Earth's Plasma Research Articles

Differences between CME‐driven storms and CIR‐driven storms

Twenty one differences between CME‐driven geomagnetic storms and CIR‐driven geomagnetic storms are tabulated. (CME‐driven includes driving by CME sheaths, by magnetic clouds, and by ejecta; CIR‐driven includes driving by the associated recurring high‐speed streams.) These differences involve the bow shock, the magnetosheath, the radiation belts, the ring current, the aurora, the Earth's plasma sheet, magnetospheric convection, ULF pulsations, spacecraft charging in the magnetosphere, and the saturation of the polar cap potential. CME‐driven storms are brief, have denser plasma sheets, have strong ring currents and Dst, have solar energetic particle events, and can produce great auroras and dangerous geomagnetically induced currents; CIR‐driven storms are of longer duration, have hotter plasmas and stronger spacecraft charging, and produce high fluxes of relativistic electrons. Further, the magnetosphere is more likely to be preconditioned with dense plasmas prior to CIR‐driven storms than it is prior to CME‐driven storms. CME‐driven storms pose more of a problem for Earth‐based electrical systems; CIR‐driven storms pose more of a problem for space‐based assets.

Magnetospheric influence on the Moon's exosphere

Atoms in the thin lunar exosphere are liberated from the Moon's regolith by some combination of sunlight, plasma, and meteorite impact. We have observed exospheric sodium, a useful tracer species, on five nights of full Moon in order to test the effect of shielding the lunar surface from the solar wind plasma by the Earth's magnetosphere. These observations, conducted under the dark sky conditions of lunar eclipses, have turned out to be tests of the differential effects of energetic particle populations that strike the Moon's surface when it is in the magnetotail. We find that the brightness of the lunar sodium exosphere at full Moon is correlated with the Moon's passage through the Earth's magnetotail plasma sheet. This suggests that omnipresent exospheric sources (sunlight or micrometeors) are augmented by variable plasma impact sources in the solar wind and Earth's magnetotail.

Dissipation scales in the Earth's plasma sheet estimated from Cluster measurements

Abstract. In order to estimate the dissipation time-scale in magnetic turbulence in the plasma sheet a novel method is introduced for classification of velocity dependent patterns of two-point probability density functions' shapes near their maxima. For the first time, we provide evidence for Reynolds number (velocity) dependent widening of the inertial range in the plasma sheet. Since spectral widening of the inertial range is a generic feature of many turbulent flows, its examination can facilitate the recognition of intermittent turbulence in the plasma sheet.

Statistical survey of magnetic and velocity fluctuations in the near‐Earth plasma sheet: International Sun Earth Explorer (ISEE‐2) measurements

International Sun‐Earth Explorer (ISEE‐2) satellite observations of low‐frequency magnetic field and ion velocity fluctuations in the Earth's magnetotail are compared with corresponding observations from the Active Magnetospheric Particle Tracer Explorers/Ion Release Module (AMPTE/IRM). Solar activity during these two missions is different and thus also considered. The ISEE‐2 analysis is performed on the data taken from satellite passes through the Earth's plasma sheet in 1978 and 1979 near solar maximum. The data is divided into 2298 ten‐minute‐long intervals where the averages of the magnetic field, ion velocity, and plasma parameter's amplitudes are statistically calculated for each interval. We find no significant dependence of the amplitude of the ion velocity and magnetic field fluctuations on the magnetotail distance or on the distance from the neutral sheet. The correlation coefficient between the geomagnetic activity and the amplitude of both ion velocity and magnetic field fluctuations in the ISEE‐2 measurements is smaller than the one found for the AMPTE/IRM passes through the plasma sheet. When comparing the two satellite data sets for the same plasma sheet region, we obtain a significantly higher upper bound for the ISEE‐2 fluctuating magnetic field energy density. Our results suggest that the significantly increased magnetic field fluctuation amplitude in the plasma sheet during the ISEE‐2 satellite passes is due to the increased solar activity near solar maximum.

