Abstract. The diurnal, seasonal and latitudinal variations of electron temperature T e , measured by the SROSS C2 satellite at equatorial and the low-latitudes during the low solar activity period of 1995-1997 are investigated. The average height of the satellite was ∼500 km and it covered the latitude belt of -31 • to 34 • and the longitude range of 40 • -100 • . T e varies between 700-800 K during nighttime (20:00-04:00 LT), rises sharply during sunrise (04:00-06:00 LT) to reach a level of ∼3500 K within a couple of hours and then falls between 07:00-10:00 LT to a daytime average value of ∼1600 K. A secondary maximum is observed around 16:00-18:00 LT in summer. Latitudinal gradients in T e have been observed during the morning enhancement and daytime hours. Comparison of measured and International Reference Ionosphere (IRI) predicted electron temperature reveals that the IRI predicts nighttime T e well within ∼ 100 K of observation, but at other local times, the predicted T e is less than that measured in all seasons.

The present work describes the low-latitude ionospheric variability during an unusually prolonged (~33 h) geomagnetically disturbed condition that prevailed during 15-16 July 2012. The low-latitude electron density in summer hemisphere, investigated using ground-and satellite-based observations, responded to this by generating strong negative ionospheric storm on 16 July. The maximum electron density on 16 July over Indian low latitudes was reduced by more than 50% compared to that on a geomagnetically quiet day (14 July 2012). In contrast to the extreme reduction in total electron content (TEC) in the Northern Hemisphere, TEC from a winter hemispheric station revealed substantial (~23 total electron content unit, 1 TECU = 10 16 el m À2) enhancements on the same day. This contrasting hemispherical response in TEC is suggested to be due to the combined effects of strong interhemispheric and solar-driven day-night winds. Further, very weak equatorial electrojet (EEJ) strength on 16 July indicated that the westward electric field perturbations in the low-latitude ionosphere were possibly due to the disturbance dynamo effect associated with meridional circulation from polar to equatorial latitudes. Interestingly, despite reduction in the integrated EEJ strength on 15 July, the low-latitude electron density showed substantial enhancement, highlighting the significant effect of the positive ionospheric storm on the low-latitude ionosphere. The roles of electrodynamical/neutral-dynamical and compositional disturbances are discussed in view of these observations to understand low-latitude ionospheric response when geomagnetic disturbance persists for longer duration.

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