P. Subrahmanyam

Diurnal, seasonal and latitudinal variation of electron density in the topside F-region of the Indian zone ionosphere at solar minimum and comparison with the IRI

Abstract The diurnal, seasonal and latitudinal variation of electron density (Ne) measured by the Indian SROSS C2 satellite from December 1994 to January 1997 at low and equatorial latitudes along 75°E meridian are investigated. The satellite covered the latitude belt of 31°S to 34°N and longitude range of 40°E to 100°E at an average altitude of ∼500 km . Electron density is minimum before sunrise and reaches the diurnal maximum in the afternoon hours. A secondary enhancement is observed in the sunset hours of June solstice. The well-known equatorial ionization anomaly is found to be asymmetric at the altitude tracked by the satellite in this period of low solar activity (mean sunspot number=13.9). The location of the crest of the anomaly varies with season. There are seasonal and latitudinal variations of Ne. Further, electron density is found to be positively correlated with the strength of the equatorial electrojet. Comparison of measured density with that predicted by the International Reference Ionosphere (IRI) reveals that the IRI overestimates Ne at about all local times and in all seasons. The IRI also fails to produce the secondary enhancement of electron density.

Electron and ion temperature and electron density at ±10° magnetic latitudes from SROSS C2 measurements over India and comparison with the IRI

Abstract Diurnal and seasonal variation of electron and ion temperature and electron density at ∼500 km altitude measured by the Indian SROSS C2 satellite during 1995–96 are investigated. Two representative magnetic latitudes ±10° are considered. Electron temperature (Te) varies between 800°K and 900°K during nighttime, rises sharply at sunrise to 3000°K and then falls to a daytime average of 1500°K. A secondary enhancement of Te within 1500–1800 LT was observed in summer and equinox. Ion temperature (Ti) was found to be less than Te at both the locations and in all seasons. Electron density (Ne) shows a minimum before sunrise and a maximum in the afternoon. Seasonal as well as spatial differences in the diurnal behaviour were observed in case of all three parameters. Comparison of density and temperature measured by SROSS C2 with the IRI reveals that IRI underestimates Te during daytime and overestimates Ne at about all local times. Predicted and observed Ti also differ.

Evening enhancements in F-region electron temperature at subtropical latitudes during June solstice in the Indian SROSS C2 RPA data

Abstract In-situ measurments of the topside F-region ionospheric electron and ion temperatures are very few over the low and equatorial latitudes during the last two solar cycles, particularly in the Indian sector. The SROSS C2 satellite has provided some valuable data on the thermal structure of the topside ionosphere over the Indian region. This article reports a typical evening enhancement in the topside F-region electron temperatures around 18:00 IST observed in the subtropical latitudes of 15–20°N. These enhancements that are seen during the low sunspot activity periods show a latitudinal difference with an early and sharp peak at higher latitudes (23°N). The observed features are explained on the basis of equatorial plasma dynamics associated with the Appleton ionization anomaly.