Electrodynamics of the equatorial F-region ionosphere during pre-sunrise period

The electrodynamics of the pre-sunrise equatorial F-region is investigated using HF Doppler radar and digital ionosonde. The observations are limited to those days for which the radar probing frequency is below the ƒoF2 value. The ionosphere observation using HF Doppler radar exhibit interesting features during pre-sunrise period similar to the post sunset pre-reversal enhancement. The most striking feature observed during pre-sunrise period is the sudden downward excursion in the vertical drift around local sunrise followed by an upward turning. Pre-sunrise observations of vertical plasma drift and the sunrise downward excursion followed by an upward turning after the ground sunrise related to the zonal electric field at the equatorial F-region are the most significant results not reported earlier

Lidar Observations of aerosol layers just below the tropopause level during IFP-INDOEX

A lidar system has been used at Gadanki (13.5º, 79.2ºE) to study the characteristics of aerosol layer (cloud) occurring just below the tropical tropopause. The preliminary results of the lidar observations indicate that the cloud occurs ~ 2 km below the tropopause. The top and bottom edges of the cloud have propensity for ice crystal presence with liquid droplets/ vapours in-between. The clouds show temporal fluctuations (in their backscattering ratio) with temporal scales of the order of 30–90 min

Momentum flux associated with gravity waves in the low-latitude troposphere

The vertical fluxes of horizontal momentum at tropospheric heights are calculated for four days, 25–28 August 1999. The mean zonal wind during these days show the presence of strong westward wind at the upper troposphere. Both the symmetric beam radar method and the power spectral method of evaluation of vertical flux of zonal and meridional momentum shows nearly the same result for quiet conditions. The temporal evolution of the momentum flux is estimated for a day with strong zonal shear and convection. These results indicate that on 28 August 1999, the strong downward vertical wind in the lower altitude range is associated with upward vertical flux of zonal momentum, and strong upward vertical wind is associated with downward vertical flux. Similarly, the strong shear in zonal wind is associated with the increase in negative values in vertical flux in the upper troposphere. Analysis of the role of wave periods in the transport of momentum flux indicates that the vertical momentum flux magnitude is not evenly distributed in all wave periods, but instead it peaks at certain wave periods in the range 10 to 100 min.Key words. Meteorology and atmospheric dynamics (convective process; tropical meteorology; precipitation

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

273-276Azimuthal 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 Ap 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 Ap index in terms of Carrington longitude is found to be similar to that of the solar wind velocity variation

Solar wind velocity distribution on the heliospheric current sheet during Carrington rotations 1787-1795

The solar wind velocity distribution in the heliosphere is best represented using a v-map, where velocity contours are plotted in heliographic latitude-longitude coordinates. It has already been established that low-speed regions of the solar wind on the source surface correspond to the maximum bright regions of the K-corona and the neutral line of the coronal magnetic field. In this analysis, v-maps on the source surface for Carrington rotations (CRs) 1787–1795, during 1987, have been prepared using the interplanetary scintillation measurements at Research Institute of Atmospherics (RIA), Nagoya Univ., Japan. These v-maps were then used to study the time evolution of the low-speed (\leq450 km s–1) belt of the solar wind and to deduce the distribution of solar wind velocity on the heliospheric current sheet. The low-speed belt of the solar wind on the source surface was found to change from one CR to the next, implying a time evolution. Instead of a slow and systematic evolution, the pattern of distribution of solar wind changed dramatically at one particular solar rotation (CR 1792) and the distributions for the succeeding rotations were similar to this pattern. The low-speed region, in most cases, was found to be close to the solar equator and almost parallel to it. However, during some solar rotations, they were found to be organised in certain longitudes, leaving regions with longitudinal width greater than 30° free of low-speed solar wind, i.e. these regions were occupied by solar wind with velocities greater than 450 km s–1. It is also noted from this study that the low-speed belt, in general, followed the neutral line of the coronal magnetic field, except in certain cases. The solar wind velocity on the heliospheric current sheet (HCS) varied in the range 300–585 km s–1 during the period of study, and the pattern of velocity distribution varied from rotation to rotation

Features of long term evolution of solar wind and IMF and their signature on geomagnetic activity

134-137The geomagnetic activity is mainly controlled by variations in the solar wind and interplanetary magnetic field (IMF) features. The solar wind data available during 1965-2004 has been utilized to study large scale features in interplanetary medium and their signature on geomagnetic activity. The solar wind plasma, IMF and geomagnetic activity are found to evolve with the phase of solar cycle. The evolution and role of solar wind streams and IMF field strength (B) in shaping long term features of planetary magnetic activity index (Ap) index have been studied. The high speed streams are caused mainly by the increase in coronal hole activity around the declining phase of solar cycle and by the presence of large number of coronal transients around solar maximum. Association between coronal holes and geomagnetic activity is clear from the simultaneous presence of high negative correlation between solar wind velocity (Vsw) and density (n) and the enhancement in geomagnetic activity during declining phase of solar cycle. During this period, Vsw and Ap index also exhibit good correlation. Around solar maximum periods, good correlation is found between B and Ap index. These results suggest that Vsw has a major role in deciding the geomagnetic activity around declining phase and IMF magnitude has a major role in deciding geomagnetic activity around solar maximum

Some Aspects of the Response of Low Latitude Magnetic Field to Sector Boundary Passage during Three Solar Cycles

393-397The response of horizontal intensity to passage of solar magnetic sector boundary past the earth is computed Using extensive data of a low latitude station in the Indian region. The analysis is carried out separately for 3 solar cycles and for four classes of boundary passages. The results indicate that at low latitudes (i) the passages of -/+ boundary in vernal and +/- boundary in autumnal equinoctial seasons have no associated enhanced geomagnetic activity; (ii) the nature and magnitude of the field response is not dependent on the magnitude of the solar cycle; (Hi)' the "Hale" part of sector boundary has no associated change in the low latitude field response; and (iv) the solar-magnetic-cycle (~22 yr) has no systematic influence on the low latitude field response to boundary passage but some association may be seen during favourable conditions