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[1] Response of low-latitude ionosphere to the geomagnetic storm of 24 August 2005 has been studied using total electron content (TEC) data obtained from the Global Positioning System (GPS) receivers. These studies were carried out using the receivers that were located (1) near the northern crest (∼15°N mag. Lat.) of the equatorial ionization anomaly around 56°E, 74°E, and 102°E longitude and (2) from the northern crest of the ionization anomaly down to the magnetic equator in the longitude belt 75°E ± 3°E. These studies have been substantiated with the ground-based magnetometer data at Tirunelveli and Alibag, an equatorial and off equatorial station, respectively. The ground-based ionosonde data at New Delhi, a low-latitude station, have also been used to substantiate the TEC observations. The storm day TEC shows two well-defined humps at all stations wherein enhancements of the order of 80%-100% have been observed. While the first of the enhancements has been attributed to the prompt penetration electric field associated with an interplanetary electric field (IEF Ey) of about 35 mV/m, the other one has been attributed to the second episode of the prompt penetration electric field (IEF Ey ∼ 20 mV/m) and abnormal equatorial plasma fountain in late evening hours, respectively. During the unsteady ring current conditions when the IMF Bz was still southward, penetration of a westward electric field has been inferred. Two peaks in foF 2 have been observed whose time of occurrence coincides with those of the humps in the low-latitude TEC. Results from stations having nearly the same magnetic latitude show that the ionospheric response, in terms of GPS TEC, to the prompt penetration electric fields is longitudinally independent. Formation of the first hump in TEC is progressively delayed in time from low to near-equatorial latitudes for stations in different magnetic latitudes along nearly the same longitude. However, its time of appearance at Diego Garcia, a station magnetically conjugate to Udaipur, is the same as that at Udaipur. The results also reveal the poleward expansion of the ionization anomaly due to the storm. Disturbance dynamoelectric fields have been inferred to be responsible for the suppressed plasma fountain, resulting in suppressed TEC values and equatorward contraction of the ionization anomaly on 25 August, compared to a reference quiet day.

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Abstract.Response of low latitude ionosphere to the geomagnetic storm of 15 May 2005 has been studied using total electron content (TEC) data, obtained from three GPS stations namely, Yibal, Udaipur and Kunming situated near the northern crest of equatorial ionization anomaly at different longitudes. Solar wind parameters, north-south component of the interplanetary magnetic field (IMF B z ) and AE index data have been used to infer the strength of the geomagnetic storm. A large value of eastward interplanetary electric field at 06:15 UT, during the time of maximum southward IMF B z has been used to infer the transmission of an eastward prompt penetration electric field (PPEF) which resulted in a peak in TEC at 07:45 UT due to the local uplift of plasma in the low latitudes near the anomaly crest over a wide range of longitudes. Wave-like modulations superposed over the second enhancement in TEC between 09:15 UT to 10:30 UT have been observed at all the three stations. The second enhancement in TEC along with the modulations of up to 5 TECU have been attributed to the combined effect of super plasma fountain and traveling atmospheric disturbances (TAD). Observed large enhancements in TEC are a cause of concern for satellite based navigation and ground positioning. Increased [O/N 2 ] ratio between 09:15 UT to 10:15 UT when modulations in TEC have been also observed, confirms the presence of TADs over a wide range of longitudes.

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[1] Evidence of simultaneous penetration of storm time electric fields in the day and night sectors of the globe during the geomagnetic storm of 23 May 2002 and the response of low-latitude ionosphere to such electric fields is presented. Using the ground-based magnetometer data, simultaneous penetrations of under-shielding and overshielding electric fields, and those arising owing to the unsteady ring currents, have been inferred in the day and night sectors. The ionospheric response in the day and night sectors near the northern crest of the equatorial ionization anomaly has been studied using total electron content (TEC) data. Multiple episodes of prompt penetration electric fields arising owing to the under-shielding and unsteady ring current conditions resulted in enhancement of the TEC in the day sector. These daytime TEC enhancements were reinforced by the equatorward neutral winds that also produced modulations in TEC. The presence of storm time equatorward neutral winds has been inferred from the increased thermospheric [O/N 2 ] ratio. We also show, probably for the first time, that the same penetration fields in the night sector are responsible for the reductions in TEC on 23 May. Reduced TEC at 120°E longitude on 24 May, the day following the storm, has been attributed to the disturbance dynamo electric fields as well as to the changes in thermospheric chemical composition signified by reduced [O/N 2 ] ratio.Citation: Galav, P., S. Sharma, and R. Pandey (2011), Study of simultaneous penetration of electric fields and variation of total electron content in the day and night sectors during the geomagnetic storm

The ionospheric response to the geomagnetic storm of 15 May 2005 has been studied over midlatitude stations in the dayside and nightside, simultaneously. In the day side the ionospheric response has been studied using the ground-based GPS and ionosonde measurements from the stations POL2 and Alma-Ata, respectively. The dayside total electron content (TEC) and f o F 2 variations are characterized by two well-separated enhancements. Of which the first enhancement in both the parameters is attributed to the episode of prompt penetration electric field caused by the sudden southward turning of interplanetary magnetic field (IMF) B z around 0600 UT. The second enhancement which was also superposed by wave like modulations has been attributed to the storm-induced winds. The maximum peak-to-peak amplitude of modulation in TEC is found to be 5 TECU (total electron content unit, 1 TECU = 10 16 el m À2). The enhanced plasma density observed during the daytime at midlatitudes is found to be locally produced and not transported from the equatorial ionization anomaly region because the time of enhanced plasma density at midlatitude is earlier than that observed at low latitudes. During the storm main phase, the nightside GPS observations from the midlatitude station ALGO (Algonquin Park, Canada) show moderate to large TEC fluctuations and short duration depletions that occur in a narrow latitude zone. These fluctuations and depletions in TEC have been attributed to the combined effect of storm-induced equatorward movement of the midlatitude ionospheric trough due to the expansion of auroral oval and the storm time-enhanced density. The maximum amplitude of the TEC depletions is found to be of the order of 20 TECU. Rate of TEC Index is also found to be high with a maximum value of 2.