• A K Gwal

      Articles written in Pramana – Journal of Physics

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

      R P Patel R P Singh Ashok K Singh A K Gwal D Hamar

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      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

      Harsha Jalori Sunil K Singh A K Gwal

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      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

      Sushil Kumar S K Singh A K Gwal

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      The role of low density upflowing field-aligned electron beams (FEBs) on the growth rate of the electron cyclotron waves at the frequencies $\omega_{r}$ < $\Omega_{­e}$, propagating downward in the direction of the Earth's magnetic field, has been analysed in the auroral region at $\omega_{e}/\Omega_{e}$ < 1 where $\omega_{e}$ is the plasma frequency and $\Omega_{­e}$ 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 $\omega_{e}/\Omega_{e}$ indicating that the beams are more effective at higher altitudes.

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