Ground Magnetic Field Disturbance Caused by the Hall Current of Bubbles in the Earth's Plasma Sheet

AbstractThe bursty bulk flows observed by Baumjohann and Paschmann, and Angelopoulos et al. have been claimed by Chen and Wolf as bubbles in the Earth's plasma sheet. Most of the major observed characteristics of the bursty bulk flows (hereafter abbreviated as BBFs) can be interpreted naturally in terms of bubble picture. In the bubble model, bubbles carry a great percentage of the total potential drop in the ionosphere. Hereby we can estimate the Hall current around a bubble and its disturbance on the ground magnetic field. From the comparison with the characteristics of BBFs observed by the Scandinavian Magnetometer Array, a very good general agreement between the two disturbances can be seen. This implies that BBFs are very likely just the bubbles proposed by Chen and Wolf.

MHD turbulence in the Earth's plasma sheet: Dynamics, dissipation, and driving

The turbulent flows and tangled magnetic fields of the Earth's plasma sheet are explored theoretically and by means of ISEE‐2 plasma and magnetic‐field measurements. The goal is to obtain a basic understanding of (1) the dynamics, (2) the driving, and (3) the dissipation of the turbulence. Dynamically, the turbulence of the plasma sheet appears to be a turbulence of eddies, rather than a turbulence of Alfven waves or other MHD modes. In this respect, it is similar to the two‐dimensional turbulence of the solar wind. Previously published statistical arguments that the correlation length (= integral scale = eddy size) of the turbulence fluctuations in the plasma sheet is ℓeddy ∼ 1.6 RE are confirmed by analyzing plasma sheet magnetic‐field measurements during two special “sweeping” intervals that follow the passage of interplanetary shocks. For dissipation of the turbulence, two mechanisms appear to be important. The first is a cascade of energy in the turbulence to small spatial scales, where internal dissipation at non‐MHD spatial scales should occur. The second mechanism is electrical coupling of the turbulent flows to the resistive ionosphere, which introduces a “quasi‐viscosity” to the plasma sheet. This quasi‐viscosity is complicated owing (1) to a time delay associated with Alfven‐transit‐time coupling (introducing a viscoelasticity to the turbulence), (2) to a scalesize dependence of the coupling (introducing a hypoviscosity), and (3) to a dependence of the coupling on the sign of the flow shear (introducing a sign‐vorticity effect). For the coupling of the plasma sheet turbulence to the ionosphere, retarded‐time Reynolds numbers R* are derived to describe the importance of the resulting dissipation (quasi‐viscosity) and a Deborah number D is derived to describe the importance of the time lag (elasticity). The difference of the plasma sheet turbulence from homogeneous turbulence is discussed; this dissimilarity is owed to (a) dissipation at all wave numbers in the plasma sheet, (b) the presence of boundaries, and (c) the limited range of spatial scales that will allow scale‐invariant dynamics. A better description of the plasma sheet turbulence would be “turbulence in a box” or a turbulent wake.

Cold ions in the hot plasma sheet of Earth's magnetotail.

Most visible matter in the Universe exists as plasma. How this plasma is heated, and especially how the initial non-equilibrium plasma distributions relax to thermal equilibrium (as predicted by Maxwell-Boltzman statistics), is a fundamental question in studies of astrophysical and laboratory plasmas. Astrophysical plasmas are often so tenuous that binary collisions can be ignored, and it is not clear how thermal equilibrium develops for these 'collisionless' plasmas. One example of a collisionless plasma is the Earth's plasma sheet, where thermalized hot plasma with ion temperatures of about 5 x 10(7) K has been observed. Here we report direct observations of a plasma distribution function during a solar eclipse, revealing cold ions in the Earth's plasma sheet in coexistence with thermalized hot ions. This cold component cannot be detected by plasma sensors on satellites that are positively charged in sunlight, but our observations in the Earth's shadow show that the density of the cold ions is comparable to that of hot ions. This high density is difficult to explain within existing theories, as it requires a mechanism that permits half of the source plasma to remain cold upon entry into the hot turbulent plasma sheet.

Interchange instability in the presence of the field‐aligned current: Application to the auroral arc formation

We have revisited the ideal MHD interchange instability for the case of the nonaligned gradients of the plasma pressure p and unit magnetic flux tube volume V, i.e., the background field‐aligned current (FAC) included. Several possibilities of a directional difference between ∇p and ∇V in the Earth's plasma sheet are considered. The growth of an interchange perturbation leading to the pressure‐gradient‐driven FAC striation is illustrated through numerical simulation in a simple model of the nightside region 1 FAC by Karty et al. [1984] and Yang et al. [1994]. It is shown that the FAC is being striated by the interchange process on a timescale from minutes to tens of minutes. The application to the quiet time discrete arc formation is discussed.

