Nighttime enhancements in TEC near the crest of northern equatorial ionization anomaly during low solar activity period

Enhancement in nighttime total electron content (TEC) near the crest of equatorial ionization anomaly at Bhopal (Geog. 23.2°N, 77.4°E, and MLAT 14.2°N) has been studied for the solar minimum period March 2005- November 2006. TEC data recorded by GPS Ionospheric Scintillation and TEC Monitor (GISTM) GSV4004A receiver is used for the study and results are presented in the paper. More than 10% increase in TEC with respect to background content is considered for analysis. Out of total 138 enhancements, 65 observed during pre midnight hours and 73 during post midnight hours. It is observed that nighttime enhancement in TEC at Bhopal occurs in all seasons; it is more frequent during summer, less during equinox and least during winter months. The enhancement in nighttime TEC can be observed both in geomagnetic disturbed time and in quiet time. We found that mean peak amplitude for pre-midnight TEC enhancement are more in equinox and less in winter, while in post-midnight TEC enhancement it is highest in summer and less in winter. Post-midnight enhancements have smaller peak amplitude as compared to pre-midnight. Also the most probable values for pre- and post-midnights are 4.4 TECU and 2.17 TECU respectively. The percentage occurrence of nighttime TEC enhancement does not show any dependence on solar activity whereas the peak amplitude depends on solar activity. The percentage occurrence of nighttime TEC enhancement decreases as the magnetic activity increases whereas there is no such dependency found with peak amplitude. It is also observed that majority of nighttime TEC enhancements are occurred without scintillation. The localised nighttime TEC enhancements near the crest of equatorial ionization anomaly region have been observed most of the nights during the period of study.     

Electrostatic Waves in the Warm Magnetoplasma at the Cyclotron Harmonic Frequencies

Mode conversion and collisionless absorption of electromagnetic wave at the cyclotron harmonic frequencies in an inhomogeneous non-Maxwellian magnetoplasma have been studied. Under suitable energy transfer condition the converted electrostatic wave (plasma wave) either grows or damps. The expressions for the growth/damping rates of this wave have been derived and studied at the cyclotron harmonic frequencies. The effect of the temperature anisotropy on the growth/damping rate of the electrostatic wave at the second cyclotron harmonic frequency has been shown. Growth of such electrostatic waves at ionospheric heights may explain the observed upper hybrid resonance (UHR) echoes and noise bands at the second cyclotron harmonic frequency.     

Role of magnetic shear on the electrostatic current driven ion-cyclotron instability in the presence of parallel electric field

Recent observation and theoretical investigations have led to the significance of electrostatic ion cyclotron (EIC) waves in the electrodynamics of acceleration process. The instability is one of the fundamental of a current carrying magnetized plasma. The EIC instability has the lowest threshold current among the current driven instabilities. On the basis of local analysis where inhomogeneities like the magnetic shear and the finite width current channel, have been ignored which is prevalent in the magnetospheric environment. On the basis of non-local analysis interesting modification has been incorporated by the inclusion of magnetic shear. In this paper we provide an analytical approach for the non-local treatment of current driven electrostatic waves in presence of parallel electric field. The growth rate is significantly influenced by the field aligned electron drift. The presence of electric field enhances the growth of EIC waves while magnetic shear stabilizes the system.     

Propagation of tweek atmospherics in the earth-ionosphere wave guide

Summary The propagation characteristics of tweek atmospherics observed at the low-latitude ground station of Bhopal have been studied near the mode cut-off frequencies. The Earth-ionosphere wave guide plays an important role in the propagation of these tweek signals. It is shown that the height of the Earth-ionosphere wave guide varies from 83 km to 89 km. The sharp increase in the propagation time is found near the cut-off frequencies. The tweeks of third mode attenuate faster than the second- and first-order mode tweeks. This explains the low occurrence rate of higher-mode tweeks.     

Observation of very low frequency emissions at Indian Antarctic station,Maitri

Recently, we have succeeded in recording VLF emissions at the Indian Antarctic station, Maitri (geom. lat. 62°S, geom. long. 57.23°E,L = 45) using a T-type antenna, pre/main amplifiers and digital audio tape recorder. VLF hiss in the frequency ranges 11–13 kHz and 13–14.5 kHz and some riser-type emissions in the frequency range 3–5 kHz and magnetospheric lines at about 6.2, 8.0 and 9.2 kHz are reported for the first time. The generation and propagation mechanism of these emissions are discussed briefly.     

Upconversion of whistler waves by gyrating ion beams in a plasma

A gyrating ion beam, with a ring-shaped distribution in velocity, supports negative energy beam modes near the harmonics of beam gyro-frequency. An investigation of the non-linear interaction of high-frequency whistler waves with the negative energy beam cyclotron mode is made. A non-linear dispersion relation is derived for the coupled modes. It is shown that a gyrating ion-beam frequency upconverts the whistler waves separated by harmonics of beam gyro-frequency. The expression for the growth rate of whistler mode waves has been derived. In Case 1, a high-amplitude whistler wave decays into two lower frequency waves, called a low-frequency mode and a side band of frequency lower than that of pump wave. In Case 2 a high-amplitude whistler wave decays into two lower frequency daughter waves, called the low-frequency mode and whistler waves. Generation mechanism of these waves has application in space and laboratory plasmas.     

Effect of upflowing field-aligned electron beams on the electron cyclotron waves in the auroral magnetosphere

The role of low density upflowing field-aligned electron beams (FEBs) on the growth rate of the electron cyclotron waves at the frequencies ω _r < Ω_e, propagating downward in the direction of the Earth’s magnetic field, has been analysed in the auroral region at ω _e/Ω_e < 1 where ω _e is the plasma frequency and Ω_e is the gyrofrequency. The FEBs with low to high energy (E _b) but with low temperature (T _‖b) have no effect on these waves. The FEBs with E _b < 1 keV and T _‖b (> 1.5 keV) have been found to have significant effect on the growth rate. Analysis has revealed that it is mainly the T _‖b which inhibits the growth rate (magnitude) and the range of frequency (bandwidth) of the instability mainly in the higher frequency spectrum. The inhibition in the growth rate and bandwidth increases with increase in T _‖b. The FEBs with less E _b (giving drift velocity) reduce growth rate more than the beams with larger E _b. The inhibition of growth rate increases with the increase in the ratio ω _e/Ω_e indicating that the beams are more effective at higher altitudes.