near-Earth Plasma Research Articles

3-D hybrid simulations of magnetic reconnection in a thin current sheet

3-D hybrid simulations (ion particle, charge neutralizing massless electron fluid) of magnetic reconnection in a thin current sheet (thickness comparable to the relevant ion inertia length) are conducted. In this study, reconnection is initiated by putting localized anomalous resistivity. The thin current sheet, which models the near-Earth plasma sheet just before substorm onsets, is also unstable to the tail K-H instability that kinks the plasma sheet in the cross-tail plane. For smaller resistivity cases, the plasma sheet kink is already well developed by the time explosive reconnection starts. Despite this different circumstance, the temporal development of the reconnection jet is virtually unchanged from a high resistivity case reported previously. The structure of the reconnection jet is modified as it intrudes into the pre-existing kinked plasma sheet, but still, the essence of the ion kinetic response remains unchanged.

Development of substorms in the near-Earth tail

For substorm onsets, earthward flowing ions appear as an initial signal of the onset in the near-Earth plasma sheet in the onset sector. The flow activity in the onset sector is followed by a step-like increase in the Bz field in the equatorial plane, which results in a dipolarization in the field. The field change is usually associated with energization in ions and electrons. Outside of the onset sector the field dipolarization takes place. However, it may not be preceded by earthward flowing ions, and ions usually have the duskward flows in the evening sector and the dawnward flows in the morning sector. The onset signatures at synchronous orbit are delayed relative to those beyond 10 R E.

Quasilinear evolution of the ion cyclotron beam-anisotropy instability in a current carrying plasma

The evolution of particle velocity moments and wave spectra during the quasilinear saturation of the proton beam-anisotropy instability in a current carrying plasma is studied. For initial plasma moments characteristic of the near-Earth plasma sheet boundary layer, it is found that the saturation of the ion beam-anisotropy instability drives a field-aligned current, which self-consistently excites a kink-like mode. The driven current allows the wave fields of current driven mode to grow to amplitudes comparable to that of the anisotropy instability and facilitates more rapid isotropization of the unstable anisotropic beam components and saturation of the beam-anisotropy instability. This work has general relevance to the study of field-aligned instabilities in plasmas where the excited waves carry a net momentum density away from the source of instability.

New aspects of plasma sheet dynamics - MHD and kinetic theory

Abstract. Magnetic reconnection is a process of fundamental importance for the dynamics of the Earth's plasma sheet. In this context, the development of thin current sheets in the near-Earth plasma sheet is a topic of special interest because they could be a possible cause of microscopic fluctuations acting as collective non-idealness from a macroscopic point of view. Simulations of the near-Earth plasma sheet including boundary perturbations due to localized inflow through the northern (or southern) plasma sheet boundary show developing thin current sheets in the near-Earth plasma sheet about 810 RE tailwards of the Earth. This location is largely independent from the localization of the perturbation. The second part of the paper deals with the problem of the macroscopic non-ideal consequences of microscopic fluctuations. A new model is presented that allows the quantitative calculation of macroscopic non-idealness without considering details of microscopic instabilities or turbulence. This model is only based on the assumption of a strongly fluctuating, mixing dynamics on microscopic scales in phase space. The result of this approach is an expression for anomalous non-idealness formally similar to the Krook resistivity but now describing the macroscopic consequences of collective microscopic fluctuations, not of collisions.Key words. Magnetospheric physics (plasma sheet) · Space plasma physics (kinetic and MHD theory; magnetic reconnection)

D0.3-0051 counterstreaming electrons in the near-Earth plasma sheet

D0.3-0051 counterstreaming electrons in the near-Earth plasma sheet

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 boundary perturbations (like, e.g., Kelvin–Helmholtz modes) can result in filamentary current structures. This fundamental process is discussed with respect to extragalactic jets.

