Abstract
With data from Van Allen Probes, we investigate electromagnetic ion cyclotron (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 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.
Department
Physics
Publication Date
4-2015
Journal Title
Journal of Geophysical Research: Space Physics
Publisher
American Geophysical Union Publications
Digital Object Identifier (DOI)
10.1002/2014JA020717
Document Type
Article
Recommended Citation
Min, K., et al. (2015), Study of EMIC wave excitation using direction measurements, J. Geophys. Res. Space Physics, 120, 2702–2719, doi:10.1002/2014JA020717.