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 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 similar to 4 Hz with 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 generate frequencies similar to(850-1450) Hz with transverse wavelengths similar to(2-20) km. The typical frequencies and transverse wavelengths associated with the resonant instability are (950-1250) Hz and (30-65) km. Both the Mode 2 nonresonant instability and the resonant instability can occur in the ring current region with L = (4-6). 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.