Survey of pancake‐shaped warm ion distributions at geosynchronous orbit

It has been proposed that the electromagnetic proton cyclotron instability is a strong source of heating for the anisotropic warm ions observed at geosynchronous orbit. We present here the results of a statistical study of pancake‐shaped warm ion distributions using a 1‐year interval of data observed with the Los Alamos magnetospheric plasma analyzer on the geosynchronous satellite 1994–084. Our results support previous findings that pancake‐shaped warm ion distributions occur more frequently on the dayside of the magnetosphere and occur very close to the plasmapause. We also confirm that the electromagnetic proton cyclotron instability is operating and is constraining the hot proton temperature anisotropy. However, our results indicate that the pancake‐shaped warm ion distributions observed at geosynchronous orbit are probably not generated by this instability but must be due to a different mechanism, possibly heating by lower hybrid waves.