Counter-streaming alpha proton plasmas in an eroding magnetic cloud: new insights into space plasma evolution from Wind

Tuesday, 16 December 2014: 2:40 PM
Michael Louis Stevens, Smithsonian Astrophysics Observatory, Cambridge, MA, United States, Justin Christophe Kasper, University of Michigan, Ann Arbor, MI, United States, Bennett Maruca, University of California Berkeley, Berkeley, CA, United States, Kelly E Korreck, Smithsonian Observatory, Cambridge, MA, United States and Robert Wicks, University of Maryland College Park, College Park, MD, United States
Following the observation of a strong interplanetary mass ejection on February 19, 2014, the Faraday Cup instrument on the Wind spacecraft recorded an unusual ion configuration consisting of two interpenetrating alpha-proton plasmas that were well-separated in phase space. The two co-present plasmas were observed for four extended time periods, beginning shortly after the apparent passage of a magnetic cloud with indications of eroding magnetic structure on its sunward side. The observed phase-space distributions are shown to be unstable to several electromagnetic resonances, including the low-beta oblique Alfven II mode, heretofore not directly observed in nature. A spectral analysis of accompanying ion-gyroscale fluctuations in the magnetic field indicates ongoing exchange of energy between the plasma and the magnetized wave field through the local excitation of these modes. We hypothesize that the erosion of the magnetic cloud via reconnection with the trailing high-speed stream is the driver for this unusual observation, and that the de-excitation of beamed ion distributions such as this could be a typical dissipation path for large-scale magnetic reconnection outflows in the interplanetary plasma.