Whistler observations during nighttimes made at low latitude Indian ground stations Jammu (geomag. lat., 29°26'N; L = 1.17), Nainital (geomag. lat., 19°1'N; L = 1.16) and Varanasi (geomag. lat., 14°55'N; L = 1.11) are used to deduce electron temperatures and electric field in the vicinity of the magnetospheric equator. The accurate curve fitting and parameter estimation technique are used to compute nose frequency and equatorial electron densities from the dispersion measurements of short whistlers recorded at Jammu, Nainital and Varanasi. In this paper, our aim is to estimate the Magnetospheric electron temperatures and electric field from the dispersion analysis of short whistlers observed at low latitudes by using different methods. The results obtained are in good agreement with the results reported by other workers.
It is well known that lightning discharges are accompanied by the generation of electromagnetic waves in a wide frequency range [1,2]. Wave energy can penetrate into the magnetosphere and propagate almost along geomagnetic field lines to the opposite hemisphere where it is recorded by a radio receiver called whistler. The dynamic spectrum of the recorded signal is typically dispersed in the spectrogram. These signals sometimes proceeded by an associated signal with an undispersed dynamic spectrum and are generated during the same lightning discharges but propagate in the Earth-ionosphere waveguide [2,3]. When this signal is recorded and coincides approximately with the moment of lightning discharge, its time delay does not usually exceed 0.04 s [
Whistlers represent an inexpensive and effective method for obtaining various plasmaspheric parameters like electron density, electron temperature, electric field etc. in the magnetosphere, but the experimental results published up to now refer mainly to higher latitudes [5-7], and a systematic description of the main features of the plasmaspheric electron density based on large quantities of whistler data is still lacking at high latitudes, with the exception of work by Park et al. [
At middle and high latitudes, both satellite and groundbased whistler data were exploited fully to reveal new facts about the structure and dynamics of the ionosphere and magnetosphere. These achievements included the discovery of the plasmasphere, plasmapause, and bulge [
We consider the methods of “traditional” diagnostics of magnetospheric parameters, such as electron plasma density, the large scale electric field and possible temporal variations of the magnetic field at the magnetospheric equator, when both fn and tn are known. When one or both of these parameters are not known the dynamic spectra of whistlers and/or sferics need to be extrapolated. Method of this extrapolation is subsequently considered. Then we estimate the equatorial electron density, electron temperature, and electric field in the equatorial magnetosphere based on the analysis of the dynamic spectra of whistlers. Whistler studies in India, which have been in progress since 1963, have made significant contribution to the propagation of low latitude whistlers and understanding of the structure and dynamics of the low latitude ionosphere [16-18].
For the analysis of non-nose whistlers, a number of methods have been proposed [
Electric fields are closely related to and control most of observed gyophysical phenomena such as the bulk motion of the magnetospheric plasma, the current systems in the magnetosphere and the ionosphere, and to the acceleration of plasma particles in the Earth’s magnetosphere. The role of the electric field in controlling the bulk motion of the plasma has been recognized in all the theoretical studies of the various dynamic processes taking place in the Earth’s magnetosphere although adequate experimental techniques for the precise measurements of such fields in the ionosphere and magnetosphere were not available for quite some time. The observed cross-L motions of the whistler ducts are being used currently for obtaining the east-west component of the electric fields in the plasmasphere during substorm periods as well as quiet times [4,5].
The tidal forces in the Earth’s atmosphere cause motion of the plasma across the magnetic field lines and give rise to electromotive forces. The generation of electric field by the motion of conducting plasma across the magnetic field is analogous to dynamo action and the theory dealing with the electric field generation by this mechanism is known as dynamo theory. The electric field generation mechanism in the ionosphere has been developed by various workers [23,24]. Electric field measurements have been carried out in the equatorial Eregion of the ionosphere by many workers. These measurements reveal the existence of east-west electrostatic field raging from 1 to 2 mV/m. The whistler method of obtaining the east-west component of the electric field has the advantage of extended time coverage and remarkable property of being directly involved in the motion of magnetospheric tubes or “ducts” of ionization. Further, the ground-based whistler determinations of electric fields are comparatively easier and the equipment used can be monitored with relative ease on a routine basis. It is precisely for this reason that the ground-based whistler studies of electric fields are still continued at a number of stations spread all over the world.
In this paper we first present the whistler data used for the analysis recorded at Jammu, Nainital and Varanasi. This is followed by a presentation of an outline of the method developed by Tarcsai [
At low latitudes, the whistler occurrence rate is low and sporadic. But once it occurs, its occurrence rate becomes comparable to that of mid-latitudes (Hayakawa et al., 1988). Similar behavior has also been observed at our low latitude Indian stations. All the Indian stations are well equipped for measurements of VLF waves from natural sources. For the present study, the whistler data chosen corresponds to June 5, 1997 for Jammu, 25 March 1971 for Nainital and 19 February 1997 for Varanasi. On 5 June 1997 at Jammu station whistler activity started around 2140 h IST (Indian Standard Time) and lasted up to 2245 h IST. During this period about 100 whistlers have been recorded [