Violation of the Guiding Center Approximation for Energetic Ions in the Deep Inner Magnetosphere during Magnetic Storms

Details Violation of the Guiding Center Approximation for Energetic Ions in the Deep Inner Magnetosphere during Magnetic Storms Violation of the Guiding Center Approximation for Energetic Ions in the Deep Inner Magnetosphere during Magnetic Storms

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmsm11a1540s&link_type=abstract

American Geophysical Union, Fall Meeting 2009, abstract #SM11A-1540

Physics

[2720] Magnetospheric Physics / Energetic Particles: Trapped, [2778] Magnetospheric Physics / Ring Current

Scientific paper

During a magnetic storm, the ring current is developed in the inner magnetosphere. It is believed that the decrease of Dst index during a strom is mainly due to the development of the ring current. Recent numerical simulations have revealed how the ring current grows and decays (Lemon et al., 2004; Zaharia et al., 2006) in the self-consistent field with the guiding center approximation (GCA). This approximation breaks down if the spatial scale of the magnetic field change is comparable to the gyroradius. From the following two reasons, we suspect an approach to treat motion of ring current oxygen ions with the GCA. (1) During magnetic storms a dominant part of energy density of the ring current is carried by oxygen ions (Daglis at al., 1999) which are 16 times heavier than protons; that is, gyroradius of oxygen ions is 4 times larger than that of protons with the same kinetic energy. (2) Magnetic field configuration becomes far deviated from dipole field even in the deep inner magnetosphere. In this study, we examine whether the GCA is valid for ring current ions, in particular for oxygen ions. We compared energetic ion trajectories computed by the Lorentz equation and those by the GCA equation (Northrop, 1963). We used the TS04 (Tsyganenko and Sitnov, 2005) magnetic field model as well as the Volland-Stern (Volland 1973; Stern 1975) electric field model. In case of integration of the Lorentz equation, we found that some of ions have different trajectories from those of the GCA equation. Most remarkable results can be summarized as follows. First, ions having a pitch angle of ~90 degrees do not conserve the first adiabatic invariant and gradually increase their amplitude of bouncing motion around the magnetic equator. Second, an ion with a large gyroradius (i.e, an energetic oxygen ion) can be accelerated thorough the meandering motion even in the deep inner magnetosphere. Above-mentioned results lead some interest phenomena, for example, pitch angle distribution or the development of the ring current. Therefore, we suggest that GCA may not describe the ring current development and decay appropriately.

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