Locations of auroral kilometric radiation bursts inferred from multispacecraft wideband Cluster VLBI observations. 1: Description of technique and initial results

Abstract

The Cluster Wideband Data instrument has been used to determine the locations of auroral kilometric radiation (AKR) using very long baseline interferometry. The technique involves cross‐correlating individual AKR bursts from all six Cluster baselines using time and frequency filtered waveforms. The resulting differential delay peaks are used to determine source locations with an uncertainty as small as 500 km in a plane perpendicular to the source‐spacecraft line of sight or about 200 km when the burst position is projected onto the auroral zone along the magnetic field lines passing through the source. The uncertainty along the line of sight is much larger, but this is mitigated by assuming that the emission arises from a height corresponding to the electron gyrofrequency. We report the locations of over 1700 individual AKR bursts during six observing epochs between 10 July 2002 and 22 January 2003 when the Cluster constellation was high above the Southern or Northern Hemisphere. In general we find that the AKR burst locations lie along magnetic field lines which map onto the nighttime auroral zone as expected from previous AKR studies. For the three observing epochs viewing the Northern Hemisphere, there is a strong tendency for AKR burst locations to be centered within the auroral oval and in the evening sector. The Southern Hemisphere burst locations favor magnetic local midnight to early morning and have somewhat higher invariant magnetic latitudes. The distribution of AKR auroral footprint locations at each epoch had a overall spatial scale between 1000 and 2000 km, much larger than the positional uncertainty of an individual AKR burst location magnetic footprint, but a small fraction of the auroral oval. This indicates that on a timescale of 1–3 hours conditions for suitable generation of AKR emission are found on only a fraction of all magnetic field lines crossing through the auroral oval. For two of the six epochs, there was a significant drift in the mean location of AKR activity over a period of 1–2 hours. The drift was predominantly in latitude at one epoch and in longitude at the other, with average drift speed V ∼ 80–90 m s−1 at the AKR emission location.