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

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 RE. 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 RE. 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 RE, the fields are about twice as high and spread over a broader MLT range. These fields occur in a region where the (ΔE/ΔB)/VA 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)/VA 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). Key Points Quasi‐static accelerationAlfvénic accelerationStatistical distribution