Non‐thermal plasma observations using EISCAT: Aspect angle dependence

Recent observations with the EISCAT incoherent scatter radar have shown large rises in dayside, auroral plasma velocities (>2 km s−1) over a wide range of latitudes and lasting about an hour. These are larger than the neutral thermal speed, and allow, for the first time, observations of a non‐thermal plasma over a range of observing angles, revealing a clear angular dependence. The observed ion temperature anisotropy, deduced by assuming a Maxwellian line‐of‐sight ion velocity distribution, is at least 1.75, which exceeds the theoretical value for a bi‐Maxwellian based on a realistic ion‐neutral collision model. The aspect angle dependence of the signal spectra also indicates non‐Maxwellian plasma.