Evidence of drift waves at the plasmapause

Abstract

As the Hawkeye 1 spacecraft crosses the plasmapause at high altitudes (R > 3 RE), a band of electric field noise is often detected in the frequency channels from 1.7 to 178 Hz. No corresponding magnetic field noise is detected, indicating that the noise is electrostatic (or at least quasi‐electrostatic), and the electric field is polarized perpendicular to the plasma density gradient. The noise is only detected when the scale length of the plasmapause is 0.1 RE or less, indicating that a large density gradient is required to produce the noise. These characteristics are all consistent with the interpretation that this noise consists of electrostatic waves excited by the drift mode instability. By using reasonable assumptions concerning the wavelengths of these waves the observed frequency spectrum can be explained as being due to Doppler shifts caused by spacecraft motion through the plasma.