IMF Direction Derived from Cycloid-Like Ion Distributions Observed by Mars Express |
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| Authors: | M. Yamauchi Y. Futaana A. Fedorov E. Dubinin R. Lundin J.-A. Sauvaud D. Winningham R. Frahm S. Barabash M. Holmstrom J. Woch M. Fraenz E. Budnik H. Borg J. R. Sharber A. J. Coates Y. Soobiah H. Koskinen E. Kallio K. Asamura H. Hayakawa C. Curtis K. C. Hsieh B. R. Sandel M. Grande A. Grigoriev P. Wurz S. Orsini P. Brandt S. Mckenna-Lawler J. Kozyra J. Luhmann |
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| Affiliation: | (1) Swedish Institute of Space Physics, Box 812, S-98 128 Kiruna, Sweden;(2) Centre d’Etude Spatiale des Rayonnements, BP-4346, F-31028 Toulouse, France;(3) Max-Planck-Institut für Sonnensystemforschung, D-37191 Katlenburg-Lindau, Germany;(4) Southwest Research Institute, San Antonio, TX 7228-0510, USA;(5) Mullard Space Science Laboratory, University College London, Surrey, RH5 6NT, UK;(6) Finnish Meteorological Institute, Box 503, FIN-00101 Helsinki, Finland;(7) Institute of Space and Astronautical Science, 3-1-1 Yoshinodai, Sagamichara, Japan;(8) Department of Physics, University of Arizona, Tucson, AZ 85721, USA;(9) Lunar and Planetary Lab, University of Arizona, Tucson, AZ 85721, USA;(10) Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX, UK;(11) University of Bern, Physikalisches Institut, CH-3012 Bern, Switzerland;(12) Instituto di Fisica dello Spazio Interplanetari, I-00133 Rome, Italy;(13) Applied Physics Laboratory, Johns Hopkins University, Laurel, MD 20723-6099, USA;(14) Space Technology Ltd., National University of Ireland, Maynooth, Co. Kildare, Ireland;(15) Space Physics Research Lab., University of Michigan, Ann Arbor, MI 48109-2143, USA;(16) Space Science Lab., University of California in Berkeley, Berkeley, CA 94720-7450, USA;(17) Department of Physical Sciences, University of Helsinki, Box 64, FIN-00014 Helsinki, Finland |
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| Abstract: | Although the Mars Express (MEX) does not carry a magnetometer, it is in principle possible to derive the interplanetary magnetic field (IMF) orientation from the three dimensional velocity distribution of pick-up ions measured by the Ion Mass Analyser (IMA) on board MEX because pick-up ions' orbits, in velocity phase space, are expected to gyrate around the IMF when the IMF is relatively uniform on a scale larger than the proton gyroradius. During bow shock outbound crossings, MEX often observed cycloid distributions (two dimensional partial ring distributions in velocity phase space) of protons in a narrow channel of the IMA detector (only one azimuth for many polar angles). We show two such examples. Three different methods are used to derive the IMF orientation from the observed cycloid distributions. One method is intuitive (intuitive method), while the others derive the minimum variance direction of the velocity vectors for the observed ring ions. These velocity vectors are selected either manually (manual method) or automatically using simple filters (automatic method). While the intuitive method and the manual method provide similar IMF orientations by which the observed cycloid distribution is well arranged into a partial circle (representing gyration) and constant parallel velocity, the automatic method failed to arrange the data to the degree of the manual method, yielding about a 30° offset in the estimated IMF direction. The uncertainty of the derived IMF orientation is strongly affected by the instrument resolution. The source population for these ring distributions is most likely newly ionized hydrogen atoms, which are picked up by the solar wind. |
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| Keywords: | IMF Mars ion gyration pick-up process |
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