Statistical altitude distribution of Cluster auroral electric fields, indicating mainly quasi-static acceleration below 2.8 R_E and Alfvénic above

Abstract

Results are presented from a statistical study of high-altitude electric fields and plasma densities using Cluster satellite data collected during 9.5 years between 2 and 4 R_E. The average electric fields are most intense on the nightside and associated with an extensive plasma density cavity, with densities of 1 cm^-3 or less. The intense electric fields are concentrated in two regions, separated by an altitude gap at about 2.8 R_E. Below this, the average electric field magnitudes reach about 50 mV/m (mapped to the ionosphere) between 22 and 01 magnetic local time (MLT). Above 3 R_E, the fields are about twice as high and spread over a broader MLT range. These fields occur in a region where the (ΔE/ΔB)/V_A ratio is close to unity, which suggests an Alfvénic origin. The intense low-altitude electric fields are interpreted to be quasi-static, associated with the auroral acceleration region. This is supported by their location in MLT and altitude, and by a (ΔE/ΔB)/V_A ratio much below unity. The local electric field minimum between the two regions indicates a partial closure of the electrostatic potentials in the lower region. These results show similarities with model results of reflected Alfvén waves by Lysak and Dum (1983), and with the O-shaped potential model, with associated wave-particle interaction at its top, proposed by Janhunen et al. (2000).