The nonlinear interactions between O+ ions and oxygen band EMIC waves

During geomagnetically active times, O+ ions from the ionosphere become an important ion component in the ring current, which changes the dispersion relations and significantly enlarges the growth rate of oxygen band electromagnetic ion cyclotron (EMIC) waves. Motivated by a large number of oxygen band EMIC wave observations in the inner magnetosphere, we study the nonlinear motions of ring current O+ ions caused by the cyclotron resonance with oxygen band EMIC waves. A dimensionless parameter R is used to characterize the competition between wave-induced and adiabatic motions. The numerical calculations based on gyroaveraged test particle equations are performed. For typical 20 keV O+ ions at L = 5, two kinds of nonlinear processes occur simultaneously when alpha(eq) > 59 degrees (R < 2.16): the phase trapping and phase bunching. The phase trapping tends to transport the O+ ions away from the loss cone and reduce the overall loss rate estimated from the quasi-linear theory. Instead, phase bunching tends to increase the overall loss rate. The phase-trapped O+ ions have chance to be accelerated drastically, reaching 750% of the initial energy. These O+ ions move forward and backward alternatively along the field lines when they are bouncing back to the equator, forming a periodic energy variation which has not been reported before. The results suggest that the oxygen band EMIC waves, which appear frequently during storms, should be considered in the ring current dynamics in terms of nonlinear acceleration and resonance scattering of ring current particles.