Magnetic reconnection: on new aspects of the microscopic cause of localized dissipation

In our paper we support the idea that at first glance very different topics in space physics and astrophysics, respectively, can include similar aspects such that progress made in one field may help to solve open questions in the other one. For these purposes we present recent results considering the occurrence of thin current sheets in the near-earth plasma sheet which are discussed to be of importance in the context of magnetospheric substorms. Studying this problem one comes upon a substantial open question referring to resistivity. An answer to that question may be found by the help of a theory for the collisionless relaxation which has been formulated first with respect to gravitating systems.

Interchange and kink modes in the near‐Earth plasma sheet and their associated plasma flows

Three‐dimensional (3‐D) particle simulations are used to investigate the response of the near‐Earth plasma sheet to the imposition of an external convection electric field for the case where magnetic flux is blocked from leaving the region. Under these conditions an interchange mode with characteristic wavelength of 1–2 RE (determined by the ion gyroradius) in the east‐west direction is excited before the plasma sheet is severed by the formation of a neutral line. The nonlinear growth of the interchange mode produces a strong (10–20 mV/m), localized ( ≲ 0.5 RE) Ey field which drives earthward flows of both electrons and ions. These flows pinch off regions of southward Bz which eventually are expelled tailward in narrow fingers around the edges of the earthward flow bursts.

Highly collimated electron beams observed during quiet times

Using 3D particle data from GEOTAIL, we have studied the occurrence of highly‐collimated bidirectional electrons in the magnetotail at −50 ≤ XGSM ≤ −9 Re during quiet intervals. They are found to appear frequently in the near‐Earth plasma sheet (−20 ≤ XGSM ≤ −10 Re), and more frequently on the dawn side than on the dusk side, with the maximum occurrence frequency of about several percentage. Both parallel/anti‐parallel components possess a characteristic flat‐top/bump shaped spectrum, which extends upto several hundreds eV to keV energy range. The balanced fluxes in both directions suggest that the electron beams are bouncing on closed field lines. Although these distribution shapes closely resemble those observed around the geosynchronous orbit and above the auroral zone, whose origins have been attributed to the active auroral acceleration, we report events that are detected in very quiet times (Kp ≤ 1°), which may cast a new question on the auroral processes during quiet intervals.

Helicon modes driven by ionospheric O+ ions in the plasma sheet region

The presence of ionospheric‐origin oxygen ions in the plasma sheet region results in only partial cancellation of the electron Hall current leading to the occurrence of the helicon mode rather than the Alfvén mode. It is shown that the presence of ionospheric‐origin oxygen ion beams with anisotropic pressure can excite helicon mode instability in the near‐Earth plasma sheet region provided their Alfvénic Mach numbers lie in a certain range. The helicon modes are easily excited under the conditions when the usual long wave‐lengths fire‐hose modes are stable. The typical real frequencies of the excited helicon modes are between 1 to 10 mHz, and the typical e‐folding time of the instability is about 3 to 15 minutes at wavelengths of 1 to 5 RE. Therefore these modes are likely to attain saturation during enhanced convection events lasting for a few hours. Large amplitude helicon modes would distort the ambient magnetic field and may be observable as flux ropes. Low‐frequency turbulence produced by these modes could scatter electrons and help excitation of the ion tearing modes leading to substorm onset.

Braking of high‐speed flows in the near‐Earth tail

We have studied possible braking mechanisms of high‐speed ion flows in the near‐Earth central plasma sheet for radial distances between 9 and 19 Earth Radii (RE) on the basis of observations made by the AMPTE/IRM satellite. Flows with velocities in excess of 400 km/s are almost always Earthward for this range, indicating that the source of the flows is beyond 19 RE. Though the occurrence rate of the high‐speed flows substantially decreases when the satellite comes closer to the Earth, high‐speed flows with velocities higher than 600 km/s are still observed. We suggest that the high‐speed flows are stopped at a clear boundary between the regions of dipolar field and tail‐like field in the plasma sheet. The boundary corresponds to the inner edge of the neutral sheet. The average jump of the magnetic field at the boundary, which is estimated from the observations by assuming a pressure balance, is 6.7 nT. The inertia current caused by the braking of the flow and the current caused by pileup of the magnetic flux at the stopping point are quantitatively estimated and discussed in relation to the formation of the substorm current wedge.

