MODULATION OF ELECTROMAGNETIC ION-CYCLOTRON INSTABILITY DUE TO INTERACTION WITH RING CURRENT O+ DURING MAGNETIC STORMS

We demonstrate that the observed enhancement in the fractional composi tion eta(O+) of ring current O+ ions during magnetic storms can have a strong controlling effect on the excitation of electromagnetic ion cy clotron (EMIC) waves. For modest storms, when eta(O+) less than or equ al to 30%, strong EMIC excitation can occur in the frequency band abov e the oxygen gryofrequency, Omega(O+), due to cyclotron resonance with anisotropic ring current H+ ions. The path-integrated gain obtained f rom ray tracing is sufficient to drive wave amplitudes into the nonlin ear regime in a region near the equatorial duskside plasmapause. The e xcited wave energy is found to be absorbed efficiently at high latitud es via cyclotron resonant interactions with energetic O+ leading to pe rpendicular heating of the O+ population. Intense waves generated near the equator should therefore not be detectable at low altitudes once the density of O+ has been enhanced during the main phase of a storm. Cyclotron absorption will also enhance the anisotropy of energetic res onant O+ ions. We show that such enhanced anisotropy can excite cyclot ron instabilities at frequencies below Omega(O+) which are able to pro pagate to low altitudes and be detectable either on the ground or on l ow altitude satellites during the storm main phase. For the most inten se storms, when the concentration of O+ can attain values eta(O+) grea ter than or equal to 60%, cyclotron absorption by resonant O+ can beco me so severe as to totally suppress wave excitation in the band above Omega(O+). The most rapid loss process for the ring current (i.e., tha t due to wave particle scattering) could therefore be suppressed durin g the main phase of such storms. This raises the interesting question of whether the main phase Dst depression might be modulated by the rel ative concentration of energetic O+ through the process of resonant in teraction with EMIC waves.