Debye Length Scale Research Articles

Explanation for the simultaneous occurrence of bipolar structures and waves near ion‐cyclotron harmonics in the auroral ionosphere

In the downward current region of the auroral ionosphere, the FAST spacecraft has observed bipolar electrostatic structures on Debye length scales and waves at frequencies between the H+ ion cyclotron harmonics. Such bipolar structures have been previously identified with the nonlinearly evolved state of a two‐stream electron instability. In this paper, we present the results of long‐duration and large‐scale particle‐in‐cell (PIC) simulations which produce, from one set of initial conditions, both bipolar electrostatic structures and, at later times, ion Bernstein waves with peak intensities between multiples of the ion cyclotron frequency. The ion Bernstein waves are driven by a weaker beam instability caused by a residual positive slope in the nonlinearly evolved (nonthermal) electron distribution. Although there are a variety of processes which can produce ion Bernstein modes, we show that a common source (an electron beam) can produce both of these observed phenomena in the downward current region.

Formation of electrostatic potential drops in the auroral zone

In order to examine the self-consistent formation of large-scale quasi-static parallel electric fields in the auroral zone on a micro/meso scale, a particle in cell simulation has been developed. The code resolves electron Debye length scales so that electron micro-processes are included and a variable grid scheme is used such that the overall length scale of the simulation is of the order of an Earth radii along the magnetic field. The simulation is electrostatic and includes the magnetic mirror force, as well as two types of plasmas, a cold dense ionospheric plasma and a warm tenuous magnetospheric plasma. In order to study the formation of parallel electric fields in the auroral zone, different magnetospheric ion and electron inflow boundary conditions are used to drive the system. It has been found that for conditions in the primary (upward) current region an upward directed quasi-static electric field can form across the system due to magnetic mirroring of the magnetospheric ions and electrons at different altitudes. For conditions in the return (downward) current region it is shown that a quasi-static parallel electric field in the opposite sense of that in the primary current region is formed, i.e., the parallel electric field is directed earthward. The conditions for how these different electric fields can be formed are discussed using satellite observations and numerical simulations.

Photonic band gaps in dusty plasma crystals

A two-dimensional model for electromagnetic wave propagation in a dusty plasma crystal is solved using plane wave expansion techniques. We show that photonic band gaps, where propagation is forbidden, can occur for a range of Debye length scale sizes with respect to the dust particle size. We find that band gap features are dependent on the plasma sheath characteristics of the dusty plasma crystal. In addition we compute the damping due to the conductivity of the dusty plasma crystal.

Reconnection: Space plasma simulations for multi-spacecraft satellite observations in the ISTP Era

In the ISTP era multi-spacecraft measurements have not only become a tool for phenomenological and correlative studies of the solar- terrestrial coupling. They also open new opportunities to test, verify or falsify basic physical concepts. For the use of multi-spacecraft measurements in this sense numerical modelling and simulations are becoming more and more a necessary tool for interpreting the observations correctly. We take the reconnection process as an example. Reconnection is supposed to release magnetic field energy from the sun, to couple the Earth's magnetosphere to the solar wind and to suddenly release previously stored magnetic energy in the course of magnetospheric substorms. Hence, reconnection is a key concept in the solar-terrestrial relationship. On the other hand it is difficult to theoretically grasp collisionless reconnection in space plasmas. This complicates its experimental verification.In this paper we present some selected essentially kinetic features of spontaneous reconnection through collisionless thin current sheets in two and three dimensions: These should be observable, e.g., at the onset of magnetospheric substorm. The kinetic approach is necessary due to the cross-scale coupling, inherent to collisionless reconnection. We demonstrate that microscopic magnetic islands of the Debye length scale are always present through thin current sheets. They are, however, reversible and occur at the level of thermal fluctuations. Macroscopic reconnection and island structure formation starts with irreversible energy transfer from fields to plasma. We demonstrate the different routes to macroscopic merging in two and three dimensions. In two dimensions Landau damping in the potential wells of magnetic islands causes the classical slow tearing mode instability. In three dimensions a sausage mode bulk current instability accelerates the transition to macroscopic reconnection. As a result three-dimensional magnetic islands are formed, which are limited in all three spatial directions. We, finally, demonstrate how multi-spacecraft observations of the ISTP era together with well posed computer simulations can help to understand col lisionless reconnection by means of in situ measurements in the Earth's magnetosphere.