The ion cyclotron group delay for source regions near the plasmapause

Abstract

Electromagnetic ion cyclotron waves are generated near the magnetic equator through an instability driven by the energetic ion temperature anisotropy. Many of these waves are observed on the ground as Pc 1 magnetic pulsations, which often exhibit a distinct wave packet structure (pearl pulsations). We use the warm dispersion relation to calculate the group delay of ion cyclotron waves from the equatorial source region near the plasmapause to the foot of the field line and compare the results to both cold theory and observations. Our results differ significantly from the cold plasma result for frequencies less than 0.85 fnof;cHe. In general we find that the presence of the ring current produces longer delays and greater wave dispersion. We model three particular ion cyclotron wave events each of which was observed simultaneously by DE 1 and the Air Force Geophysics Laboratory Magnetometer Network. Plasma data from three other instruments on DE 1 provide input parameters to the model. In two out of three cases our predicted delays agree with ground/satellite correlation delays. The third event does not agree as well with theory possibly due to the presence of a significantly greater O+ concentration during this event. The enhanced O+ concentration introduces an additional stop band below fnof;cHe along the propagation path which the linear theory is unable to model.