A STATISTICAL-SURVEY OF AURORAL SOLITARY WAVES AND WEAK DOUBLE-LAYERS .1. OCCURRENCE AND NET VOLTAGE

Weak double layers have been proposed to contribute to the buildup of a quasi-static potential drop along the geomagnetic field lines at the auroral latitudes. The discussions, however, have been based on very limited amounts of analyzed data. In this work we show results of a statistical study of the occurrence and average potential drop of weak double layers (WDLs) and solitary waves (SWs) in data from the Viking spacecraft. We conclude that these structures are found in connection with beams of moderate-energy, upward-flowing ions (UFI): Practically all observations are in UFI regions and these structures have been identified in approximately two thirds of the analyzed UFI beam regions. They are most typical at altitudes between 7000 and 11,000 km. The SWs/WDLs with largest amplitudes and net potential drops appear at the same altitude range where most structures are detected. The polarities and net voltages vary between different analyzed orbits and in a smaller scale also inside single ion beam regions. The structures tend to appear in groups with group sizes corresponding to a few kilometers extension perpendicular to the magnetic field. We conclude that the observed SW/WDLs are not sufficient as an explanation of parallel electrostatic fields with perpendicular extensions of hundreds of kilometers, but they may be important for structures with perpendicular sizes on the kilometer scale.