Interactions between interplanetary (IP) shocks and the Earth's magnetosphere manifest many important space physics phenomena including low-energy ion flux enhancements and particle acceleration. In order to investigate the mechanisms driving shock-induced enhancement of low-energy ion flux, we have examined two IP shock events that occurred when the Van Allen Probes were located near the equator while ionospheric and ground observations were available around the spacecraft footprints. We have found that, associated with the shock arrival, electromagnetic fields intensified, and low-energy ion fluxes, including H+, He+, and O+, were enhanced dramatically in both the parallel and perpendicular directions. During the 2 October 2013 shock event, both parallel and perpendicular flux enhancements lasted more than 20 min with larger fluxes observed in the perpendicular direction. In contrast, for the 15 March 2013 shock event, the low-energy perpendicular ion fluxes increased only in the first 5 min during an impulse of electric field, while the parallel flux enhancement lasted more than 30 min. In addition, ionospheric outflows were observed after shock arrivals. From a simple particle motion calculation, we found that the rapid response of low-energy ions is due to drifts of plasmaspheric population by the enhanced electric field. However, the fast acceleration in the perpendicular direction cannot solely be explained by E × B drift but betatron acceleration also plays a role. Adiabatic acceleration may also explain the fast response of the enhanced parallel ion fluxes, while ion outflows may contribute to the enhanced parallel fluxes that last longer than the perpendicular fluxes.
Bibliographical noteFunding Information:
This work was supported by the NASA Living With a Star Jack Eddy Postdoctoral Fellowship Program, administered by the UCAR Visiting Scientist Programs, NASA grants NNX15AI62G, NNX13AI61G, and NNX14AI18G, NSF grants PLR-1341359, AGS-1405054, and 1564510, and AFOSR grant FA9550-15-1-0179. We acknowledge use of Van Allen Probes data, made publicly available through NASA prime contract number NAS5-01072, including the Level 3 HOPE flux data obtained from the RBSP-ECT website (www.rbsp-ect.lanl.gov/data_pub/rbspb/hope/level3/PA/), the Level 3 magnetic field data obtained from the RBSP EMFISIS website (emfisis.physics.uiowa.edu/Flight/RBSP-B/L3), and the Level 3 electric field data were obtained from the RBSP EFW website (rbsp.space.umn.edu/data/rbsp/rbspb/l3/). We thank the Space Physics Data Facility at the NASA Goddard Space Flight Center for providing the OMNI data (ftp://spdf.gsfc.nasa.gov/pub/data/omni/omni_cdaweb/) and the Applied Physics Laboratory at the Johns Hopkins University and the William B. Hanson Center for Space Sciences at the University of Texas at Dallas for DMSP data. Contact the authors to access the PFISR and DMSP data.
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- adiabatic accelerations
- enhancement of low-energy ion flux
- ionospheric ion outflows
- response to IP shocks