Effect of Spatial Density Variation and O+ Concentration on the Growth and Evolution of Electromagnetic Ion Cyclotron Waves

Abstract:

We simulate electromagnetic ion cyclotron (EMIC) waves, which were observed during June 9, 2001 by Geostationary Operational Environmental Satellite (GOES) spacecraft. First we use a ring current simulation with a plasmasphere model to model the particle populations that give rise to the instability. Then, using two different models for the cold ion composition, we do a full scale hybrid code simulation in dipole coordinates of the EMIC waves on a meridional plane at MLT = 18 and at 1900 UT within a range of L shell from L = 4.9 to 6.7. While an exact comparison between observed and simulated spectra is not possible here, we do find significant similarities between the two, at least in certain regions. We simulate the EMIC wave growth and evolution within three regions, the plasmasphere (or plasmaspheric plume), the plasmapause, and the low density plasmatrough outside the plasmapause. We find that the plasmapause is not a preferred region for EMIC wave growth, though waves can grow in that region. There is a preference for EMIC waves to be driven in the He+ band (frequencies between the O+ and He+ gyrofrequencies) within the plasmasphere, although they can also grow in the plasmatrough. If present, H+ band waves are more likely to grow in the plasmatrough. This fact, plus L dependence of the frequency and possible time evolution toward lower frequency waves can be explained by a simple model. Large O+ concentration limits the frequency range of or even totally quenches EMIC waves. This is more likely to occur in the plasmatrough at solar maximum. Such large O+ concentration significantly affects the H+ cutoff frequency, and hence the width in frequency of the stop band above the He+ gyrofrequency. EMIC wave surfaces predicted by cold plasma theory may not be valid.