Low‐energy plasma observations at synchronous orbit

Abstract

The University of California at San Diego Auroral Particles Experiment on the ATS 6 satellite in synchronous orbit has detected a low‐energy plasma population which is separate and distinct from both the ring current and the plasma sheet populations. The density and temperature of this low‐energy population are highly variable, with temperatures in the range kT = 1–30 eV and densities ranging from less than 1 cm−3 to more than 10 cm−3. The occurrence of a dense low‐energy plasma is most likely in the afternoon and dusk local time sectors, whereas n > 1 cm−3 is seen in the local night sector only during magnetically quiet periods. These observations suggest that this plasma is the outer zone of the plasmasphere. During magnetically active periods this low‐energy plasma is often observed flowing sunward. In the dusk sector, strong sunward plasma flow is often observed for 1–2 hours prior to the onset of a substorm‐associated particle injection.