Evidence for Strong Pitch-Angle Diffusion within the Polar Cusps

Abstract

AS is well known, the classical zero-Larmor radius Chapman-Ferraro theory of the magnetosphere^1–3 neither accounts for the observed penetration of low-energy solar wind plasma through the dayside magnetopause into the polar magneto-sphere^{4, 5} nor explains the specific properties^5–9 of this plasma entry. On the other hand, because gradients of the magnet field³ tangential to the magnetopause are an inherent property of the zero-Larmor radius theory of the magnetosphere, the former can probably be understood on the basis of finite Larmor radius theory that takes into account the drift motion of the particles. The dominant role of the tangential gradients in the formation of the theta-model of the geomagnetic tail has recently been established¹⁰. The tangential gradients on the dayside magnetopause point in the direction of the polar neutral points, which are the sources and sinks of almost all of the magnetic field lines that make up the magnetopause. Thus the geometry of the magnetospheric field allows for lines of maximum gradient drift perpendicular to the magnetopause that connect the neutral points along the line of demarcation with the neutral sheet of the tail. We propose that magneto-sheath plasma enters the magnetosphere along these zones of maximum tangential gradient drift velocity, thereby identifying them with the polar cusps. Then some of the characteristics of the observed particle population within the polar cusps can be explained if one further assumes that the inward drift motion of the particles couples to strong pitch-angle diffusion¹¹. This assumption is partially justified by the measured pitch-angle distributions in the distant polar cusps which suggest that the distribution near the loss cone is more isotropic than that of the magnetosheath. It is also natural to assume that the polar cusp region will be highly unstable against various kinds of plasma waves, and hence wave-particle interactions leading to pitch-angle diffusion could occur there.