Production of gyrating ions from nonlinear wave-particle interaction upstream from the Earth's bow shock: A case study from Cluster-CIS
Lundin, R.; Balogh, A.; Dandouras, I.; Eastwood, J. P.; Klecker, B.; Kistler, L. M.; Rème, H.; Bosqued, J. M.; McCarthy, M.; Korth, A.; Mazelle, C.; Bavassano-Cattaneo, M. B.; Wilber, M.; Sauvaud, J. A.; Meziane, K.; Pallocchia, G.; Le Quéau, D.
France, Canada, United States, United Kingdom, Germany, Italy, Sweden
Abstract
We present observations of E<40 keV/e backstreaming ions measured by the Cluster Ion Spectrometry experiment at 1-2 RE upstream of the bow shock. The ions are simultaneously observed at all three spacecrafts for which CIS measurements are available. The proton distributions are analyzed using 4 s time resolution. They are observed in association with low-frequency quasi-monochromatic waves with substantial amplitudes (δ B/B∼1) . When the waves are present the ion distributions appear as gyrophase-bunched gyrating distributions while field-aligned beams are also observed just adjacent to the interval of wave occurrence. The gyrating distributions are observed at distances larger than an ion Larmor radius and they have pitch-angles inconsistent with a specular reflection mechanism at the bow shock. The properties of the ULF waves reveal that they are in cyclotron resonance with the ion parallel beams that could drive a right-hand ion/ion instability responsible for the wave occurrence. Moreover, the observed pitch-angles for the gyrating ion distributions are consistent with the theoretical value expected if they are produced by a coherent nonlinear wave-particle interaction.