Characteristics of the Pitch-Angle Anisotropy of Energetic Protons in the Daytime Magnetosphere due to Particle Drift in the Nondipole Magnetic Field

Abstract

This paper analyzes the effect of azimuthal proton drift from the nighttime sector of the Earth’s magnetosphere on the characteristics of the pitch-angle anisotropy of proton fluxes in the daytime sector. The drift in the magnetic field caused by curvature of the field lines and the magnetic field gradient, in the absence of external forces, has been considered. The external geomagnetic field has been described with Tsyganenko’s T96 model. The proton drift has been calculated in the approximation of the guiding center motion in the equatorial magnetosphere plane, i.e., after averaging over the bounce oscillations of particles between the mirror points. The influence of two effects on the anisotropy value has been studied. The first effect is related to changes in the proton pitch angle in the course of azimuthal drift. This effect can significantly (up to approximately six times, depending on the T96 model input parameters) increase the transverse anisotropy of protons when they drift from the nighttime to the daytime side. The second effect is caused by drift-shell splitting, as a result of which protons from the nighttime sector from different radial distances come to the same field line on the daytime side: the lower the pitch angle of the particle is, the larger is the distance it starts to drift from. As a result, the radial proton flux gradient on the magnetosphere nighttime side can lead to a pitch-angle anisotropy of fluxes in the daytime sector that is sufficient to generate electromagnetic ion cyclotron (EMIC) waves, even if the fluxes in the nighttime sector were isotropic in the pitch angles. The dependence of this anisotropy on the radial proton flux gradient on the nighttime side has been studied. The maximum anisotropy in the daytime sector due to the longitudinal drift in the geomagnetic field can be achieved at a distance of 5.3–10 RE depending on the Т96 model input parameters.