Observation of a new type of low-frequency waves at comet 67P/Churyumov-Gerasimenko
Altwegg, K.; Tsurutani, B. T.; Glassmeier, K. -H.; Nilsson, H.; Eriksson, A.; Volwerk, M.; Carr, C.; Henri, P.; Goldstein, R.; Mokashi, P.; Richter, I.; Rubin, M.; Vallat, C.; Götz, C.; Burch, J.; Cupido, E.; Frühauff, D.; Koenders, C.; Perschke, C.; Motschmann, U.; Heinisch, P.; Lebreton, J. -P.; Auster, H. -U.; Nemeth, Z.; Szegö, K.; Stoll, B.
Germany, Switzerland, United States, United Kingdom, Sweden, France, Hungary, Spain, Austria
Abstract
We report on magnetic field measurements made in the innermost coma of 67P/Churyumov-Gerasimenko in its low-activity state. Quasi-coherent, large-amplitude (δ B/B ~ 1), compressional magnetic field oscillations at ~ 40 mHz dominate the immediate plasma environment of the nucleus. This differs from previously studied cometary interaction regions where waves at the cometary ion gyro-frequencies are the main feature. Thus classical pickup-ion-driven instabilities are unable to explain the observations. We propose a cross-field current instability associated with newborn cometary ion currents as a possible source mechanism.