Generation of low-frequency electric field fluctuations in the magnetosphere

Abstract

Observations from many spacecrafts have established the presence of low-frequency electric field fluctuations (LEF) in the'regions of magnetospheric flow boundaries like magnetopause, plasma sheet boundary layer, polar cusp/cleft and the auroral region. It is shown that in the presence of cold background electrons, a low freqency electrostatic instability can be excited on the auroral field lines by the upward moving ion beams. For typical auroral plasma parameters the instability can generate LEF in the frequency range of 10' Hz to about 60 Hz. The low frequency waves propagate nearly transverse to the auroral field lines, and can amplitude -, 10 m V/m to as high.as 1 V/m. On the other band, ion beams observed in the plasma sheet boundary layer and in the polar cusp have sufficient velocity shears to drive ion cyclotron instabilities propagating nearly transverse to the magnetic field Bo. In the plasma sheet boundary layer, LEF generated by the velocity shear instabilities have frequencies in the range from 0.4 Hz to 4 Hz with wavelengths ti 30 km to 400 km. In the polar cusp, these velocity shear instabilities can excite braodband LEF with frequencies varing from 2 Hz to 35 Hz.