Observations of a current pulse in the near‐Earth plasma sheet associated with a substorm onset

The onset of an isolated magnetospheric substorm occurred at about 0437 UT on 9 February 1995 while the Geotail spacecraft was located in the premidnight sector at a geocentric radial distance of about 30 RE. The onset occurred during a slow northward turning of the interplanetary magnetic field after a period of weakly southward fields as observed by the Wind spacecraft upstream from Earth's bow shock. For several minutes immediately following the onset of the substorm an intense field‐aligned current, with maximum current densities of 30 nA/m², was observed at the Geotail spacecraft. This is the first report of a directly detected current pulse coincident with onset. Unlike most of the magnetotail field‐aligned currents which are carried primarily by electrons, this impulsive current at onset is carried by both electrons and protons. It is possible that the onset current pulse is a signature of a “trigger” mechanism for the subsequent expansion phase of a substorm.

Solar wind control of density and temperature in the near‐Earth plasma sheet: WIND/GEOTAIL collaboration

A statistical survey of GEOTAIL observations reveals the following properties of the near‐Earth plasma sheet (−15 < XGSM′ < −50 Re): During the periods when the northward IMF dominates, (1) the plasma sheet becomes significantly cold and dense, (2) the best correlations between the plasma sheet and the IMF parameters occur when the latter quantities are averaged over 9−4+3 hours prior to the plasma sheet observations, and (3) temperatures diminish and densities increase near the dawn and dusk flanks of the plasma sheet. We suggest that during prolonged northward IMF periods (∼ several hours) there is a slow diffusive transport of the plasma from the solar wind into the plasma sheet through the the magnetotail flanks.

Plasma sheet pressure changes during the substorm growth phase

This paper reports clear evidence for increases in plasma pressure in the plasma sheet at 20–50 Re during the growth phase of substorms. The spacecraft WIND has provided the IMF Bz and other solar wind conditions, while the spacecraft GEOTAIL has observed plasma and magnetic field in the plasma sheet. We have examined 19 isolated substorms after clear southward turnings of the IMF Bz in steady solar wind conditions. Pressure increases start immediately after the southward turnings of the IMF Bz. Increases in plasma pressure are caused mainly by increases in plasma number density. Temperature does not show any significant changes. These observations suggest that the near‐Earth plasma sheet is compressed by enhanced magnetic field intensity in the tail lobes during the substorm growth phase.

The fate of the outer plasmasphere

Both the solar wind and the ionosphere contribute to Earth's magnetospheric plasma environment. However, it is not widely appreciated that the plasmasphere is a large reservoir of ionospheric ions that can be tapped to populate the plasma sheet. We employ empirical models of high‐latitude ionospheric convection and the geomagnetic field to describe the transport of outer plasmasphere flux tubes from the dayside, over the polar cap and into the magnetotail during the early phases of a geomagnetic storm. We calculate that this process can give rise to high densities of cold plasma in the magnetotail lobes and in the near‐Earth plasma sheet during times of enhanced geomagnetic activity, and especially during storms. This model can help explain both polar cap ionization patches and the presence of cold flowing ions downtail.

Cluster - Science and Mission Overview

The European Space Agency's Cluster programme is designed to study the small-scale spatial and temporal characteristics of the magnetospheric and near-Earth solar wind plasma. The programme is composed of four identical spacecraft which will be able to make physical measurements in three dimensions. The relative distance between the four spacecraft will be varied between 200 and 18000 km during the course of the mission. This paper provides a general overview of the scientific objectives, the configuration and the orbit of the four spacecraft and the relation of Cluster to other missions.

