Characteristics of spectral fine structure in Auroral Kilometric Radiation

Auroral Kilometric Radiation (AKR) is a special type of nonthermal radio emission that is produced along auroral magnetic field lines at several thousand kilometers altitude above Earth's surface. Strong upward currents inside the AKR source region can lead to plasma instabilities and further to electrostatic solitary waves in the form of electron holes and ion holes. These small-scale plasma structures can modify the electron distribution function which is usually supposed to amplify the free-space wave modes, introducing various kinds of fine spectral features into the AKR emission pattern. We will discuss these fine spectral features based on observations from the Cluster Wideband Receiver (WBD). Spectral fine structure in AKR is frequently observed as fast frequency-drifting bursts of emission with time scales of milliseconds, or as features drifting more slowly over several seconds or a few minutes, like the well-known striations and banded emissions. The physical properties and parameter ranges of associated electron holes and ion holes are estimated based on statistics of observed WBD spectral patterns.