The occurrence frequency of auroral potential structures and electric fields as a function of altitude using Polar/EFI data

Abstract. The aim of the paper is to study how auroral potential structures close at high altitude. We analyse all electric field data collected by Polar on auroral field lines in 1996–2001 by integrating the electric field along the spacecraft orbit to obtain the plasma potential, from which we identify potential minima by an automatic method. From these we estimate the associated effective mapped-down electric field E_i, defined as the depth of the potential minimum divided by its half-width in the ionosphere. Notice that although we use the ionosphere as a reference altitude, the field E_i does not actually exist in the ionosphere but is just a convenient computational quantity. We obtain the statistical distribution of E_i as a function of altitude, magnetic local time (MLT), K_p index and the footpoint solar illumination condition. Surprisingly, we find two classes of electric field structures. The first class consists of the low-altitude potential structures that are presumably associated with inverted-V regions and discrete auroral arcs and their set of associated phenomena. We show that the first class exists only below ~3R_E radial distance, and it occurs in all nightside MLT sectors (R_E=Earth radius). The second class exists only above radial distance R=4R_E and almost only in the midnight MLT sector, with a preference for high K_p values. Interestingly, in the middle altitudes (R=3–4R_E) the number of potential minima is small, suggesting that the low and high altitude classes are not simple field-aligned extensions of each other. This is also underlined by the fact that statistically the high altitude structures seem to be substorm-related, while the low altitude structures seem to correspond to stable auroral arcs. The new finding of the existence of the two classes is important for theories of auroral acceleration, since it supports a closed potential structure model for stable arcs, while during substorms, different superposed processes take place that are associated with the disconnected high-altitude electric field structures.

Key words. Magnetospheric physics (electric fields; auroral phenomena) – Space plasma physics (electrostatic structures)