Solar–wind–magnetosphere–ionosphere interactions in the Earth's plasma environment

The properties of the Earth’s coupled magnetosphere-ionosphere system are dominated by its interaction with the solar wind plasma, mediated by magnetic reconnection processes at the magnetopause interface. In this paper we focus on recent progress and remaining questions concerning the nature of these interactions, how they depend on the orientation of the interplanetary magnetic field (IMF), and the consequences which follow for internal dynamics. The progress we report, on flux transfer events and substorms during intervals of southward IMF, and magnetopause and tail processes during intervals of northward IMF, emphasise the great diagnostic power of combined in situ and remote sensing observations from space and on the ground.

Acceleration of dust grains by means of electromagnetic cyclotron waves

It is shown that charged dust grains can be accelerated by the nonuniform space charge electric fields that are created by the ponderomotive force of circularly polarized electromagnetic cyclotron waves. The relevance of our investigation to dust energization in the Earth's mesospheric plasma is stressed.

Modified electron whistler dispersion law

A modified electron whistler dispersion law is derived in the cold-plasma approximation for analytical treatment and simplified numerical calculations of wave propagation in a wide range of ratios ωc/ωp of electron gyro- to plasma frequencies if the wave frequency is much less than ωp. The net contribution of ions to the wave dispersion law is expressed through the value of the lower-hybrid resonance frequency ωlhr only. This approximate dispersion law is valid in a wide frequency domain, that is, from the range of ωlhr until the domain where the contribution of ions can be neglected. A comparison of geometrical-optics ray trajectories calculated by the use of modified and total cold-plasma electron whistler dispersion laws is presented for the case of the Earth's plasma environment. Computer simulations of dynamical spectra of whistler waves excited by lightning discharges and registered in remote regions of the Earth's plasmasphere reveal good numerical stability of the developed ray-tracing code.

Development of a compact EUV photometer for imaging the planetary magnetosphere

We have developed a lightweight, compact, and highly sensitive Extreme ultraviolet (EUV) photometer for a space‐borne instrument. The photometer is onboard Japan's Mars orbiter, Planet‐B, which was successfully launched on July 4, 1998, and stayed in the parking orbits around the Earth for 6 months. The new photometer is designed for studying the spatial distribution of helium ions and atoms by detecting He II (304 Å) and He I (584 Å), which are major emission lines detectable around a planet. The photometer is a type of normal‐incidence telescope that consists of a mirror, filters, and a detector. The mirror employs a new technology of a molybdenum/silicon multilayer coating and is designed to have peak reflectivity at 304 Å. The photometer is capable of taking two‐dimensional images by using the spin and orbital motions of the spacecraft. There are open questions concerning the Earth's plasmasphere and plasma sheet that cannot be resolved by in situ observations alone (e.g., global shape of the plasmasphere and cold ions in the plasma sheet). Our observations with the EUV photometer over a reasonable number of orbits will answer them. The photometer also shows an outstanding performance of measuring the Martian helium.

A revised forest-fire cellular automaton for the nonlinear dynamics of the Earth's magnetotail

Recent observations and numerical simulations seem to suggest that the Earth's magnetotail plasma could exist in a near-criticality configuration, and that many features of the magnetospheric response to solar wind changes could be described by avalanche models displaying self-organized criticality. Further evidences of this near-criticality dynamics have been found analyzing the statistical features of the auroral electrojet (AE) index. Here, we present a cellular automaton, based on a revised version of the well-known forest-fire model, for the nonlinear dynamics of the Earth's magnetotail. This simple model, chaotically driven using a 1-d coupled map, is able to capture many of the statistical features of the magnetospheric response to solar wind changes. The results, compared with previous analyses of the AE-index features, are discussed in the framework of a near-criticality dynamics of the magnetospheric tail plasma.

