We have studied the spatial location relative to the plasmapause (PP) of the most intense electromagnetic ion cyclotron (EMIC) waves observed on Van Allen Probes A and B during their first full precession in local time. Most of these waves occurred over an L range of from −1 to +2 RE relative to the PP. Very few events occurred only within 0.1 RE of the PP, and events with a width in L of < 0.2 RE occurred both inside and outside the PP. Wave occurrence was always associated with high densities of ring current ions; plasma density gradients or enhancements were associated with some events but were not dominant factors in determining the sites of wave generation. Storm main and recovery phase events in the dusk sector were often inside the PP, and dayside events during quiet times and compressions of the magnetosphere were more evenly distributed both inside and outside the PP. Superposed epoch analyses of the dependence of wave onset on solar wind dynamic pressure (Psw), the SME (SuperMAG auroral electrojet) index, and the SYM-H index showed that substorm injections and solar wind compressions were temporally closely associated with EMIC wave onset but to an extent that varied with frequency band, magnetic local time, and storm phase, and location relative to the PP. The fact that increases in SME and Psw were less strongly correlated with events at the PP than with other events might suggest that the occurrence of those events was affected by the density gradient.
Bibliographical noteFunding Information:
Work at Augsburg College was supported by NSF grants PLR-1341493 and ATM-1202267, and work performed by M.J.E. at NASA/GSFC was supported by the Van Allen Probes mission. Work at Dartmouth College by R.E.D. was supported by NASA grant NNX13AD65G. Craig Kletzing is the P.I. of the EMFISIS experiment on the Van Allen Probes. Van Allen Probes research at the University of Minnesota, University of New Hampshire, Los Alamos National Laboratory, and the Aerospace Corporation was supported by NASA prime contract NAS5-01072 to The Johns Hopkins University Applied Physics Laboratory. We thank David Sibeck, Mark Adrian, Brian Anderson, Viacheslav Pilipenko, and both referees for helpful comments. We gratefully acknowledge use of NASA/GSFC's Space Physics Data Facility's OMNIWeb, SSCweb, and CDAWeb data. Van Allen Probes CDF data files are available at http://rbsp.space.umn.edu/rbspdata/ (EFW) and http://emfisis.physics.uiowa.edu/dta/index (EMFISIS). Plots of HOPE and MagEIS data are available from the CDAWeb site at NASA Goddard Space Flight Laboratory (http://cdaweb.gsfc.nasa.gov/).
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- EMIC waves