[1] A case of the drastic effects of an eastward prompt penetration and a westward overshielding electric field successively affecting the daytime equatorial ionosphere during the space weather event that occurred on 24 November 2001 is presented. Under the influence of the strong eastward prompt penetration electric field starting from 11:25 Indian standard time (IST), the equatorial electrojet (EEJ) strength reached the maximum value of 225 nT at 12:42 IST, almost 7 times greater than the monthly quiet time mean at the same time. This peak EEJ value exceeds the maximum observed values during the month of November for the entire solar cycle by more than 100 nT, irrespective of quiet or disturbed conditions. Further, owing to an ensuing overshielding event that occurred during the main phase of the storm rather than the end of the main phase, this unusually large EEJ showed an equally strong polarity reversal along with a weakening of the sporadic E layer over the equator. The EEJ strength was reduced from +225 to À120 nT at $13:45 IST, resulting in a strong counter electrojet condition. The latitudinal variation of the F region electron density data from the CHAMP satellite reveal an ill-developed equatorial ionization anomaly at 17:00 IST (11:24 UT) over the Indian sector due to this significant weakening of the zonal electric field. These observations showcase the significant degree to which the low-latitude ionosphere can be affected by the interplanetary electric field.
This paper investigates the response of the equatorial, and near equatorial, ionosphere to geomagnetic disturbances during the period November [8][9][10] 2004. Ionosonde data from Trivandrum (8.5• N 77• E and dip 0.5• N) and SHAR (13.5 • N, 80.2 • E, dip ∼5.5• N), magnetic field data from Tirunelveli (8.7• N, 76.9• E, dip latitude 0.5 • S) and Alibag (18.64 • N, 72.87• E), and GUVI O/N 2 data in the Indian longitude sector, are used for the study. The behavior of interplanetary parameters is also examined in conjunction with the ionospheric data. On 8 November, the EIA around noontime is not fully inhibited even though the electrojet strength an indicates inhibition of EIA due to a disturbance dynamo electric field effect. It is the enhanced O/N 2 over TRV and SHAR, with a larger increase over SHAR, which results in a larger (than expected) value of the EIA proxy parameter. On 9 November, the comparable values of f o F 2 at TRV and SHAR around noon time is due to the combined effect of a weakened anomaly in the presence disturbance dynamo electric field effects leading to the EIA crest being near SHAR, and increased O/N 2 values at TRV and SHAR with a larger increase at TRV. On 10 November, the very strong values of the EIA proxy-SHAR parameter is attributed to the combined effects of prompt penetration electric field related modulations of EIA, and significant O/N 2 changes at the equatorial, and near equatorial, latitude. Thus, the study reveals the important role of storm-induced O/N 2 changes, along with prompt penetration electric fields and disturbance dynamo electric fields in modulating the ionization distribution in the equatorial ionization anomaly (EIA) region during this period.
Perturbations in the solar atmosphere are the major origins of geomagnetic storms. Reconfiguration of magnetic fields in the solar atmosphere causes uplift of materials from the solar chromosphere into the corona. These relatively cool, but dense materials are suspended against gravity at greater heights by magnetic tension in the dips of the field lines, appearing by absorption against the hotter and brighter background (Carlyle, 2016). These materials could be elongated in structures, to the order of thousands of kilometers in length to form filaments, which could, in turn erupt from the solar coronal surface as Coronal Mass Ejection (CME). CMEs, particularly the Earth-directed ones are the sources of space weather events (e.g., geomagnetic storms) on the Earth. High Speed Streams (HSSs) from the Sun's coronal holes are the sources of the Corotating Interaction Regions (CIRs) which also known to cause geomagnetic storms (Burlaga & Lepping, 1977;Gosling, 1993). The occurrences of geomagnetic storms do influence the electrodynamics of
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