Study of EMIC wave excitation using direct ion measurements

TitleStudy of EMIC wave excitation using direct ion measurements
Publication TypeJournal Article
Year of Publication2015
AuthorsMin, K, Liu, K, Bonnell, JW, Breneman, AW, Denton, RE, Funsten, HO, Jahn, öerg-M, Kletzing, CA, Kurth, WS, Larsen, BA, Reeves, GD, Spence, HE, Wygant, JR
JournalJournal of Geophysical Research: Space Physics
Date Published03/2015
KeywordsEMIC wave excitation; observation, linear theory and hybrid simulation; Van Allen Probes
AbstractWith data from Van Allen Probes, we investigate EMIC wave excitation using simultaneously observed ion distributions. Strong He-band waves occurred while the spacecraft was moving through an enhanced density region. We extract from Helium, Oxygen, Proton, and Electron (HOPE) Mass Spectrometer measurement the velocity distributions of warm heavy ions as well as anisotropic energetic protons that drive wave growth through the ion cyclotron instability. Fitting the measured ion fluxes to multiple sinm-type distribution functions, we find that the observed ions make up about 15% of the total ions, but about 85% of them are still missing. By making legitimate estimates of the unseen cold (below ~2 eV) ion composition from cutoff frequencies suggested by the observed wave spectrum, a series of linear instability analyses and hybrid simulations are carried out. The simulated waves generally vary as predicted by linear theory. They are more sensitive to the cold O+ concentration than the cold He+ concentration. Increasing the cold O+ concentration weakens the He-band waves but enhances the O-band waves. Finally, the exact cold ion composition is suggested to be in a range when the simulated wave spectrum best matches the observed one.
URLhttp://doi.wiley.com/10.1002/2014JA020717
DOI10.1002/2014JA020717
Short TitleJ. Geophys. Res. Space Physics


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