Thermal and suprathermal protons and alpha particles in the Earth's plasma sheet

We have investigated thermal and suprathermal protons (from ∼13 keV to ∼1 MeV) and alpha particles (from ∼30 keV/ Q to 130 keV/ Q ) in the quasi‐stable plasma sheet near local midnight at geocentric distances of ∼12–20 R E . The data, obtained during five different plasma sheet penetrations with the Max‐Planck‐Institut/University of Maryland sensor system on ISEE 1, represent the first measurements of plasma sheet ion composition in this energy range. Below ∼16 keV the proton spectra can be represented by a Maxwellian distribution. Above ∼16 keV we find a suprathermal tail which cannot be represented by a single power law. In some cases the suprathermal population consists of two distinct components: a low‐energy component falling off sharply at ∼100 keV and a high‐energy component extending up to ∼1 MeV. The high‐energy component often decays with time during expansion of the plasma sheet. Alpha particle to proton ratios average ∼3% and are observed to be constant at either equal energy per nucleon or energy per charge.