Evidence for non-Maxwellian ion velocity distributions in the F-region

A study has been made of data taken with EISCAT using the Common Program CP-3-C (F-region meridian scan) which shows that regions of enhanced ion temperature (in excess of 3000K at all three EISCAT stations) are found on most days when K_p exceeds 2 or 3, usually accompanied by ion drift velocities of more than 1 km s⁻¹. These periods are often accompanied by anisotropy of the ion temperature and abnormally low apparent electron temperature, consistent with the presence of a non-Maxwellian ion velocity distribution such as would result from large but not exceptional ion drifts. Data for a selected period have been fitted using theoretical ion velocity distributions based on the relaxation collision model and assuming that the ion composition is 100% O⁺. The results confirm the presence of non-Maxwellian distributions, but a detailed comparison with theory reveals some discrepancies, indicating that the analysis may need to be extended to include effects due to, for example, molecular ions and instabilities.