Lumped Element Model for Spacecraft Surface Charging Studies

The external surfaces of a spacecraft exposed to Earth's magnetospheric plasma can build up differential voltages which can result in an electrostatic discharge (ESD). This discharge may induce transient electromagnetic interference (EMI) in the electronics circuitry and may give rise to switching anomalies or system failure under certain environmental conditions. This paper outlines the development of Lumped Element Model (LEM) of a spacecraft for surface charging studies. A numerical solution procedure using method of moments and triangular patch modelling to determine the capacitance of conducting objects in free space has been developed. Differential equations of spacecraft charging have been derived and the surface charging of INSAT-2E spacecraft has been simulated. The results obtained for INSAT-2E using LEM concept are compared with those obtained using the NASA Charging Analysis Program (NASCAP) for a similar satellite.Using LEM of a conducting plate located in free space, and with the help of circuit simulation software, the transient structural current due to a blow-off discharge is predicted. Power supply lines, command and control lines, lines connected to antenna probes, switches and thence to microwave devices can pick up ESD transients. Hence ESD immunity should be built into microwave subsystem as well. Techniques of hardening microwave subsystem against on-board ESD have also been outlined.

The Lunar Poles: Water Ice or Chemically Trapped Hydrogen?

An alternative explanation is proposed for the hydrogen excess observed by the Lunar Prospector neutron spectrometer: solar wind protons trapped on radiation defects in regolith particles and effectively retained at the temperature of the lunar poles can be misinterpreted as water. Protons from the Earth's magnetotail plasma can be a source of hydrogen atoms in the regolith of permanently shadowed areas of the lunar surface.

Multiple‐spacecraft observation of a narrow transient plasma jet in the Earth's plasma sheet

We use observations from five magnetospheric spacecraft in a fortuitous constellation to show that narrow transient plasma flow jets of considerable length formed in the tail can intrude into the inner magnetosphere and provide considerable contribution to the total plasma transport. A specific auroral structure, the auroral streamer, accompanied the development of this narrow plasma jet. These observations support the ‘boiling’ plasma sheet model consisting of localized underpopulated plasma tubes (bubbles) moving Earthward at high speeds as a realistic way to resolve the ‘convection crisis’ and to close the global magnetospheric circulation pattern.

Medium Energy Neutral Atom (MENA) Imager for the Image Mission

The Medium Energy Neutral Atom (MENA) imager was developed in response to the Imaging from the Magnetopause to the Aurora for Global Exploration (IMAGE) requirement to produce images of energetic neutral atoms (ENAs) in the energy range from 1 to 30 keV. These images will be used to infer characteristics of magnetospheric ion distributions. The MENA imager is a slit camera that images incident ENAs in the polar angle (based on a conventional spherical coordinate system defined by the spacecraft spin axis) and utilizes the spacecraft spin to image in azimuth. The speed of incident ENAs is determined by measuring the time-of-flight (TOF) from the entrance aperture to the detector. A carbon foil in the entrance aperture yields secondary electrons, which are imaged using a position-sensitive Start detector segment. This provides both the one- dimensional (1D) position at which the ENA passed through the aperture and a Start time for the TOF system. Impact of the incident ENA on the 1D position-sensitive Stop detector segment provides both a Stop-timing signal and the location that the ENA impacts the detector. The ENA incident polar angle is derived from the measured Stop and Start positions. Species identification (H vs. O) is based on variation in secondary electron yield with mass for a fixed ENA speed. The MENA imager is designed to produce images with 8 4 angular resolution over a field of view 140 360, over an energy range from 1 keV to 30 keV. Thus, the MENA imager is well suited to conduct measurements relevant to the Earth's ring current, plasma sheet, and (at times) magnetosheath and cusp.

Solitary Waves in a Collisionless Plasma with Isothermal Pressure

Wave resonances in the hydrodynamic model of an isotropic collisionless quasi-neutral hot plasma with isothermal ions and electrons are considered. These resonances lead to the formation of two types of solitary waves: solitary waves proper and generalized solitary waves. The latter result from the nonlinear resonance of the proper solitary waves with magnetosonic and Alfven periodic waves. The possibility of observing these waves in the Earth's magnetospheric plasma is discussed.

Evolution of Electron Phase-Space Holes in a 2D Magnetized Plasma

The nonlinear stage of the two-stream instability in a 2D magnetized plasma produces electron phase-space tubes, the counterpart of phase-space holes in a 1D plasma. These tubes align primarily perpendicular to the magnetic field ${\mathbf{B}}_{0}$ and have self-consistent bipolar electric fields parallel to ${\mathbf{B}}_{0}$. Such bipolar electric fields have recently been observed in four different regions of the Earth's space plasma environment. Massively parallel 2D kinetic simulations show the dynamics of tube formation, evolution, and breakup, accompanied by the generation of electrostatic whistler waves. We focus on the breakup of the tubes and describe a new numerical study of tube stability.

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)