A quantitative study of the high latitude ionospheric trough using EISCAT's common programmes

Eighteen days of EISCAT data were used in a systematic study of the high latitude trough. Apart from a few days at midwinter, the pattern was the same in all cases. Near midnight the reversal of plasma flow from westward to eastward caused significant frictional heating of the ion population. At the same time a strong plasma velocity was observed upwards along the magnetic field line. This was the result of

• 1.(i) a southward neutral wind
• 2.(ii) a vertical wind driven by Joule heating
• 3.(iii) diffusion. Both enhanced recombination—associated with the increase in ion temperature—and the escape of plasma along the field line contribute to the drop in electron density.