An alternative drift shell parameter for modeling trapped particles

Abstract

The McIlwain L parameter has been widely used to label magnetic drift shells of trapped particles. In a dipolar magnetic field, L gives the equatorial radius of a drift shell. When the total magnetospheric magnetic field is significantly altered by the presence of external magnetospheric currents, particularly during geomagnetically active periods, L has to be calculated by using an appropriate magnetic field model that includes the external fields. Thus, an L value computed at a given local time does not always give the position of a drift shell in configuration (real) space, especially at high L values (>4). Since L is one of the magnetic coordinates typically used 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 field given by the sum of the IGRF and the Tsyganenko T89c models, a configuration-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 quantity I is empirically related to the radial distance r t to the point of minimum field strength along that field line. By conserving I, r t measured in units of Earth radii ( R E ) at any local time can be mapped to its equivalent value L tn at noon, and vice versa. We show that L tn can be used as a parameter to label drift shell in configuration space, analogous to the use of L in magnetic space.