An alternative drift shell parameter for modeling trapped particles

The McIlwain L parameter has been widely used to label magnetic drift shell s of trapped particles. In a dipolar magnetic field, L gives the equatorial radius of a drift shell. When the total magnetospheric magnetic field is s ignificantly altered by the presence of external magnetospheric currents, p articularly during geomagnetically active periods, L has to be calculated b y using an appropriate magnetic field model that includes the external fiel ds. Thus, an L value computed at a given local time does not always give th e position of a drift shell in configuration (real) space, especially at hi gh L values ( > 4). Since L is one of the magnetic coordinates typically us ed to organize trapped particle data, the external current effects on the L calculation need to be assessed. In this paper, we show that by conserving the second adiabatic invariant, I, in a model magnetospheric magnetic held given by the sum of the IGRF and the Tsyganenko T89c models, a configurati on-space-based drift shell parameter can be defined. It has been found that for a given mirroring field strength B-m on a magnetic field Line, the qua ntity I is empirically related to the radial distance r, to the point of mi nimum field strength along that field line. By conserving I, r, measured in units of Earth radii (R-E) at any local time can be mapped to its equivale nt value L-m at noon, and vice versa. We show that L-m can be used as a par ameter to label drift shell in configuration space, analogous to the use of L in magnetic space. Published by Elsevier Science Ltd.