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SCAR Report No 21, January 2002

The Response of Polar Ionosphere to a magnetic storm
obtained from GPS observations

L.W.Baran , P.Wielgosz
Institute of Geodesy, WM University at Olsztyn, Poland

J.Cisak
Institute of Geodesy and Cartography, Warsaw, Poland

I.I.Shagimuratov
WD IZMIRAN, Kaliningrad, Russia

Abstract

GPS measurements at IGS network were used to study the ionospheric effects of September 12 to 16 1999 magnetic storm on Arctic and Antarctic regions. The GPS data from more than 60 stations of EPN (EUREF Permanent Network) were applied to create Total Electron Content (TEC) maps over Europe. The dense GPS network in Europe enabled to release TEC maps with high spatial and temporal resolution. 15 minutes averages of TEC values were taken to produce TEC maps. The storm consisted of the positive as well as the negative phase. The positive effect took place during the first day of the storm. A short daytime enhancement of TEC was observed at all latitudes. The maximum enhancement reached a factor of 1.3-1.5. On the second and third days the negative phase of the storm appeared. The decrease of TEC was registered regardless of time of a day and exceeded 70% relative to the quiet days. On September 15 and 16 once again an essential daytime enhancement of TEC was observed.

The complex nature of the ionospheric storm was related to the features of development of magnetic storm. We found out that during the storm the large and medium-scale irregularities occurred in the high-latitude ionosphere. The multi-stations technique was used to create TEC maps and it was successful in particular to study the midlatitude ionospheric trough. We also found out that the essential changes of TEC registered in the auroral and subauroral ionosphere were attributed to the effect of the trough. Data on dynamics of the trough in dependence on geomagnetic activity is presented. The horizontal gradients in the ionosphere during the disturbances were pronounced. These gradients may have an impact on ambiguity resolution while processing GPS data.

Introduction

Nowadays GPS measurements are commonly used to investigate the structure and dynamics of the ionosphere. Recently, several authors have applied the GPS network data to study an occurrence of TEC during a storm (Ho et al., 1998; Musman et al., 1998; Jakowski et al., 1999; Baran et al., 2001). Those studies basically concern analyses of winter events during solar minimum. It is known, that the ionospheric effects of a storm essentially depend on the season (Fuller-Rowel and Codrescu, 1996). Here we present data on the ionospheric response to a September 1999 storm. The paper stresses especially on the response of TEC in polar and subauroral regions of the ionosphere. The data of Arctic and Antarctic GPS stations were used for analysis of ionospheric TEC behavior during the storm. The detail picture of development of the storm in TEC was obtained for European sector by producing TEC maps. The dense network of GPS stations in Europe enabled to conduct TEC measurements with high temporal and spatial resolution and find out the dramatically changes of TEC during the storm. The reason of these phenomena is discussed.

Geomagnetic Conditions

September 1999 included a few magnetic active periods. We shall discuss the intensive disturbances of 12 to16 September. The variations of Kp and Dst indices are presented in Fig.1. The sudden commencement of storm occurred September 12 at 04 UT. The maximum sum of Kp reached 39 on September 13. The storm consisted of series of intensive geomagnetic bays.

 Fig.1. Kp and Dst variations during September 11 - 16 1999

Data Source

The GPS measurements collected at the stations of IGS and EPN networks were used in this study. The EUREF Permanent Network observations were used to produce TEC maps. Data from more than 60 European GPS stations were processed (Fig.2).

  Fig.2. Map of stations used in this paper

Estimation Technique

When estimating TEC from GPS observations the ionosphere was approximated by a spherical shell at fixed height of 350 km above the Earth surface. The simple geomagnetic factor was used to convert the slant TEC into a vertical one. The high precision phase measurements were used when processing GPS observations. The phase ambiguities were removed by fitting phase measurements to code data collected along individual satellite pass. After pre-processing the phase measurements contained an instrumental bias only. The absolute TEC and the instrumental bias were estimated using the single site algorithm (Baran et al., 1997). The biases were determined for every individual stations using GPS measurements of all satellite passes over site in 24-hour period. The diurnal variations of TEC over site and biases for all satellites were simultaneously estimated. After the technique had been run on all stations the instrumental biases were removed in all satellite passes. Using this procedure the absolute line of sight TEC for all satellite-receiver paths were calculated.

