Dawn-dusk asymmetry in particles of solar wind origin within the magnetosphere

Abstract

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 He 2+ 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 ( ) 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 B Y component of the interplanetary magnetic field, IMF) or by Fermi acceleration of He 2+ at the bow shock (and so depends on the IMF ratio B X /B Y ). 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 He 2+ 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.

Daily variation at Concordia station (Antarctica) and its dependence on IMF conditions

“…Indeed, for B z <0, the B y component creates an asymmetry about the noon-midnight meridian in the evolution of newly opened field lines (Svalgaard-Mansurov effect; see for example Zhou et al, 2000;Stubbs et al, 2001). Moreover, Figure 6.…”

Daily variation at three Antarctic geomagnetic observatories within the polar cap

“…In general, for negative values of B y , the magnetic activity in the high latitude Southern Hemisphere tends to be higher (Nishida, 1978). Moreover, when B z assumes negative values, the B y component creates an asymmetry about the noon-midnight meridian in the evolution of newly opened field lines (Svalgaard-Mansurov effect; see for example McEwen, 1998;Zhou et al, 2000a;Stubbs et al, 2001). More recently, B y has been shown to be important also for positive B z conditions, when the existence and the geometry of the polar cap, as well as the local current geometry, is strongly connected to the value of the B y component (Vennerstrom et al, 2002(Vennerstrom et al, , 2005Park et al, 2006): the introduction of an eastward IMF component gradually opens the magnetosphere poleward of the cusp (as originally suggested by Burch, 1973), with newly open field lines which in the Northern Hemisphere are displaced toward dawn (dusk) for positive (negative) B y (the contrary in the Southern Hemisphere).…”

“…B z is important in that positive and negative values generally correspond to closed or open magnetospheric conditions and, at these high latitudes, to reduced or expanded polar cap conditions, respectively (Iijima, 2000). Furthermore, also B y plays a main role in determining geomagnetic variations because it can create an asymmetry about the noon-midnight meridian, the well known Svalgaard-Mansurov effect (see for example McEwen, 1998;Zhou et al, 2000a;Stubbs et al, 2001). More recently the importance of B y for the polar cap currents also for positive B z conditions has been found (Vennerstrom et al, 2002;Vennerstrom et al, 2005;Park et al, 2006): the eastward IMF component, during northward IMF conditions, gradually opens the magnetosphere poleward of the cusp (Burch, 1973), changing the pattern of the ionospheric currents that become asymmetric respect to the noon-midnight meridian.…”

CLUSTER encounters with the high altitude cusp: boundary structure and magnetic field depletions

“…Paschmann et al, 1976;Farrell and Van Allen, 1990;Kessel et al, 1996;Eastman et al, 2000;Dunlop et al, 2000), and significant recent advances have also come from the POLAR and Interball spacecraft (e.g. Zhou and Russell, 1997;Merka et al, 2000;Sandahl et al, 2000;Stubbs et al, 2001;Scudder et al, 2002). POLAR, in particular, has presented a picture of an extended cusp (dimensions many R E : Grande et al, 1997;Fritz et al, 2003), populated by particles of solar wind and ionospheric origin (e.g.…”