Initial Measurements of O-ion and He-ion Decay Rates Observed from the Van Allen Probes RBSPICE Instrument Andrew Gerrard, Louis Lanzerotti et al. Center for Solar-Terrestrial Research, New Jersey Institute of Technology JGR 2014

Introduction RBSPICE: Radiation Belt-Storm Probes Ion Composition Experiment Target: examine the compositional changes of the Earth's ring current during geomagnetically disturbed times. Class: ToF Detector (composition) + Solid State Detector (energy) Systematic studies of inner magnetospheric H-ion, He-ion, and O-ion ring current abundances and their variations during quiet and moderately active times are relatively scarce. AMPTE/CCE (around 1993) CRRES/MICS (around 2001) Difficult to find absolute flux measurements. reports of He-ion/H-ion ratios and O-ion/H-ion ratios are likewise scarce.

Introduction Physical nature of loss mechanisms of ring current ions. Coulomb interactions (i.e., drag and scattering) Charge exchange Drift/convective loss Wave-particle interactions Each mechanism being present or even dominant at particular particle energies, particular particle masses, or particular spatial locations. Between keV, the dominate loss mechanism present for ring current ions is likely charge exchange.

Introduction RBSPICE energy ranges along with typical integrated flux values. L ~ 5.5 vertical bars: common energies keV The fluxes reported herein are integrated over all pitch angles and over the total energy ranges quoted above. He-ions are largely composed of He+. O-ions are expected to be all O+.

Observation Panels Dst Index H+ Ions He+ Ions*100 O+ Ions*100 Data binning:30min*0.1L RBSPICE instrument was shut o below L3 to reduce anomalous currents and instrument alarming.

Observation Orbital plots in the GSE X-Y plane.

Observation Enhancement of the flux across all ionic species during active periods, as measured by drops in the Dst. Direct coupling of the solar wind without induced terrestrial outflow.

Observation He/H Ratio O/H Ratio large increases occur in the abundance of He-ion relative to H-ion and O-ion relative to H- ion in the equatorial magnetosphere at the time of all geomagnetic disturbances as indicated by the Dst index.

Observation H-ions are still the dominate ion species for the events reported herein. The persistence of the increases in He-ion abundance depends strongly on the L value. The persistence is quite limited. L~3: 1 Day L~6: 10 Days The persistence of the increases in O- ion abundance is not associated with L value. (< 1 Day) Other Loss processes.

Observation 2013 DoY Black: L=3~4 Green: L=4~5 Red: L=5~6 He+ Exponential fits result: (1/e time, 20 events) L=3~4: 0.8 Days L=4~5: 1.1 Days L=5~6: 1.7 Days Uncertainties: ~0.2 Days Max increase: 5%~10% For O+: 1.5 ±0.1 Hours, L value independent.

Observation Same as before but use common energy range data. O+ Exponential fits result: (1/e time, 20 events) L=3~4: 0.3 Days L=4~5: 0.5 Days L=5~6: 0.8 Days High energy O+ lost rapidly!

Discussion Loss rate is inferred based on the persistence time of ions after an injection event. It is possible that there are some continued source of heavier ions (particularly at high L) even during the recovery phase? L-shell dependence of decay rate is due to the fallout of neutral hydrogen density with altitude. (charge exchange loss processes is dominant) The measured He-ion lifetimes reported herein seem to support this approach and correspond to previously published lifetime calculations. The rapid O-ion loss rate (especially at high energy) is not expected based on our current understanding of O-ion charge exchange. This is a first-time observation result. Could be due to diffusive or convective outflow processes or perhaps field line curvature scattering.