Source of O+ in the ring current: Van Allen Probes observations during the 1 June 2013 storm
Thursday, 18 December 2014
During magnetic storms, the concentration of O+ ions in the ring current can increase substantially. The mechanisms that energize these O+ ions so that they gain enough energy to escape from the ionosphere into the magnetosphere have long been debated. The highly sensitive HOPE (Helium, Oxygen, Proton, and Electron) instrument onboard the Van Allen Probe satellites provide an opportunity to investigate this problem. The two satellites are in nearly identical highly-elliptical, low-inclination (~10°) orbits with a perigee of 1.1 Earth radii (RE), an apogee of 5.8 RE, and a period of ~9 hours. We study a storm event whose SSC occurred near 15:00 UT on 31 May 2013 followed by a lengthy initial phase. The main phase began early on 1 June, reaching a Sym-H minimum of - 134 nT at 07:54 UT. We have identified a large energetic proton enhancement event which is marked by a significant increase in the fluxes of high-energy ions in the deep portion of the main phase. We concentrate on the relative dynamics of H+ and O+. From 03:00 UT to 06:50 UT, there were a few quasi-periodic enhancements of O+ fluxes in the 200 eV to 3.5 keV energy range. A Fourier analysis shows clear power around 30 min periods and excellent coherence among these channels. These enhancements further show clear dispersion with high-energy channels increasing first followed by lower energy channels. During the event, the satellites were near the apogee in the pre-midnight local time sector. The location of the source region can be inferred by energy dispersion based on the time-of-flight effect. One may show that if the particles in different energies were produced at the same time and location, time is linearly correlated with the reciprocal of square-root of the energy for the enhanced fluxes and the source distance can be derived by the slope of the lines of enhanced fluxes. During the event, we identified 8 such enhancements. The sources of these O+ ions appear to be from similar distance to the satellite. Other features during the event will be presented and discussed.