Estimation of Magnetospheric Plasma Parameters from Whistlers Observed at Low Latitudes

Mohd Altaf, Mohammad Mushtaq Ahmad, Jan Mohammad Banday

Department of Civil engineering, National Institute of Technology, Srinagar, India.
Department of Physics, National Institute of Technology, Srinagar, India.

DOI: 10.4236/ijaa.2013.34047                

Abstract

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.

Keywords

Magnetosphere; Electron Temperature; Electric Field; Electron Density

1. Introduction

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 [4] as the velocity of wave propagation in the Earth-ionosphere waveguide is close to velocity of light, this signal is called an atmospheric or sferic. The whistler signal intensity is normally greatest at few kHz within the ELF/ VLF band; 1 - 20 kHz (Carpenter, 1962).

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. [8]. Recently Tracsai et al. [9] have processed whistlers recorded at Tihany, Hungary (L = 1.9) between December 1970 and May 1975 in order to study the distribution of equatorial electron density and total electron content in flux tubes having Lvalue, lying in the range L = 1.4 - 3.2. At low latitudes, the exploration of whistlers for electron density determination has only been carried out by Lalmani et al. [10]. In this paper the equatorial electron density, equatorial electron temperature and east-west component of electric field at low latitudes using the whistler data observed at our ground stations Jammu, Nanital and Varanasi have been estimated.

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 [11], identification of the mechanism of ionosphere-protonosphere coupling [5,12] and the measurement of the magnetospheric electric field [13]. Although the application of whistlers to diagnostic of electron temperature of high latitudes has been discussed since the early 1960s [6,14,15], this problem still seems to be at an early stage of its development at low latitudes. At low latitudes, whistler data have been used for determining electron temperatures, electric field etc. for understanding the magnetospheric phenomena.

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 f_n and t_n 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 [19]. The nose frequency of the whistler data used in estimating electron density, electron temperature and electric field has been computed by means of accurate curve fitting method developed by Tarcsai [19] based on least squares estimation of the two parameters, zero frequency dispersion D_o, equatorial electron gyrofrequency f_He in Bernard’s approximation. This matched filtering technique developed for the analysis of whistler waves increases the accuracy of analysis and speed of data processing [17,18,20]. The technique employs dispersive digital filters whose frequency-time response is matched to the frequency-time response of the signal to be analyzed. Due to high resolution and time domain, many fine structure components with amplitudes differing in frequency and time are seen in dynamic spectra [20]. The accuracy and effectiveness of the technique have been discussed at length by analyzing a large number of whistlers both on the ground stations (from the low to the high latitudes) and onboard rockets/satellites [18,20-22].

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 [19] from which electron density, electron temperatures, and electric field in the vicinity of magnetospheric equator are evaluated. Finally the results are discussed and compared with those reported by other workers.

2. Data Selection and Method of Analysis

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 [25]. On 25 March 1971 at Nainital station whistler activity commenced around 0020 IST and lasted up to 0520 IST. Altogether more than hundred whistlers were recorded and the occurrence rate showed a feeble but discernible periodicity [26]. On 19 February 1997 at Varanasi station whistler activity started around 2300 IST and lasted for about one hour up to 0030 IST. During this period several whistlers were recorded [27].

Figure 1(a) presents dynamic spectrum of short whistlers (marked A, B, C, D, E, F and G, selected for the analysis) in the frequency band 3 - 4.5 KHz recorded at Jammu at 2212 IST on June 5, 1997. In the frequency band 1.7 - 3 KHz large number of frequency components are missing and signals resemble more like emissions rather than whistlers. Further, VLF waves in both the frequency bands do not appear simultaneously, rather they appear alternately. Figure 1(b) shows dynamic spectrums of short whistlers (marked 1, 2, 3 and 4, selected for the analysis) and VLF emissions recorded at Jammu at 2147 IST. Whistlers are banded and diffused in the frequency range 2.7 - 3.7 KHz and are repeated in time. The time interval between the events is not con-

Conflicts of Interest

The authors declare no conflicts of interest.

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