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ULF waves in the Martian foreshock: MAVEN observations


Mazelle Christian




Auteur(s) supplémentaire(s)Lican Shan, Karim Meziane, César Bertucci, Jasper Halekas, Jack Connerney, Jin McFadden, Dave Mitchell, Davin Larson, Dave Brain, Bruce Jakosky
Institution(s) supplémentaire(s)IGGS, CAS, Beijing, Chine; UNB, Fredericton; Canada; IAFE, Buenos-Aires, Argentine; Iowa University, Iowa City, USA; GSFC, Greenbelt, MD, USA; SSL, Berkeley CA, USA; LASP, Univ. Boulder, CO, USA


Foreshock ULF waves constitute a significant physical phenomenon of the plasma environment for terrestrial
planets. The occurrence of these ULF waves, associated with backstreaming ions reflected and accelerated at
the bow shock, implies specific conditions and properties of the shock and its foreshock. Using measurements
from MAVEN, we report clear observations of this type of ULF waves in the Martian foreshock. We show from
different case studies that the peak frequency of the wave case in spacecraft frame is too far from the local ion
cyclotron frequency to be associated with local pickup ions taking into account the Doppler shifted frequency
from a cyclotron resonance, the obliquity of the mode, resonance broadening and experimental uncertainties. On
the opposite their properties fit very well with foreshock waves driven unstable by backtreaming field-aligned ion
beams. The propagation angle is usually less than 30 degrees from ambient magnetic field. The waves also display
elliptical and left-hand polarizations with respect to interplanetary magnetic field in the spacecraft frame. It is
clear for these cases that foreshock ions are simultaneous present for the ULF wave interval. Such observation
is important in order to discriminate with the already well-reported pickup ion (protons) waves associated with
exospheric hydrogen in order to quantitatively use the later to study seasonal variations of the hydrogen corona.

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