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 geo synchronous orbit. We present here the results of a statistical study of pa ncake-shaped warm ion distributions using a 1-year interval of data observe d with the Los Alamos magnetospheric plasma analyzer on the geosynchronous satellite 1994-084. Our results support previous findings that pancake-shap ed warm ion distributions occur more frequently on the dayside of the magne tosphere and occur very close to the plasmapause. We also confirm that the electromagnetic proton cyclotron instability is operating and is constraini ng the hot proton temperature anisotropy. However, our results indicate tha t the pancake-shaped warm ion distributions observed at geosynchronous orbi t are probably not generated by this instability but must be due to a diffe rent mechanism, possibly heating by lower hybrid waves.