The role of ULF waves interacting with oxygen ions at the outer ring current during storm times

Abstract

The modulations of the outer ring current O+ ion fluxes by ULF Pc5 waves are investigated by multisatellite observations during storm times. The O+ ions have energies up to tens of keV. We concentrate on the process in terms of drift-bounce resonance of O+ ions with ULF standing waves to understand whether the ring current O+ ions could be accelerated/decelerated by ULF waves. Two case studies are performed, in which the Cluster satellites travel the outer ring current region in the morning sector with radial distances of about 5.5 RE. Distinct O+ ion flux oscillations are observed associated with fundamental mode ULF standing waves. On 25 October 2002, both satellites SC1 and SC4 observe strong poloidal and toroidal standing waves at approximately the same region one by one with a time lag of 45 min. The O+ ion flux oscillations at around 20 keV are dominantly coherent with the poloidal standing wave at 3.4 mHz with cross phases of near 90 with respect to the magnetic field waves. The O + phase space density spectra at 10 to 25 keV, measured by both satellites, deviate significantly from the typical power law distribution. We suggest that the O+ ions at 10 to 25 keV are accelerated due to drift-bounce resonance with the poloidal standing wave. On 4 November 2002, satellite SC1 observes considerable poloidal and toroidal standing waves. The O+ ion flux oscillation at around 7 keV is well correlated with both of the two wave modes at 3.7 mHz with cross phases of about 90 with respect to the magnetic field waves. The O+ spectra at 4 to 8 keV deviates remarkably from the background power law distribution. When satellite SC4 closely encounters the same region 40 min later, the wave activities at 3.7 mHz are found to be rather weak and the O+ spectra is close to the background power law distribution. We suggest that the spectra variation of SC1 results from the deceleration of O+ ion at 4 to 8 keV via drift-bounce resonances during the strong wave activities. The observations made in this study reveal the effective role of ULF standing waves in accelerating/decelerating the ring current O+ ions. © Copyright 2011 by the American Geophysical Union.