LOW-FREQUENCY ELECTRIC-FIELD FLUCTUATIONS EXCITED BY RING CURRENT PROTONS

Energetic protons having ring type distributions are shown to generate low-frequency electrostatic waves, propagating nearly transverse to t he geomagnetic field lines, in the ring current region by exciting Mod e 1 and Mode 2 nonresonant instabilities and a resonant instability. M ode 1 nonresonant instability has frequencies around similar to 4 Hz w ith transverse wavelengths of similar to(8-80) km, and it is likely to occur in the region L = (7-8). Mode 2 nonresonant instability can gen erate frequencies similar to(850-1450) Hz with transverse wavelengths similar to(2-20) km. The typical frequencies and transverse wavelength s associated with the resonant instability are (950-1250) Hz and (30-6 5) km. Both the Mode 2 nonresonant instability and the resonant instab ility can occur in the ring current region with L = (4-6). The low-fre quency modes driven by energetic protons could attain maximum saturati on electric field amplitude varying from 0.8 mV/m to 70 mV/m. It is su ggested that the turbulence produced by the low-frequency modes may ca use pitch angle scattering of ring current protons in the region outsi de the plasmapause resulting in the ring current decay.