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.

Abstract.A GSV 4004A GPS receiver has been operational near the crest of the equatorial anomaly at Udaipur, India for some time now. The receiver provides the line-of-sight total electron content (TEC), the phase and amplitude scintillation index, σ φ and S 4 , respectively. This paper presents the first results on the nighttime TEC depletions associated with the equatorial spread F in the Indian zone. The TEC depletions are found to be very well correlated with the increased S 4 index. A new feature of low-latitude TEC is also reported, concerning the observation of isolated and localized TEC enhancements in the nighttime low-latitude ionosphere. The TEC enhancements are not correlated with the S 4 index. The TEC enhancements have also been observed along with the TEC depletions. The TEC enhancements have been interpreted as the manifestation of the plasma density enhancements reported by Le et al. (2003).

We have performed a detailed analysis of the Czernik 3 (Cz3) open cluster by using deep near-infrared photometry taken with TIRCAM2 on 3.6m Devasthal optical telescope along with the recently available high quality proper motion data from the Gaia DR2 and deep photometric data from Pan-STARRS1. The cluster has a highly elongated morphology with fractal distribution of stars. The core and cluster radii of the cluster are estimated as 0.5 pc and 1.2 pc, respectively. We have identified 45 stars as cluster members using the Gaia proper motion data. The distance and age of the cluster are found to be 3.5 ± 0.9 kpc and $0.9^{+0.3}_{-0.1}$ Gyr, respectively. The slope of the mass function `Γ′ in the cluster region, in the mass range ∼0.95 <M/M⊙<2.2, is found to be −1.01 ± 0.43. The cluster shows the signatures of mass-segregation and is dynamically relaxed (dynamical age=10 Myr). This along with its small size, big tidal radius, low density/large separation of stars, and elongated and distorted morphology, indicate that the Cz3 is a loosely bound disintegrating cluster under the influence of external tidal interactions.

[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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