H. Wiechen

Shear flow instabilities in magnetized partially ionized dense dusty plasmas

Shear flow instabilities may play an important role in the dynamics of partially ionized dusty plasmas. Within a multifluid approach the onset criteria with and without electrical resistivity are derived. Long wave length modes can be stabilized by dust–neutral gas collisional momentum transport. Self-consistent numerical simulations of the multifluid plasma dynamics illustrate the nonlinear development of vortical structures in the dust, ion, and neutral velocity as well as in the magnetic field. The unstable modes lead to a significant local amplification of the magnetic field strength. Moreover, electric current sheets form that also show vortices.

Reconnection in the near‐Earth plasma sheet: A three‐dimensional model

We present results of three-dimensional MHD simulations of reconnection in the near-Earth plasma sheet. We use two-dimensional start configurations which include the Earths dipole field explicitly and three-dimensional resistivity profiles. These equilibria are stable against large-scale tearing instability, that is, the mode with the maximum growth rate. For nonhomogeneous resistivity profiles, however, the system may become unstable against nonlinear, resistive modes. We discuss onset and nonlinear evolution of reconnection due to these instabilities. One result of our three-dimensional simulations is that earthward flow can be found even tailward of the reconnection region. This confirms a typical feature of previous two-dimensional simulations of reconnection in the near-Earth plasma sheet. The nonlinear resistive instabilities found in our simulations could be relevant for phenomena like pseudobreakups.

Radiative condensation modes in dense dusty plasmas

Thermal condensation modes in static partially ionized dense dusty plasmas are investigated by linear mode analysis. A necessary and sufficient onset criterion for a coupled dust-neutral gas condensation mode is found with the help of the Hurwitz theorem. In the long wavelength limit one finds a coupled condensation mode and a new dust mode in the short wavelength limit. The growth rates of the modes are limited by elastic collisions between the different plasma components.

Reconnection in a dipole‐dominated magnetosphere: A two‐dimensional model

We study the problem of onset of reconnection and the corresponding nonlinear evolution of reconnection in the near-Earth plasma sheet, where the influence of the Earths dipole field is essential. This is done by the help of two-dimensional resistive MHD simulations. The simulations start from a two-dimensional equilibrium model for the near-Earth plasma sheet, including the Earths dipole field explicitly. The stability properties of this start configuration differ significantly from the stability of two-dimensional tail equilibria, including both plasma sheet and lobe. This allows us to study onset and evolution of resistive modes that are linearly stable but non-linearly unstable. We find qualitatively significant differences, as compared with simulations of tail configurations. For instance, we find sunward flow both tailward and earthward of the reconnection X line. The corresponding reconnection rates come out to be about an order of magnitude lower compared with reconnection as a result of a large-scale tearing mode in tail configurations. We show that the reconnection rates can be significantly increased by suitable modifications of the start configuration.

Current filamentation in astrophysical magnetohydrodynamic jets

Astrophysical plasmas typically have to be considered as highly collision-free from a kinetic point of view or highly ideal in a fluid description. This refers to near-Earth space plasmas, like the magnetosphere, or extragalactic objects like radio jets, as well. On the other hand, magnetic reconnection is discussed to play an important role as an effective mechanism for conversion of magnetic energy into kinetic energy by particle acceleration. As magnetic reconnection demands for local deviations from adiabatic plasma conditions, in the highly collision-free plasma regime inertia driven reconnection should be expected. In the framework of a magnetohydrodynamic description, anomalous resistivity due to current driven microinstabilities is discussed to provide sufficient nonidealness. Both classes of processes demand for sufficiently thin current sheets. This constraint can be fulfilled by current filamentation. In the present paper numerical magnetohydrodynamical studies are used to show that ideal bound...

Self-magnetization of protoplanetary accretion disk matter

Shear-flow induced friction between neutral gas and charged particle components of different mass can yield a significant self-magnetization of matter. In this paper this process is discussed with respect to protoplanetary disk matter consisting of charged massive dust grains, neutral gas, ions and electrons. Self-consistent three-dimensional multi-fluid plasma—neutral gas—dust simulations are presented taking into account typical parameters for protoplanetary accretion disk matter like that of our early solar system. The results of the simulations show that self induced magnetic fields of 10−5 Tesla up to 10−3 Tesla can be expected in a protoplanetary accretion disk on very short time scales of a few years. Thus, shear-flow induced self-magnetization can yield a significant contribution to the magnetization of the early solar system.

The origin of primeval magnetic fields: Self-consistent simulation of magnetic field generation and amplification

A self-consistent numerical model of self-generation and amplification of magnetic fields in weakly ionized protogalactic matter is presented. This model is based on three-dimensional two-fluid simulations of the dynamics expected for protogalactic plasma–neutral gas clouds characterized by shear flow. Due to the friction between the ionized and neutral gas components self-generated magnetic fields of seed field order, i.e., 10−17 G are created on time scales of 105 yr. Moreover, unstable Kelvin–Helmholtz modes are excited yielding a significant amplification of the field.

A high-latitude boundary layer crossing-INTERBALL measurements and MHD model results

Abstract We analyze the highly variable magnetic field behavior after a high-latitude outbound boundary layer crossing of the INTERBALL-1 main satellite on August 26, 1995, using data obtained by the fluxgate magnetometer FGM-1. We suggest that the oscillations, observed immediately after the first magnetopause crossing, are due to a Kelvin-Helmholtz instability. We verify this suggestion by appropriate MHD simulations. For the sake of a direct comparison we present the simulation results in a frame moving through the propagating structure at the relative speed of the satellite along its orbit. As a result we have found that the observed signatures compare well with the simulated crossing of a Kelvin-Helmholtz unstable magnetopause.

Driven reconnection in the near-Earth plasma sheet

Abstract In some recent MHD simulations of the near-Earth plasma sheet we studied onset and evolution of reconnection due to non-linear resistive instabilities. In our present contribution we show that these non-linear instabilities can be amplified significantly by inflow through the plasma sheet boundary and we discuss the consequences of that driving mechanism on the global dynamics of the instabilities. For high magnetic Reynolds numbers we find thin current sheets developing.

INTERBALL-1 plasma sheet encounters and three-dimensional MHD modeling results

Abstract We use magnetic field observations by the fluxgate magnetometer FGM-I during two substorm related INTERBALL-1 plasma sheet encounters to discuss signatures of reconnection in the Earths magnetotail. In the first case the satellite stayed northward of the current sheet; in the second case for most of the substorm time it stayed in the northern lobes until it approached the plasma sheet and, finally, crossed the current sheet. In order to determine whether the observed magnetic field signatures can be understood in the framework of a resistive magnetohydrodynamic (MHD) approach to reconnection, we generated data using a three-dimensional numerical MHD model of the tail. In the model, reconnection takes place due to a localized finite resistivity. Comparing the modeled data with the measured ones, we find that, although the substorm-related dipolarization-like evolution of the B x and the B z field components can be well understood in terms of a resistive MHD approach to reconnection with dipolarization starting before the substorm ground onset and geostationary injection tailward of the satellite position, all attempts failed to explain the observed transient dawnward drop of the shear component B y by a resistive MHD model of three-dimensional reconnection. We conclude that the dawnward drop has to be caused by effects beyond the resistive MHD approach.