Low-frequency electric field fluctuations excited by ring current protons

Abstract

Energetic protons having ring type distributions are shown to generate low-frequency electrostatic waves, propagating nearly transverse to the geomagnetic field lines, in the ring current region by exciting Mode 1 and Mode 2 nonresonant instabilities and a resonant instability. Mode 1 nonresonant instability has frequencies around -4 Hz with transverse wavelengths of +%SO) km, and it is likely to occur in the region L = (7-8). Mode 2 nonresonant instability can generate frequencies +85&1450)Hz with transverse wavelengths ~(2-20) km. The typical frequencies and transverse wavelengths associated with the resonant instability are (950-1250) Hz and (3@65) km. Both the Mode 2 nonresonant instability and the resonant instability can occur in the ring current region with L = (46). The low-frequency modes driven by energetic protons could attain maximum saturation electric field amplitude varying from 0.8 mV/m to 70 mV/m. It is suggested that the turbulence produced by the low-frequency modes may cause pitch angle scattering of ring current protons in the region outside the plasmapause resulting in the ring current decay.

Microinstabilities excited by energetic ring current protons

“…We model the ring current region following Singh et al (1996) by a plasma system consisting of electrons, cold protons and energetic protons (hot) embedded in the geomagnetic field which we treat as uniform, i.e. B 0 = B 0ẑ .…”

“…Here erf (u/α ⊥h ) is the error function with argument u/α ⊥h . A dispersion relation for the ring current instability is obtained by making the following assumptions (Singh et al 1996). We consider waves that propagate nearly transverse to the ambient magnetic field, i.e.…”

“…Under these conditions, the electrons are strongly affected by the magnetic field while the protons are largely unaffected by it. Then for wave perturbations varying as exp[−i(ωt − k • r)], where ω = ω r + iγ is the wave frequency, the dispersion relation for the ring current instability, as obtained by solving the linearized Vlasov equation along with the full set of Maxwell's equations, turns out to be (Bhatia and Lakhina 1980;Singh et al 1996)…”

“…The relative drift between electrons and ions due to the ring current provides a source of free energy to excite linear plasma instabilities. In addition, the velocity distributions of the charged particles in the ring current region are found to be anisotropic (Singh et al 1996), which also serves as a source for microinstabilities.…”

“…In this paper we examine microinstabilities excited by ring current protons having a given anisotropic ring-type velocity distribution, by extending the parameter study of Singh et al (1996). Both resonant and non-resonant instabilities are considered.…”