Temperature anisotropy of drifting ions in the auroral F-region, observed by EISCAT

Abstract

During relative drifts between the ions and the neutrals perpendicular to the geomagnetic field, the ion temperature in the auroral F-region becomes anisotropic with a higher temperature perpendicular than parallel to the magnetic field ( T ⊥ > T ∥). It has been shown that for a gyrotropic ion velocity distribution the ion temperatures T ⊥ and T ∥ can be expressed as a function of the neutral temperature and of the squared normalized relative ion-neutral drift, with parameters β ⊥ and β ∥ describing the anisotropy and the collision process. In this paper, five increases of the F-layer ion temperature and ion drift velocity, found in EISCAT-CP1F data, were analyzed to obtain information about the anisotropy and the collision process. In the CP1F experiment, the angles between the magnetic field line ending in Tromsø and the antenna directions remain small, and the ion drift velocities of the investigated events in general were below 1500 m/s. Thus the ion velocity distributions were approximated by a bi-Maxwellian, and NO + was assumed to remain a minor constituent at the F- layer maximum. For a quantitative analysis, generalized theoretical β-values for a bi-Maxwellian ion velocity distribution drifting through a mixture of different neutral components and for arbitrary observation directions were calculated. With these expressions it was possible to compare the drift dependence of the measured ion temperature for every antenna position directly with the theory. A statistical analysis of the heating events showed a good correlation between the ion temperatures of Tromsø, Kiruna and Sodankylä and the squared normalized ion drift, and values β T , β K , β S could be calculated by linear regression. The fitted curves corresponded well with theoretical curves for a bi-Maxwellian velocity distribution of O + ions drifting through a neutral atmosphere consisting of O and N 2.