Energetic neutral atoms in the outer magnetosphere: An upper flux limit obtained with the HEP-LD spectrometer on board GEOTAIL

Abstract

The energetic particle spectrometer HEP-LD on board the Japanese mission GEOTAIL is the first instrument in the energy range above 77 keV which can discriminate between energetic neutral atoms (ENA) and charged particles. The principle for particle identification is based on a time-of-flight (T) and energy (E) measurement which uniquely determines the particle species. A rather compact and highly anisotropic active collimation system in front of the (E,T)-detector head acts as an effective charged particle rejector (CPR) in the ENA mode. The resulting high-pass filter function for charged particles allows the detection of ENAs with energies below 200 keV. This report concentrates on ENA observations in the outer magnetosphere on the day-side (GSE x= -3, y = +12, z = +1 RE) and establishes an upper limit for the ENA flux in quiet conditions. The measurements agree reasonable well with predictions, however, the statistical uncertainties are considerable .