Cluster observations of Pc 1–2 waves and associated ion distributions during the October and November 2003 magnetic storms |
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| Institution: | 1. Department of Physics, Augsburg College, Minneapolis, MN 55454, USA;2. Space Science Laboratory, University of California-Berkeley, Berkeley, CA 94720, USA;3. Institut fur Geophysik und Meteorologie, Technical University of Braunschweig, Braunschweig D-83106, Germany;4. School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55454, USA;5. Princeton Plasma Physics Laboratory, Princeton, NJ 08543, USA;6. CESR, Toulouse, Cedex 4, France;1. University of California, Los Angeles, USA;2. The Johns Hopkins University, Laurel, USA;3. Dartmouth College, Hanover, USA;4. University of Minnesota, Minneapolis, USA;1. Serena University, La Serena, Chile;2. Space Research Institute, Moscow, Russia;3. Geological and Geophysical Institute of Hungary, Tihany, Hungary;4. University of Santiago de Chile, Santiago, Chile;5. GFZ German Research Centre for Geosciences, Potsdam, Germany;6. Kyushu University, Fukuoka, Japan;1. Nanjing University of Information Science and Technology, Jiangsu 210044, China;2. National Centre for Space Weather, China Meteorological Administration, Beijing 100081, China;3. Wuhan University, Hubei 430072, China;1. Department of Radio Physics, St. Petersburg State University, Ulyanovskaya 1, Petrodvorets, St. Petersburg 198504, Russia;2. Space Research Centre PAS, Bartycka 18A, 00-716 Warsaw, Poland;3. Department of Engineering, University of Leicester, Leicester LE1 7RH, UK;1. Department of Physics, University of Lagos, Akoka, Yaba, Lagos, Nigeria;2. Institute for Scientific Research, Boston College, Chestnut Hill, MA, USA;3. Research Institute for Sustainable Humanosphere, Kyoto University, Kyoto, Japan;4. Department of Physics, Lagos State University, Ojo, Lagos, Nigeria;1. GPHI UMBC/NASA GSFC, Code 673, Greenbelt, MD 20771, USA;2. Faculty of Problems of Physics and Power Engineering, Moscow Institute of Physics and Technology, Russia;3. NASA Goddard Space Flight Center, Code 674, Greenbelt, MD 20771, USA |
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| Abstract: | Unusual wave activity in the Pc 1–2 frequency band (0.1–5 Hz) was observed by the Cluster spacecraft in association with the two large geomagnetic storms of late 2003. During the onset of the Hallowe’en storm on October 29, 2003, intense broadband activity between ～0.1 and 0.6 Hz appeared at all 4 spacecraft on both sides of the magnetic equator at perigee (near 1400 UT and 08:45 MLT). Power was especially strong and more structured in frequency in the compressional component: a minimum in wave power was observed at 0.38 Hz, corresponding to the oxygen ion cyclotron frequency. Poynting vector calculations indicated that wave power was primarily directed radially inward rather than along the magnetic field. Narrowband purely compressional waves near 0.15 Hz appeared at higher dayside latitudes in the southern hemisphere. CIS ion spectrometer data during this pass revealed that O+ was the dominant energetic ion. During the recovery phase of the November storm, on November 22, 2003, predominantly transverse 1.8 Hz waves with peak-to-peak amplitude of 10 nT were observed by all four spacecraft near perigee at L=4.4. During this more typical Pc 1 event, wave power was directed along B, toward the northern ionosphere. An unusually polarized 2.3 Hz emission (with power in the radial and compressional, but not azimuthal directions) was observed at L=5.4–5.9, 10–15° south of the magnetic equator. We infer that this wave event may have been generated on lower L shells and propagated obliquely to Cluster's location. Consistent with other recent observations, anisotropic plasma sheet/ring current proton distributions appeared to be a necessary condition for occurrence of waves during both passes, but was not always a sufficient condition. The transverse waves of November 22 occurred in regions which also contained greatly increased fluxes of cool ions (E<1 keV). On both days, Cluster observed features not previously reported, and we note that the purely compressional nature of the October 29 events was not anticipated in previous theoretical studies. The fact that these unusually polarized waves occurred in association with very intense geomagnetic storms suggests that they are likely to be extremely rare. |
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