To solve the spatial and temporal variations of TEC in GPS data and to produce TEC maps the measurements were fitted to a spherical harmonic expansion in Φ and Θ . The coordinate system used is geographic latitude (Φ ) and longitude (Θ ). Only TEC observations with elevation angles above 20° were used in the fits. 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 (Manucci et al., 1998). The large number of GPS stations in Europe provide a good coverage for GPS data and enable to get high accuracy TEC maps with an error at the level of 0.5 - 2TECU. Fig. 3 shows the shell coverage for data arcs of 15 min. length. The adequate shell coverage yields a reasonable surface harmonic fit and provides TEC spatial resolution of 100-200km with time resolution of 15 min.

Fig.3. Spatial coverage by data provided by GPS network at arc of 15 min

In order to clearly identify the ionospheric changes during the storm the percentage change of storm-time TEC relative to TEC maps for quiet conditions was computed. To obtain the quiet time data we averaged 5 magnetically quiet day of TEC measurements. The maps over Europe in this case were produced every 15 minutes. To discuss the storm behavior in detail, various temporal and spatial TEC profiles were obtained from TEC maps. All TEC data was presented in TEC units (1TECU = 1016 el/m2).

Diurnal Variations of TEC

General idea of storm development can be seen in day-by-day diurnal variations of TEC. Fig. 4 shows the example of TEC variations over two stations of northern and southern hemispheres during the storm. The TEC behavior at KIRU, VAAS and SYOG stations is very similar. On the first day of the storm the positive effect took place. The short time increase of TEC was observed near local noon. On the second and third days the negative phase of the storm was developed. The main feature of the storm under consideration is a significant positive effect during daytime on 15 and 16 September. This series of TEC enhancement is probably related to the feature of development of magnetic storm, which included the sequence of magnetic bays (Fig.1).

It is interesting that at VESL station the storm effect is weakly pronounced. It can be seen that at VESL the absolute level of TEC is lower than at higher latitude SYOG station. It appears that VESL was located near the ionospheric trough. In the trough the TEC is minimal and it is increasing in direction of both the equator and the pole. Unfortunately the longitudinal arrangement of Antarctic stations does not represent the latitudinal distribution of TEC in Antarctic area and does not enable to determine the latitudinal location of trough. Difference in behavior of TEC at SYOG and VESL can partially be attributed to the longitudinal effect of storm development.

Fig.4. Diurnal variation of TEC during 11-16 September 1999 (solid curves) with median values (dots). The geomagnetic coordinates are given in the figure.

Spatial Variations of TEC during Storm

The fine spatial structure of the ionosphere is well traced in GPS phase observable on individual satellite passes. Fig. 5 demonstrates the variations of TEC along single satellites tracks on quiet day of September 11 (Σ Kp=16) and on disturbed day of September 12 (Σ Kp=31). Because the orbital revolution period Of GPS satellite is 12h of sidereal time the track repeats on successive days except the satellites arrive 4 minutes earlier each day. In Fig. 5 one can see that during storm the auroral and subauroral ionosphere was essentially modified. The large and medium scale structures with deep changes of TEC developed after noon during the storm in the Northern and Southern hemispheres.

The behavior of TEC at stations spaced out on 300-500 km is also different. It is evident that spatial correlation of TEC during the storm deteriorated. The large scale ionospheric structure we attributed to the occurrence of the main ionospheric trough, which during storm was lowering towards the equator. The storm-time horizontal gradient in ionosphere also increased. The severe ionospheric conditions, which occurred during the storm, can prevent ambiguity resolution and influence on accuracy of GPS positioning (Wanninger, 1993).

 

Fig.5. Time variations of TEC along individual satellite passes on quiet day 11 (solid curve) and disturbed day 12(dots) September 1999. The location of the satellite traces on ionospheric height are also presented (crosses)

Storm-time TEC Distribution over Europe

The temporal evaluation of TEC distribution over Europe on the first disturbed day i.e. on September 12 is presented in Fig. 6 via the series of TEC maps. When producing the maps we used 15-min averaged TEC data. This provides analysis of ionospheric response to the storm in detail. The presence of the trough is the significant feature of latitudinal variations. In Fig. 6 one can see that the trough occurred at the east and after that moved to the west. As compared to quiet geomagnetic conditions the daytime ionization enhanced at high latitudes and after that moved to low latitudes. The enhancement of TEC amounted to 30-50% at high latitudes; towards the equator the percentage deviation have increased to 60-80%. The positive effect in TEC lasted till 15-16 UT and then the negative phase of the storm started.

