High resolution observations of the collision frequency and temperatures with the Eiscat UHF Radar

Abstract

Ion-neutral collision frequencies and ion and electron temperatures have been deduced from high resolution (600 m) EISCAT UHF radar measurements. The results are based on 80 h of data measured in February, July and August 1984 and August 1985. They are compared with other EISCAT observations and with collision frequencies and neutral temperatures based on the MSIS-86 model neutral atmosphere. Theoretical error estimates for the plasma parameters are presented and they are used to study the possibility of deducing the collision frequency and/or the electron-ion temperature ratio from the data. Collision frequency results have been obtained between 93 and 110 km altitudes using the assumption of equal ion and electron temperatures. These observations agree with the other published collision frequency observations by EISCAT. The observations indicate that the MSIS-86 model gives correct collision frequencies in July but underestimates them in February and August. The temperature observations from each of the three months show an event-to-event variation which exceeds that predicted by the MSIS-86 model. It is proposed that, although Joule and particle heating can explain the variation partly, the observations indicate larger changes in the neutral temperature than those given by the model. This is especially evident below 100 km, where the heating mechanisms are inefficient, but large changes in the ion temperature are nevertheless observed. The ion temperature exceeds the electron temperature above 110 km but near that height the temperatures are closely equal. This fact justifies the assumption of equal ion and electron temperatures used in the analysis below 110 km.