Magnetosonic wave instability analysis for proton ring distributions observed by the LANL magnetospheric plasma analyzer

Energetic proton (1–45 keV) distributions measured by the magnetospheric plasma analyzer detector on Los Alamos National Laboratory geosynchronous orbiting satellites are analyzed to study the characteristics of proton ring development during the 2001 April storm. Distinct proton rings are formed over a broad spatial region from noon to premidnight, associated with individual or multiple substorm injections on the nightside. On the basis of observed proton distributions, the convective growth rate of magnetosonic waves is calculated. We find that these proton ring distributions can provide a source of free energy for exciting magnetosonic waves, with convective growth rate mostly in the range 10−5–10−6 m−1, over a broad range of frequency from a few times ΩH+ up to ~35 ΩH+ and over a broad magnetic local time range from 1000 to 2200.