Showing papers by "Satyavir Singh published in 2019"

Kinetic Alfvén waves generated by ion beam and velocity shear in the Earth's magnetosphere

Abstract: Generation of Kinetic Alfven Waves (KAWs) in a generalized three component plasma model consisting of the background cold ions, hot electrons, and hot ion beams, where all the three species have non-uniform streaming and velocity shear, is discussed. First, the role played by the ion beam solely in exciting KAWs is analyzed. Next, how this behavior gets modified when the velocity shear is present along with the streaming ion beam is discussed. The effects of other parameters such as temperature, number density, and propagation angle on the growth of KAWs are explored. It is found that when shear is positive and ions are streaming along the ambient magnetic field, KAWs are stabilized. On the other hand, with positive shear and an anti-parallel ion beam or vice-versa, KAWs with a larger growth rate are excited as compared to the case of waves excited by the ion beam alone. Also, for the first time, we have shown the combined effect of the ion beam and velocity shear on the generation of KAWs. The theoretical model can generate ultra-low frequency waves with frequencies up to ≈60 mHz for the plasma parameters relevant to auroral/polar cusp field lines.

Resonant instabilities of kinetic Alfvén waves in the Earth's magnetosphere with superthermal electrons

Abstract: A theoretical plasma model for the generation of kinetic Alfven waves (KAWs), having background Maxwellian ions, κ-electrons, and drifting Maxwellian beam ions, is discussed. The ion beam streams along the ambient magnetic field, whereas velocity shear is perpendicular to it. The role played by nonthermal electrons in the excitation of resonant KAWs with the velocity shear in the ion beam as the free energy source is examined. In the presence of κ-electrons, the effect of plasma parameters such as propagation angle, ion beam temperature, number density, and ion plasma βi on the growth of the KAWs is analyzed. It is found that nonthermal electrons restrict the excitation of KAWs by reducing the growth rate of the waves. It is inferred that a high velocity shear and ion beam density are required to excite KAWs in the presence of nonthermal electrons. The model is capable of producing waves with frequencies up to ≈18 mHz in the auroral region of Earth's magnetosphere.

Higher harmonic instability of electrostatic ion cyclotron waves

Abstract: Electrostatic ion cyclotron instability pertaining to the higher harmonics of proton and helium cyclotron modes is investigated in three-component magnetised plasma consisting of beam electrons, protons and doubly charged helium ions. The effect of different plasma parameters, namely, angle of propagation, number density and temperature of helium ions and electron beam speed, has been studied on the growth of proton and helium cyclotron harmonics. It is found that an increase in angle of propagation leads to the excitation of fewer harmonics of proton cyclotron waves with decreased growth rates and higher number of helium harmonics with decreased growth rates. Also, largely odd helium harmonics are excited, except for one particular case where the second harmonic also becomes unstable. The number density and temperature of ions have significant effect on the helium cyclotron instability compared to the proton cyclotron instability. Further, as the speed of electron beam is increased, the peak growth rate increases. Our results are relevant to laboratory and space plasmas where field-aligned currents exist.

A theoretical model for the generation of Kinetic Alfvén Waves (KAWs) in the Earth’s magnetosphere by ion beam and velocity shear

Abstract: A generation mechanism of the Kinetic Alfven Waves (KAWs) by the ion beam and velocity shear will be discussed. For this, a three component plasma model consisting of cold background ions, hot electrons and hot ion beams is considered. The model is very general in the sense that all the three species have drifting Maxwellian distribution, non-uniform streaming and velocity shear and can be applied to magnetospheric regions where velocity shear is present. The effect of ion beam alone and the combined effect of the ion beam as well as the velocity shear in exciting the KAWs will be discussed. It is found that the ion beam alone can excite these KAWs. However, in the presence of ion beam along the ambient magnetic field and negative velocity shear or antiparallel ion beam and positive shear, the wave growth is much larger as compared to ion beam case alone. Also, the anti-parallel ion beam and positive shear can excite the KAWs with significantly higher growth rate as compared to the negative shear and parallel ion beam. The effect of plasma parameters such as species temperature, number density, angle of propagation etc. on the growth of the waves will be discussed. The present model is applied to auroral region of Earth's magnetosphere and it can explain several characteristic properties of the observed ultra low-frequency waves. The mechanism presented here can excite the KAWs upto the frequency of 30 mHz, which can explain the ultra-low-frequency waves observed in the auroral/polar cusp region. The model can be applied to any region of Magnetosphere, where, velocity shear is found.