The mosaic structure of plasma bulk flows in the Earth's magnetotail

Abstract

Moments of plasma distributions observed in the magnetotail vary with different time scales. In this paper we attempt to explain the observed variability on intermediate timescales of ∼10–20 min that result from the simultaneous energization and spatial structuring of solar wind plasma in the distant magnetotail. These processes stimulate the formation of a system of spatially disjointed, highly accelerated filaments (beamlets) in the tail. We use the results from large‐scale kinetic modeling of magnetotail formation from a plasma mantle source to calculate moments of ion distribution functions throughout the tail. Statistical restrictions related to the limited number of particles in our system naturally reduce the spatial resolution of our results, but we show that our model is valid on intermediate spatial scales Δx × ΔZ ∼ 1 RE × 1000 km. For these spatial scales the resulting pattern, which resembles a mosaic, appears to be quite variable. The complexity of the pattern is related to the spatial interference between beamlets accelerated at various locations within the distant tail which mirror in the strong near‐Earth magnetic field. Global motion of the magnetotail results in the displacement of spacecraft with respect to this mosaic pattern and can produce variations in all of the moments (especially the x‐component of the bulk velocity) on intermediate timescales. The results obtained enable us to view the magnetotail plasma as consisting of two different populations: a tail ward‐Earthward system of highly accelerated beamlets interfering with each other, and an energized quasithermal population which gradually builds as the Earth is approached. In the near‐Earth tail, these populations merge into a hot quasi‐isotropic ion population typical of the near‐Earth plasma sheet. The transformation of plasma sheet boundary layer (PSBL) beam energy into central plasma sheet (CPS) quasi‐thermal energy occurs in the absence of collisions or noise. This paper also clarifies the relationship between the global scale where an MHD description might be appropriate and the lower intermediate scales where MHD fails and large‐scale kinetic theory should be used.

Interball-tail observations of vertical plasma motions in the magnetotail

“…The large-scale kinetic model of ion dynamics in the magnetotail (Ashour-Abdalla et al, 1995) suggests that in addition to thermal ion population in the plasma sheet, there exists an accelerated plasma population in the form of smallsize beamlets. In such a case, flow variations are due to multiple crossings of these spatial structures.…”

Resonances and Particle Stochastization in Nonhomogeneous Electromagnetic Fields

“…This explains tremendous practical interest in understanding the physical processes governing its storage/release cycle, controlling auroral emissions, radio communications and radiation hazards. At the same time the magnetotail is very interesting as a large laboratory for exploration of hot collisionless plasma whose quite a complex behavior can be at least partially deduced from simple mathematical models (Büchner and Zelenyi, 1989;Ashour-Abdalla et al, 1995;Lui, 2001). Magnetic field there is rather weak and trajectories of ions could not be described by the guiding center approximation.…”

Ion shell distributions as free energy source for plasma waves on auroral field lines mapping to plasma sheet boundary layer

“…First, there are critical values of κ given by κ crit n ≈0.8/(n+1/2), n=1, 2, 3, ... where the ions most easily spend a long time in the current sheet gaining energy from the electric field. Second, each beamlet bounces multiple times (labelled by k) across the equatorial plane while being convected earthward by the E×B drift (Ashour-Abdalla et al, 1995).…”

“…VDIS energy-latitude slopes have been used to find the location of the X-line, for example, Zelenyi et al (1990). Test particle calculations and observations show that deep in the magnetotail, the VDIS ions are decomposed of beamlets that can be labelled by two integers: the order of the κ parameter resonance and the number of bounces the beamlet has made (Ashour-Abdalla et al, 1995). Each beamlet has a different energy (field-aligned velocity) and, therefore, the perpendicular convection mixes and reorders the beamlets, causing a mosaic structure (Ashour-Abdalla et al, 1995).…”

“…Test particle calculations and observations show that deep in the magnetotail, the VDIS ions are decomposed of beamlets that can be labelled by two integers: the order of the κ parameter resonance and the number of bounces the beamlet has made (Ashour-Abdalla et al, 1995). Each beamlet has a different energy (field-aligned velocity) and, therefore, the perpendicular convection mixes and reorders the beamlets, causing a mosaic structure (Ashour-Abdalla et al, 1995). At lower altitude, the three-dimensional ion distribution function corresponding to VDIS has not been directly addressed in the literature.…”