Forced current sheets in the Earth's magnetotail: Their role and evolution due to nonadiabatic particle scattering

Abstract

Current sheets in the Earth's magnetosphere may be considered as the regions of the acceleration and heating of solar wind and ionospheric plasmas. We devote our attention to the formation of very thin current sheets (TCSs) in the magnetotail. Various models of TCS equilibrium are discussed. We extend our previous self-consistent analysis of TCS formation. We delineate the relative role of two types of ion populations: (1) current- carrying Speiser's ions, impinging from the mantle, (2) nonadiabatic ions resulting from chaotic scattering of ions on transient Speiser-type orbits. To take into account the effect of population (2) we use the centrifugal impulse model, which treats the scattering of the particle magnetic moments as the result of perturbation of the gyromotion by an impulsive centrifugal force. Using several “snapshots” of the scattered distribution function, we obtain a series of quasi-stationary TCS equilibria. It is shown that nonadiabatic effects do not influence significantly the TCS thickness but generally alter the structure of the “young” Speiser-orbit based current sheet, “polluting” the sheet by particles with large magnetic moments, which suppress the positive current in the center of the sheet. In other words, TCS are “aging” due to scattering. When the quasi-trapped population becomes too abundant, the equilibrium solutions decay. The characteristic lifetime of such TCS due to intrinsic nonadiabaticity of the system is found to be about 10–60 minutes.