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Center for Nonlinear Studies |
The Earth's inner magnetosphere (r < 8 Earth's radii) can be characterized by nearly dipolar magnetic field. The inner magnetosphere contains populations of particles trapped in the magnetic field. Understanding the dynamics of the trapped particles is essential for understanding evolution of the radiation belts and ring current. Both are of critical importance to space-weather science, which attempts to protect our technological infrastructure from the adverse effects of the space environment. The problem of trapped charged particle motion in a dipole magnetic field, however, is intrinsically insoluble and full solution of this problem can only be achieved by numerical integration. The theory of adiabatic invariants (i.e., approximate integrals of motion associated with different periodic motions of particles) is often employed to describe particle orbits. The first adiabatic invariant, magnetic moment, is associated with fast gyromotion across the magnetic field. It leads to the concept of 'adiabatic loss cone', which determines when trapped particles can become lost by scattering into the atmosphere. Furthermore, magnetic moment is the most robust among the three adiabatic invariants and thus its possible breaking is never considered unless plasma waves with frequency comparable to the gyro-frequency are present in the environment. In this talk, I will show that, even in absence of plasma waves, motion of charged ions in the inner magnetosphere can become nonadiabatic and even small deviations from adiabaticity will result in a number of important changes. First, I will show that when ion energy increases or radius of curvature of equatorial magnetic field decreases, adiabatic loss cone modifies noticeably. This modification can be described by a simple quasi-adiabatic model which works well even for significant deviations from adiabaticity. Second, nonadiabatic effects can lead to diffusion of charged particles which occur when cyclotron motion of particles around magnetic field line resonates with longitudinal oscillations of bounce motion between mirror points and widths of neighboring resonances overlap. I will show that such diffusion becomes important for the ring current decay during geomagnetic storms.
Host: Chris Neale