Drift shell bifurcation near the dayside magnetopause in realistic magnetospheric magnetic fields

Abstract

We study trapped energetic particles in the terrestrial magnetosphere undergoing drift shell bifurcation in the magnetic field lacking north‐south and east‐west symmetry. Drift shell bifurcation occurs near the dayside magnetopause, where, due to the solar wind compression, the field strength has a local maximum near the equatorial plane. As a result, a charged particle may become temporarily trapped in one of the hemispheres while traversing the region. Although this phenomenon has been known for a long time, only recently were the associated second invariant changes quantified for the magnetic field with north‐south and east‐west symmetry. Here we show that if the magnetic field lacks such symmetry, the effect is more significant. We calculate changes to the second invariant of keV to MeV electrons in Tsyganenko magnetic fields with nonzero interplanetary magnetic field (IMF) BY component. The changes are on the order of the invariant itself, and thus, this effect is much larger than for the case of symmetric magnetic field (when the particle gyroradius is much less than the magnetospheric scale length). We also quantify the effect for different values of the solar wind dynamic pressure, IMF BZ component, and the Dst index with the Tsyganenko magnetic field T02. We find that Dst has no noticeable role, while larger solar wind ram pressure increases the second invariant changes. We verify our calculations by numerical integration of the guiding center drift equations and discuss properties of different versions of these equations.