One of the most important sources of magnetospheric plasma is particle entry through the distant magnetotail boundary, the nightside magnetopause. This entry mechanism depends on the magnetopause configuration. Off the equator, the strong lobe magnetic field renders the magnetopause a tangential or a rotational discontinuity, and thus the magnetosheath field orientation predominantly controls particle entry through magnetic reconnection. At the equatorial, distant tail magnetopause, however, the magnetic field's control of particle entry is diminished because the plasma beta there is large on both sides of the boundary. Thus, transport there can be significantly different from that at the dayside and off‐equatorial magnetopauses. Using observations from two Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun probes, we investigate plasma transport mechanisms around the distant equatorial magnetopause. We find that transport occurs as a series of abrupt transitions in density, ion and electron temperatures, and ion kinetic energy of spatial scales as small as a typical plasma sheet ion gyroradius. Analysis of the particle phase space density reveals that an energy‐selection mechanism controls electron transport across the magnetopause, whereas ion transport is likely controlled by spatial diffusion driven by low‐frequency magnetic field fluctuations. We discuss the importance of these fluctuations for the magnetopause structure (e.g., the thickness of the transitions in plasma density, ion and electron temperatures, and ion kinetic energy).

Long-term simulations of energetic electron fluxes in many space plasma systems require accounting for two groups of processes with well separated time-scales: a microphysics of electron resonant scattering by electromagnetic waves and a macrophysics of electron adiabatic heating/transport by mesoscale plasma flows. Examples of such systems are Earth's radiation belts and Earth's bow shock, where ion-scale plasma injections and cross-shock electric fields determine a general electron energization, whereas electron scattering by waves relaxes anisotropy of electron distributions and produces small populations of high-energy electrons. The application of stochastic differential equations is a promising approach for including effects of resonant wave–particle interaction into codes tracing electrons in models of large-scale electromagnetic fields. This study proposes and verifies such equations for the system with non-diffusive wave–particle interactions, i.e., the system with nonlinear effects of phase trapping and bunching. We consider electron resonances with intense electrostatic whistler-mode waves often observed in the Earth's radiation belts. We demonstrate that nonlinear resonant effects can be described by stochastic differential equations with the non-Gaussian probability distribution of random variations of electron energies.

One of the main sources of magnetospheric particles is solar wind penetration through the magnetopause-a current sheet separating the cold, dense magnetosheath from the hot, rarified magnetospheric plasmas. The mechanism responsible for magnetosheath particle transport across the magnetopause has been better investigated for the near-Earth dayside magnetopause (contrary to the nightside), where it was found that such a transport is controlled by the current sheet thickness, structure, and dynamics. Because plasma properties and magnetic field intensity in the magnetosheath significantly change as a function of radial distance from the Earth, the current sheet characteristics are different near Earth and at distant nightside magnetopause. Comparative investigations between near-Earth and distant magnetopauses, however, require statistical observations at the two locations during the same time interval, that enable control for the same upstream solar wind driving conditions. In this paper, we perform such a study using the four Magnetosheric Multiscale (MMS) and two Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) probes to compare the characteristics of magnetic field and plasma populations during magnetopause crossings, which are separated by about 50 R E. We find that the current sheet profiles is similar at two locations. We also show that the magnetopause current sheet thickness scales with the local magnetosheath ion gyroradius. A weaker magnetic field on both sides of the current sheet is correlated with smaller current density at lunar distances.

In a statistical model plasma sheet By primarily depends on interplanetary magnetic field (IMF) Byi and geodipole tilt τ. With 11 years of Geotail measurements we investigate a role of several other parameters with a linear regression model. Optimal averaging window of IMF input, maximizing correlation and regression coefficients, is found to be 2.25 hr. Influence of IMF Bzi and local Bz on IMF penetration (regression with regard to Byi) and the deviations from the predefined warp deformation are at the level 5–10% relative to the primary model coefficients. The IMF penetration beyond 25 RE is somewhat larger for northward IMF, while closer to the Earth it becomes somewhat larger for southward IMF. These smaller effects turned out to be rather uneven across the tail, making reliable quantification and physical interpretation not always possible. The major reasons of difficulties are uneven coverage and internal correlations in the input space ( Byi–τ– Bzi) due to combination of spacecraft orbit and neutral sheet dynamics, effects of coordinate transformations, etc. In particular, origins of extremely large IMF Byi penetration (order of 30–50% above the average one) for some years and tail locations remain not fully clear. A larger multispacecraft data set covering all seasons in all spatial zones is necessary to further advance in this study.

Force-free plasma equilibria are expected to form in the solar corona, while in-situ spacecraft observations have shown that force-free equilibria are formed in the planetary magnetotails. In this paper, we develop fluid models of two-dimensional axially symmetric force-free equilibria and discuss similar slab equilibria. The group theory approach is used to find the symmetry groups and reduce the Grad-Shafranov equation with exponential and power law nonlinearities to ordinary differential equations for the self-similar (automodel) solutions that we analyze analytically and numerically. Force-free equilibria of the developed class have a magnetotail-type configuration with magnetic field lines stretched in the radial direction and represent nonlinear force-free equilibria, because rot B=α(r) B with α(r)≠const. Making use of the same symmetry groups, we generalize the developed force-free equilibria by including a finite plasma pressure gradient and compare different equilibria of the developed class. These models can be useful for describing the structure and stability of current sheets observed in planetary magnetotails and formed in the solar atmosphere.

Current sheets with strong transverse (cross field) currents are commonly observed in planetary magnetospheres and serve as a natural energy source for magnetic reconnection. As the most investigated current sheet, the current sheet in the Earth's magnetotail forms in a high-β plasma, with hot ions dominantly contributing to the diamagnetic currents. Spacecraft observations have shown, however, that a superthin electron dominated current sheet can be embedded in the Earth's magnetotail current sheet. In this paper, we develop a model of such superthin current sheets with strong currents produced by anisotropic electrons. We also compare the model with spacecraft observations, which shows reasonable agreement in spatial profiles and magnitudes of the current density. The spatial scale (thickness) of the superthin current sheet is controlled by the equatorial magnetic field component, whereas the current density magnitude is controlled by the electron fire-hose parameter at the equator. Although the current density peak within the superthin current sheet can significantly exceed the background (embedding) current density, the magnetic field magnitude at the superthin current sheet boundary does not exceed 10% of the total magnetic field magnitude. These superthin current sheets are sub-ion (or even electron-scale) structures, which are not sufficiently large/intense to perturb ion dynamics. We discuss applications of the proposed model for the analysis of plasma instabilities in superthin electron-dominated current sheets.

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We study average magnetic field growth in a mirror-symmetrical Kazantsev turbulent flow near the dissipative scales. Our main attention is directed to a subcritical regime, when an exponential decrease of magnetic energy is usually expected. We show that instead of damping, transient energy growth can be obtained, for example, in stochastic processes supported by the large-scale magnetic fields. We calculate the longitudinal correlation functions and demonstrate that they can tend to nonzero stationary solutions, whose localization width is inversely proportional to the square of the magnetic Reynolds numbers and with amplitude depending on the closeness of these numbers to the critical value. We present the local generation effect without any external support, predicted by Zeldovich in 1956. Numerically solving the initial-boundary Kazantsev problem on the nonuniform grids, we simulate this process by implicit schemes and discuss the possible consequences of subcritical growth for dynamo theory.