Fig.6. TEC maps over Europe in geographic coordinates for post noon sector of 12 September Fig.7. The 15 min latitude profiles of percentage TEC changes relative to the quiet time for period of 11:45-13:15 UT, 12 September 1999. The zero level is shown for each curve

In Fig. 4 one can see the marked surge in diurnal variations of TEC near the noon on September 12. The analysis of diurnal variations of TEC at different latitudes showed that the surge moved towards the equator. The surge at the lower latitudes appeared about 2-3 hours later. TEC maps with 15min interval enabled to obtain the picture of TEC perturbation related with this surge in detail. Fig. 7 demonstrates the temporal evaluation of latitudinal profiles of TEC deviation. Here one can see as the large-scale wave-like perturbation moved towards the equator. It is interesting that the amplitude of the perturbations increased with time. The perturbance is associated with a large-scale travelling disturbance (TID) that have latitudinal scale of about 1500-2000km. Time delay of wave surge corresponds to a propagation velocity of the perturbation, i.e. to about 200ms-1. The velocity is lower than the TID velocity of winter storm of January 10, 1997 (Jakowski et al., 1999).

 
Fig.8. Latitudinal profiles of percentage TEC changes relative to the quiet time for 13 September (left panel) and 14 September (right) at different hours. The zero level is shown for each curve.

Fig.9. Diurnal profiles of percentage TEC changes relative to the quiet time for 15 September (left panel) and 16 September (right) at different latitudes (40° , 50° , 60° , 70° N). The zero level is shown for each curve

During September 13 and 14 the negative phase of the storm in TEC distribution over Europe took place. The latitudinal profiles of the percentage TEC deviation (from averaged quiet values) on September 13 and 14 at longitude of 20° E are presented in Fig.8. The daytime depressions exceeded 50% during September 13 and 30% during September 14. There are interesting strong variations of percentage deviation of TEC on latitude in evening/night sector. The effects we attribute to the difference in location of the trough for disturbed and quiet geomagnetic conditions. For September 13 the negative effects were more pronounced at latitudes over 30° -55° N. At the same time on September 14 the depression of TEC was observed at all latitudes under consideration.

It is worth to note the longitudinal dependence in development of the storm at midlatitudes during September 13. At longitudes over 20° E the positive effect while at longitudes under 20° E the negative effect were detected. Such behavior of TEC gives an evidence of the regional feature in development of the storm (Cander and Mihajlovic, 1998).

The feature of the storm is the occurrence of the daytime positive effect on a recovery stage of the storm on September 15 and 16 (Fig. 9). The attention shall be paid to time shift of percentage deviation maximum when going from high to lower latitudes. The maximum at the lower latitudes appeared about 2-3 hours later. At high latitudes the duration of the positive effect was about 3-4 hours. After 12 UT at latitude of 70° N the negative effect took place on September 15 as well as on 16. At all latitudes the positive effect was observed only in daytime. During night at high latitudes the negative effect took place on September 15 and 16.

Storm-Time Dynamics of the Ionospheric Trough

The temporal variation of TEC for individual satellite passes (Fig.5) and TEC maps (Fig.6) demonstrate the trough-like structures in behavior of TEC. The occurrence of the trough in latitudinal profiles of TEC is presented in Fig. 10 for quiet and disturbed days. For quiet geomagnetic conditions the trough-like structure was observed after 19 UT. The ratio of TEC at polar wall to bottom of the trough was less than 2, the equatorward wall is weakly pronounced. For disturbed day (September 12) the trough started after 15 UT (16:30 LT). The depth of the trough increased. Both walls of the trough were particularly pronounced. For September 13 equatorward wall was well pronounced and the polar one was weak. The ratio of TEC at equatorward wall to bottom amounted to the factor of 3-4. The value of TEC at the bottom of the trough made up only 3 TECU. The equatorward displacement of the trough is well detected during the disturbance. Location of the trough was also shifted to the equator. As the whole, the trough underwent substantial changes during the disturbance.

 

Fig.10. The dynamics of latitude profiles at post-noon sector for quiet day (left panel) and disturbed days September 12 and 13t(central and right panel)

Summary

The behavior of TEC during the equinox storm is very complex and includes many interacting ionospheric effects. Storm-time response of TEC depends on latitude, longitude, local time and it is specified by features of development of magnetic storm such as:

Acknowledgements

The authors are grateful to IGS community and WDC-C1 for the GPS data. Research was conducted within the project No 8T12E04520 supported by Polish National Committee for Scientific Research.

References

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