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Ann. Geophys., 19, 1-9, 2001
www.ann-geophys.net/19/1/2001/
© European Geosciences Union 2001
Dawn-dusk asymmetry in particles of solar wind origin within the magnetosphere
T. J. Stubbs1,2, M. Lockwood2, P. Cargill1, J. Fennell3, M. Grande2, B. Kellett2, C. Perry2, and A. Rees1
1Space & Atmospheric Physics, The Blackett Laboratory, Imperial College, London, SW7 2BW, United Kingdom
2Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX, United Kingdom
3The Aerospace Corporation, Mail Station M2-259, PO Box 92957, Los Angeles, CA 90009, U.S.A.
Correspondence to: T. J. Stubbs (tj.stubbs@ic.ac.uk)
Abstract. Solar wind/magnetosheath
plasma in the magnetosphere can be identified using a component that has a
higher charge state, lower density and, at least soon after their entry into the
magnetosphere, lower energy than plasma from a terrestrial source. We survey
here observations taken over 3 years of He2+ ions made by the Magnetospheric
Ion Composition Sensor (MICS) of the Charge and Mass Magnetospheric Ion
Composition Experiment (CAMMICE) instrument aboard POLAR. The occurrence
probability of these solar wind ions is then plotted as a function of Magnetic
Local Time (MLT) and invariant latitude (7) for various energy ranges. For all
energies observed by MICS (1.8–21.4 keV) and all solar wind conditions, the
occurrence probabilities peaked around the cusp region and along the dawn flank.
The solar wind conditions were filtered to see if this dawnward asymmetry is
controlled by the Svalgaard-Mansurov effect (and so depends on the BY component
of the interplanetary magnetic field, IMF) or by Fermi acceleration of He2+ at
the bow shock (and so depends on the IMF ratio BX /BY ). It is shown that the
asymmetry remained persistently on the dawn flank, suggesting it was not due to
effects associated with direct entry into the magnetosphere. This asymmetry,
with enhanced fluxes on the dawn flank, persisted for lower energy ions (below a
"cross-over" energy of about 23 keV) but reversed sense to give higher
fluxes on the dusk flank at higher energies. This can be explained by the
competing effects of gradient/curvature drifts and the convection electric field
on ions that are convecting sunward on re-closed field lines. The lower-energy
He2+ ions E × B drift dawnwards as they move earthward, whereas the higher
energy ions curvature/ gradient drift towards dusk. The convection electric
field in the tail is weaker for northward IMF. Ions then need less energy to
drift to the dusk flank, so that the cross-over energy, at which the asymmetry
changes sense, is reduced.
Key words. Magnetospheric physics (magnetospheric
configuration and dynamics; magnetopause, cusp, and boundary layers) – Space
plasma physics (charged particle motion and acceleration)
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