NOPR Collection:

Structure of solar wind velocity along the HCS and related geomagnetic field variations

Sun, 01 Aug 1993 00:00:00 GMT

Title: Structure of solar wind velocity along the HCS and related geomagnetic field variations Authors: Bala, B; Nayar, S R Prabhakaran Abstract: Azimuthal variation of solar wind velocity in the range 300-450 km s^-1 has been found to exist along the current sheet during 1972-1977. The possibility of using the A_p index (when the earth goes through an IMF sector boundary crossing) to study the azimuthal structure of solar wind has been analysed. The longitudinal variation of the A_p index in terms of Carrington longitude is found to be similar to that of the solar wind velocity variation. Page(s): 273-276

Multistation analysis of VHF radio wave scintillations at low latitudes

Sun, 01 Aug 1993 00:00:00 GMT

Title: Multistation analysis of VHF radio wave scintillations at low latitudes Authors: Kumar, Sushil; Singh, A K; Chauhan, Pawan; Gwal, A K; Singh, Birbal; Singh, R P Abstract: Simultaneous observations.of VHF-ionospheric scintillations at Bhopal, Varanasi and Agra, during the period (January 1991-December 1991) have been reported. Nocturnal variation .of.occurrence of scintillations is found to be higher at Bhopal as compared to that at Varanasi and Agra. Under the geomagnetic disturbances the scintiIlations are suppressed in pre-midnight period at all the three stations. Severity .of suppression is found to be more at Bhopal. The storms with the D_st value below - 100 nT, reaching in local nighttime or in very early morning hours are found to be most effective in giving rise to the scintillations in post-midnight period. The results have been interpreted in terms of (i) F-region irregularities associated with spread-F and (ii) coupling of high latitude and magnetospheric current systems with equatorial electric field. Page(s): 267-272

Study of ionospheric electron content (IEC) depletions and their association with VHF scintillations

Sun, 01 Aug 1993 00:00:00 GMT

Title: Study of ionospheric electron content (IEC) depletions and their association with VHF scintillations Authors: Rao, P V S Rama; Rao, B V Ramana; Prasad, D S V V D Abstract: Significant depletions in IEC data observed in the Indian sector from the Faraday rotation of VHF signals of the ETS-II and SIRIO satellites for high (1978-81) and low (1983-84) solar activity periods at a low latitude station, Waltair (17.7°N, 83.3°E), are studied. Their diurnal, seasonal and solar cycle variations in the occurrence characteristics are presented. The diurnal variations in the occurrence of depletions show a maximum around pre-midnight hours of 2100 hrs LT. With the available data, excluding the months of satellite eclipses, the month-to-month variation in the occurrence of depletions shows maximum in February during low solar activity period and in August during high solar activity period. The amplitudes of depletions vary from 1 to 9 IEC units (1 IEC unit = 10¹⁶ el./m²) and the duration of most depletions vary from 5 to 15 min during the high and low solar activity periods. Further, the amplitudes of depletions vary linearly with their durations indicating that bubbles with long durations have large depletions. All these depletion events are invariably accompanied by amplitude scintillations. Page(s): 259-266

Results from the MWR network of IMAP

Sun, 01 Aug 1993 00:00:00 GMT

Title: Results from the MWR network of IMAP Authors: Moorthy, K Krishna; Nair, Prabha R; Prasad, B S N; Muralikrishnan, N; Gayathri, H B; Murthy, B Narasimha; Niranjan, K; Babu, V Ramesh; Satyanarayana, G V; Agashe, V V; Aher, G R; Singh, Risal; Srivastava, B N Abstract: During the Indian Middle Atmosphere Programme (IMAP) a network of ground based passive multi-wavelenghs solar radiometers (MWR) has been established covering different geographic locations in the country. The temporal, spatial and spectral characteristics of atmospheric aerosols have been studied using the spectral solar extinction measurements made by the above network at eight wavelengths lying in the visible and near IR range. In this paper are presented the temporal variations of aerosol optical depths at the different locations as revealed by the preliminary data. An attempt has been made to explain the observed features on the basis of natural and anthropogenic effects which are of synoptic as well as of meso-scale nature. Page(s): 243-258