Near-Earth injection of MeV electrons associated with intense dipolarization electric fields: Van Allen Probes observations

Substorms generally inject tens to hundreds of keV electrons, but intense substorm electric fields have been shown to inject MeV electrons as well. An intriguing question is whether such MeVelectron injections can populate the outer radiation belt. Here we present observations of a substorm injection of MeV electrons into the inner magnetosphere. In the premidnight sector at L ∼ 5.5, Van Allen Probes (Radiation Belt Storm Probes)-A observed a large dipolarization electric field (50 mV/m) over ∼40 s and a dispersionless injection of electrons up to ∼3 MeV. Pitch angle observations indicated betatron acceleration of MeV electrons at the dipolarization front. Corresponding signals of MeV electron injection were observed at LANL-GEO, THEMIS-D, and GOES at geosynchronous altitude. Through a series of dipolarizations, the injections increased the MeV electron phase space density by 1 order of magnitude in less than 3 h in the outer radiation belt (L > 4.8). Our observations provide evidence that deep injections can supply significant MeV electrons.

Rapid acceleration of protons upstream of earthward propagating dipolarization fronts

[1] Transport and acceleration of ions in the magnetotail largely occurs in the form of discrete impulsive events associated with a steep increase of the tail magnetic field normal to the neutral plane (Bz ), which are referred to as dipolarization fronts. The goal of this paper is to investigate how protons initially located upstream of earthward moving fronts are accelerated at their encounter. According to our analytical analysis and simplified two-dimensional test-particle simulations of equatorially mirroring particles, there are two regimes of proton acceleration: trapping and quasi-trapping, which are realized depending on whether the front is preceded by a negative depletion in Bz . We then use three-dimensional test-particle simulations to investigate how these acceleration processes operate in a realistic magnetotail geometry. For this purpose we construct an analytical model of the front which is superimposed onto the ambient field of the magnetotail. According to our numerical simulations, both trapping and quasi-trapping can produce rapid acceleration of protons by more than an order of magnitude. In the case of trapping, the acceleration levels depend on the amount of time particles stay in phase with the front which is controlled by the magnetic field curvature ahead of the front and the front width. Quasi-trapping does not cause particle scattering out of the equatorial plane. Energization levels in this case are limited by the number of encounters particles have with the front before they get magnetized behind it.

Static configurations and nonlinear waves in rotating nonuniform self-gravitating fluids

The equilibrium states and low-frequency waves in rotating nonuniform self-gravitating fluids are studied. The effect of a central object is included. Two-dimensional static configurations accounting for self-gravity, external gravity, and nonuniform rotation are considered for three models connecting the pressure with the mass density: thermodynamic equilibrium, polytropic pressure, and constant mass density. Explicit analytical solutions for equilibrium have been found in some cases. The low-frequency waves arising due to the vertical and horizontal fluid inhomogeneities are considered in the linear and nonlinear regimes. The relationship between the background pressure and mass density is supposed to be arbitrary in the wave analysis. It is shown that the waves considered can be unstable in the cases of polytropic pressure and constant mass density. The additional nonlinear term proportional to the product of the pressure and mass density perturbations, which is usually omitted, is kept in our nonlinear equations. There have been found conditions for this term to be important. Stationary nonlinear wave equations having solutions in the form of coherent vortex structures are obtained in a general form. The importance of involving real static configurations in the consideration of wave perturbations is emphasized.

Interchange mode in the presence of dust

The linear and nonlinear development of an electrostatic interchange mode which involves a magnetized nonuniform electron-ion fluid in the presence of nonuniform static charged dust grains is investigated. The charge on grains is taken as spatially dependent, and the consequences of that condition are investigated. It is shown that standardly accepted stabilization of the interchange mode in the presence of negatively charged grains can be violated due to the spatial dependence of the charge on grains. Also, the ion drift, which is caused by the action of a gravity term perpendicular to the magnetic field lines, is taken as nonuniform as a result of the magnetic field nonuniformity, and it is shown that due to such a nonuniformity the instability condition can be significantly modified. In the nonlinear regime several types of coherent stationary vortex structures are found: namely, dipolar and tripolar vortices and vortex chains. The dipolar vortex is found to propagate in the direction of the ion drift, while the tripole and vortex chains are carried by the drift flow. The spatial dependence of these structures is determined by parameters describing the nonuniformity of the equilibrium plasma.