Viking observations at the source region of auroral kilometric radiation

Abstract

The orbit of the Swedish satellite Viking was optimized for in situ observations of auroral particle acceleration and related phenomena. In a large number of the orbits, auroral kilometric radiation (AKR) was observed, and in approximately 35 orbits the satellite passed through AKR source regions as evidenced by very strong signals at the local electron cyclotron frequency fce. These sources were found at the poleward edge of the auroral oval at altitudes from 5000 to 8000 km, predominantly in the evening sector. The strong AKR signal has a sharp low‐frequency cutoff at or very close to fce in the source. In addition to AKR, strong broadband electrostatic noise is measured during the source crossings. Energetic (1–15 keV) electrons are always present at and around the AKR sources. Upward directed ion beams of several keV are closely correlated with the source as are strong and variable electric fields, indicating that a region of upward pointing electric field below the observation point is a necessary condition for AKR generation. The plasma density is measured by three independent experiments and it is generally found that the density is low across the whole auroral oval. For some source crossings the three methods agree and show a density depletion (but not always confined to the source region itself), but in many cases the three measurements do not yield consistent results. The magnetic projection of the satellite passes through auroral forms during the source crossings, and the strongest AKR events seem to be connected with kinks in an arc or more complicated structures. The complete Viking data set is described in detail for a few typical source crossings, and the observations are briefly discussed in relation to theory.