The Cold-Dense Plasma Sheet: a GEOTAIL Perspective

GEOTAIL observations of the low-latitude boundary layer (LLBL) in the tail-flanks show that they are the region where the cold-dense plasma appears with stagnant flow signatures accompanied by bi-directional thermal electrons (< 300 eV). It is concluded from these facts that the tail-LLBL is the site of capturing the cold-dense plasma of the magnetosheath origin on to the closed field lines of the magnetosphere. There are also cases that strongly suggest that the cold-dense plasma entry from the flanks can be significant to fill a substantial part of the magnetotail. In such cases, the cold-dense plasma is not spatially restricted to a layer attached to the magnetopause (that is, the LLBL), but continues to well inside the magnetotail, constituting the cold-dense plasma sheet. Inspired by the fact that these remarkable cases are found for northward interplanetary magnetic field (IMF), a statistical study on the status of the near-Earth plasma sheet is made. The results show that the plasma sheet becomes significantly colder and denser when the northward IMF continues than during southward IMF periods, and that the cold-dense status appears most prominently near the dawn and dusk flanks. These are consistent with the idea that, during northward IMF periods, the supply of cold-dense ions to the near-Earth tail from the flanks dominates over the hot-tenuous ions transported from the distant tail.

Driven reconnection in the near-Earth plasma sheet

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.

Global modelling study (GSM TIP) of the ionospheric effects of excited &amp;lt;i&amp;gt;N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;&amp;lt;/i&amp;gt;, convection and heat fluxes by comparison with EISCAT and satellite data for 31 July 1990

Abstract. Near-earth plasma parameters were calculated using a global numerical self-consistent and time-dependent model of the thermosphere, ionosphere and protonosphere (GSM TIP). The model results are compared with experimental data of different origin, mainly EISCAT measurements and simultaneous satellite data (Ne and ion composition). Model runs with varying inputs of auroral FAC distributions, temperature of vibrationally excited nitrogen and photoelectron energy escape fluxes are used to make adjustments to the observations. The satellite data are obtained onboard Active and its subsatellite Magion-2 when they passed nearby the EISCAT station around 0325 and 1540 UT on 31 July 1990 at a height of about 2000 and 2200 km, respectively. A strong geomagnetic disturbance was observed two days before the period under study. Numerical calculations were performed with consideration of vibrationally excited nitrogen molecules for high solar-activity conditions. The results show good agreement between the incoherent-scatter radar measurements (Ne, Te, Ti) and model calculations, taking into account the excited molecular nitrogen reaction rates. The comparison of model results of the thermospheric neutral wind shows finally a good agreement with the HWM93 empirical wind model.

Centrifugally driven phase bunching and related current sheet structure in the near‐Earth magnetotail

We examine the role of centrifugal effects during nonadiabatic interactions of charged particles with the magnetotail current sheet. It is shown that when the parameter κ (defined as the square root of the minimum curvature radius‐to‐maximum Larmor radius) is of the order of unity, as is the case for ions traveling in the near‐Earth plasma sheet, enhanced centrifugal effects lead to prominent bunching of the particles in gyration phase. As a result of this bunching effect we demonstrate that a thin current sheet develops in the vicinity of the tail midplane. When average values of the plasma density (a few tenths of ions per cubic centimeter) and temperature (several keV) in the near‐Earth tail are used, the current sheet obtained has a characteristic thickness of the order of a few tenths of an Earth radius and leads to significant stretching of the local magnetic field lines. A further consequence of phase bunching is the buildup of a substantial current in the Earth‐tail direction at low latitudes, which leads to field line inclination in the dawn‐dusk direction. This phase bunching mechanism, which maximizes when the bulk of the ion distribution nears κ = 1, is of potential importance for the dynamics of the inner plasma sheet during the growth phase of substorms.