Abstract
Abstract. Results pertaining to the response of the equatorial and low latitude ionosphere to a major geomagnetic storm that occurred on 15 May 2005 are presented. These results are also the first from the Indian zone in terms of (i) GPS derived total electron content (TEC) variations following the storm (ii) Local low latitude electrodynamics response to penetration of high latitude convection electric field (iii) effect of storm induced traveling atmospheric disturbances (TAD's) on GPS-TEC in equatorial ionization anomaly (EIA) zone. Data set comprising of ionospheric TEC obtained from GPS measurements, ionograms from an EIA zone station, New Delhi (Geog. Lat. 28.42° N, Geog. Long. 77.21° E), ground based magnetometers in equatorial and low latitude stations and solar wind data obtained from Advanced Composition Explorer (ACE) has been used in the present study. GPS receivers located at Udaipur (Geog. Lat. 24.73° N, Geog. Long. 73.73° E) and Hyderabad (Geog. Lat. 17.33° N, Geog. Long. 78.47° E) have been used for wider spatial coverage in the Indian zone. Storm induced features in vertical TEC (VTEC) have been obtained comparing them with the mean VTEC of quiet days. Variations in solar wind parameters, as obtained from ACE and in the SYM-H index, indicate that the storm commenced on 15 May 2005 at 02:39 UT. The main phase of the storm commenced at 06:00 UT on 15 May with a sudden southward turning of the Z-component of interplanetary magnetic field (IMF-Bz) and subsequent decrease in SYM-H index. The dawn-to-dusk convection electric field of high latitude origin penetrated to low and equatorial latitudes simultaneously as corroborated by the magnetometer data from the Indian zone. Subsequent northward turning of the IMF-Bz, and the penetration of the dusk-to-dawn electric field over the dip equator is also discernible. Response of the low latitude ionosphere to this storm may be characterized in terms of (i) enhanced background level of VTEC as compared to the mean VTEC, (ii) peaks in VTEC and foF2 within two hours of prompt penetration of electric field and (iii) wave-like modulations in VTEC and sudden enhancement in hmF2 within 4–5 h in to the storm. These features have been explained in terms of the modified fountain effect, local low latitude electrodynamic response to penetration electric field and the TIDs, respectively. The study reveals a strong positive ionospheric storm in the Indian zone on 15 May 2005. Consequences of such major ionospheric storms on the systems that use satellite based navigation solutions in low latitude, are also discussed.
Highlights
In recent years, the Global Positioning System (GPS) has emerged to be a very important and powerful tool in the investigation of the ionosphere
The main phase of this storm commenced at around 06:00 Universal Time (UT) on 15 May 2005 (Fig. 1) and the high latitude convection electric field penetrated to the equatorial and low latitudes in the Indian sector, is obvious from the Fig. 2. These observations provide us with sufficient inputs to discuss the high latitude penetration electric field associated with sudden southward and northward turning of IMF-Bz on 15 May 2005
The results show that the rapid variations of about 15–20 TECU occurred in the vertical TEC (VTEC) on 15 May, compared to quiet time levels, whereas, the peak values of VTEC were about 95–100 TECU on storm day
Summary
The Global Positioning System (GPS) has emerged to be a very important and powerful tool in the investigation of the ionosphere. Sudden transformations in the primary east-west electric field in low latitudes occur at the time of penetration of high latitude electric field This disturbance electric field maps to equatorial F-region, which in turn causes sudden modifications in the F-region vertical E×B drift over the equator as observed and reported by many authors using different techniques like ionosonde, radar and satellite measurements (Sastri, 1992a, b; Fejer and Scherliess, 1995; Fejer et al, 2007; Kelley et al, 1979; Abdu, 1997; Kil, 2007). Large variations in the difference, ( HTIR− HABG), as observed after 06:00 UT, represent the complexities associated with the storm-induced variations, first, in the east-west electric field over dip equator, in EEJ current strength. It is worth mentioning here that the GPS satellites are slow east-west moving satellites and the VTEC observations should always be looked at the location of IPPs in space and time
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