longitude, latitude and time.

Key words: Ionosphere, TEC, modeling of ionosphere

1 Introduction

Two severe geomagnetic storms took place on March 19 and March 31, 2001. The main phase of the first storm started about 11 UT on March 19. The Dst index reached its minimum value -160 nT at 13 UT on March 20 (Figure 1). Simultaneously Kp index amounted to 7 and _Kp ≈ 44 (Figure 2). The second investigated storm started about 04 UT on March 31. The Dst index decreased sharply to –358 nT at 08:00 UT. The Kp index reached the value of 9 between 06:00 and 12:00 UT on March 31 (_Kp amounted of 61). The recovery phase took place after 09:00 UT on April 4, when Dst gradually returned to its regular level.
Days of March and April 2001

Fig. 1. Dst index

Fig.2. Variations of sum Kp during March 2001

2 Data source and estimation technique

The GPS data from IGS (International GPS Service) permanent network were used to obtain TEC changes on the global scale during both storms. A dense GPS network provided TEC measurements with high temporal and spatial resolution. The analyses of the storms were carried out over North America, Antarctica and Europe. To present the temporal and spatial variation of TEC during the storms, we created the instantaneous TEC maps. The data from over 80 European and 100 American GPS stations were used to create TEC maps. Precise dual frequency GPS phase measurements were used (Baran et al., 1997).
While estimating TEC, the ionosphere was approximated as a single layer at a fixed height of 400 km above the Earth’s surface. The simple geometric factor was used to convert slant TEC into vertical one. A sun-fixed reference frame was used (local time/geomagnetic latitude). In order to produce TEC maps, the TEC measurements from all stations were fitted to a spherical harmonic expansion as functions of geographic latitude and longitude. The maximum degree/order of the spherical expansion was 16. The maps were derived with a 15-minute resolution.

3 Results

3.1 The storm on March 19, 2001

Over Europe, the storm started just after local noon on March 19 (Figure 3). Storm-time effects occurred in TEC in the evening and during night, as the TEC increase at auroral and subauroral latitudes.

Fig. 3. Storm development over Europe
The analyses of TEC variations for the individual satellite passes show that during the driven phase of the storm, different scale irregularities developed at high latitudes. Patch-like structures with a strong TEC increase were observed. It is interesting, that similar structures but with decreased TEC were also observed (Baran et al., 2002, Shagimuratov et al., 2002).
During daytime on March 20, the negative effect occurred with a maximum at latitudes over 55∞N. The weak negative effect took place also at latitudes under 40∞N. The negative phase of the storm lasted through the next day (March 21) and the following night. The negative phase was mostly pronounced at latitudes over 50-55N.
On Figures 4a and 4b you can see the differential maps of TEC over Europe for March 19 and 20. Nighttime TEC increase took place on March 19/20 at auroral and subauroral latitudes. The increase reached 100-150%. The positive effect occurred also at latitudes below 50∞N. The TEC depression, observed at latitudes about 55∞N can be attributed to the effect of the midlatitude trough. On March 20, after 06:00 UT the negative effect prevailed over Europe and lasted until 06:00 UT on March 21.Fig.4a. The differential maps of TEC over Europe for March 19, 2001 (geographic coordinates).Fig.4b. The differential maps of TEC over Europe for March 20, 2001 (geographic coordinates).

3.2 The storm on March 31, 2001

Figure 5 presents storm development over North America region on March 31.

 

Fig. 5. Storm development over North America

Before the start of the storm the positive effect took place during the local daytime on March 31.The negative effect occurred during the following night and TEC depression reached 75 %. The negative effect lasted through the following local day.
During the driven phase of the storm, large-scale structures of the increased TEC were observed in the ionosphere. The structures are related to the occurrence of the midlatitude trough and strong perturbations induced in the ionization processes, such as particle precipitation at high latitudes (Figure 6a and 6b).
Fig.6a. Differential maps of TEC over North America for March 31, 2001 (geographic coordinates).
Fig.6b. Differential maps of TEC over North America for April 1, 2001 (geographic coordinates).
Fig.7 The satellite/receiver biases of OHIG and MCM4 receivers for March 2001
Fig.8 Diurnal variation of TEC over Antarctic stations for period 29 March -1 April 2001 (dashed line) and quiet day 26 March, 2001 (solid line)
In the periods of geomagnetic storms (on March 19 and 31) the satellite/receiver biases of O’Higgins and McMurdo receivers increased sharply. On March 31 satellite/receiver biases reached the value of 4 meters at OHIG and MCM4, respectively (Figure 7).
Figure 8 presents diurnal variations of the TEC over single Antarctic stations for the period of storm of March 31, 2001. For a period of quiet day - 26 March 2001 the TEC values at Antarctic stations (Casey, Davis, Syowa, Sanae) reached the values of 30, 60, 50, 40 TECU, respectively. On March 31 2001 absolute TEC values decreased sharply to 15, 25, 18, 20 TECU, respectively

4. Conclusion

The GPS observations of the IGS network were used to study the response of the ionosphere to two severe geomagnetic storms of March 2001 over European, North America, and Antarctic sectors. The following conclusions can be made:

Reference

Baran L.W., I.I. Shagimuratov, N.J. Tepenitsina, 1997, The use of GPS for Ionospheric Studies, Artificial Satellites, Vol.32, No 1, pp. 49-60.
Baran L.W., M. Rotkiewicz, P. Wielgosz, I. I. Shagimuratov; Ionospheric Effects During a Geomagnetic Storm, Vistas for geodesy in the New Millennium, International Association of Geodesy Symposia, Volume 125, Springer-Verlag Berlin Heidelberg New York 2002, pp. 291-296.
Krankowski, L. W. Baran, I. I. Shagimuratov; Influence of the Northern Ionosphere on Positioning Precision, Physics and Chemistry of the Earth 27 (2002), pp. 391-395, 2002 Elsevier Science.
Shagimuratov I.I., L.W. Baran, P. Wielgosz. G.A. Yakimova, The structure of Mid-and High-Latitude Ionosphere During September 1999 Storm Event Obtained from GPS Obserations, Annales Geophysicae (2002) 20: pp. 655-661 European Geophysical Society 2002.