Earth's Magnetic Tail Research Articles

Magnetic Fields and Plasmas

Plasmas and magnetic fields are inseparably related in numerous physical circumstances. This is not only the case in natural occurring plasmas like the solar corona and the earth magnetic tail, but also in laboratory plasmas like tokamaks and stellarators.

Magnetic Fields and Plasmas

Plasmas and magnetic fields are inseparably related in numerous physical circumstances. This is not only the case in natural occurring plasmas like the solar corona and the earth magnetic tail, but also in laboratory plasmas like tokamaks and stellarators.

Substorm Conference in Japan highlights the importance of new ISTP data

“Substorm” is the term used to designate a period of enhanced energy input into the magnetosphere from the solar wind. For over 2 decades it has been generally accepted that energy enters through the merging of the interplanetary magnetic field with the frontside Earth's field lines. Transport of magnetic flux into the Earth's magnetic tail follows, and reconnection of the tail field lines across the tail neutral sheet along a “neutral line” with ensuing Earthward magnetic flux transport back to near‐Earth regions. The return of magnetic flux from the tail was understood to occur in impulsive bursts and these were thought to be associated with the violent bursts of auroral and electromagnetic activity that defined the substorm expansive phase.

Magnetic Fields and Plasmas

Plasmas and magnetic fields are inseparably related in numerous physical circumstances. This is not only the case in natural occurring plasmas like the solar corona and the earth magnetic tail, but also in laboratory plasmas like tokamaks and stellarators.

PREFACE

It is a great pleasure to dedicate this special issue of Physica Scripta to Prof. Karl Schindler, my mentor and close friend for almost 25 years. Karl Schindler is one of the leading theorists in the field of space plasma physics. His work has largely formed our present theoretical understanding of the Earth's magnetic tail. This is the prime site of energy storage and sudden release, associated with the arguably most dramatic phenomenon in the Earth's magnetosphere, the "magnetospheric substorm." Substorms are the main cause of sudden injections of energetic charged particles into the inner magnetosphere and ionosphere and of the brightening of the auroras (the northern and southern polar lights).

Magnetospheric tail structure: concepts, problems and storm-time development of the auroral oval

This paper reviews current knowledge on links between the Earth's magnetic tail and the auroral oval, and identifies some problems remaining. It considers electrons as tracers of the geomagnetic field, boundaries between different regions, plasma flows, and pressure balance conditions. The auroral arc is considered as a standing discontinuity in the flow of central plasma sheet (CPS) plasma field-aligned current systems are also proposed. The plasma instability responsible for the breakup phase of an auroral substorm is also discussed.

Magnetohydrodynamic Riemann problem and the structure of the magnetic reconnection layer

We present a complete solution for a set of magnetohydrodynamic (MHD) Riemann problems in which the upstream and downstream states have the same total pressure, and in which the normal component of the magnetic field is very small. These solutions are pertinent to subfast flows in the earth's magnetic tail and near the magnetopause. In a coplanar situation a family of solutions exists that depend on two parameters as well as on dissipation mechanisms. In the parallel case the transverse magnetic field either does not change direction or changes the direction twice by involving two intermediate shocks. In the antiparallel case an intermediate shock is always required, except when the solution consists of two switch‐off shocks. In a noncoplanar case the solution is not self‐similar as a function of x/t, but continues to evolve. At early times the evolution is similar to the coplanar case. In general two time‐dependent intermediate shocks are required to rotate the magnetic fields. The velocity shear has strong effect on the Riemann solution. In some cases no Riemann solution can exist because of the cavitation caused by the slow rarefaction waves. The calculated magnetopause structure resembles the observed structure for northward interplanetary magnetic field (IMF). However, for southward IMF, the MHD result shows the existence of a depletion layer, which is not supported by observations. We also show that on the magnetosheath side, the Walén relation, which is exact for a rotational discontinuity, can also be well satisfied by a slow shock, an intermediate shock, or the head of a slow rarefaction wave.

Birkeland currents in the plasma sheet

A search was conducted for the signatures of Birkeland currents in the Earth's magnetic tail, using observed values of Bx and By from large sets of spacecraft data. The data were binned by x and y for −10 > xGSM > −35 and |yGSM| ≤ 20RE (≤ 30 RE for xGSM ≤ −25 RE) and in each bin their distribution in the (Bx, By) plane was fitted by least squares to a piecewise linear function. That gave average x‐y distributions of the flaring angle between Bxy and the x direction, as well as that angle's variation across the thickness of the plasma sheet. Angles obtained in the central plasma sheet differed from those derived near the lobe boundary. That is the expected signature if earthward or tailward Birkeland current sheets are embedded in the plasma sheet, and from this difference we derived the dawn‐dusk profiles of the tail Birkeland currents for several xGSM intervals. It was found that (1) the Birkeland currents have the sense of region 1 currents, when mapped to the ionosphere; (2) both the linear current density (kiloamperes/RE) and the net magnitude of the field‐aligned currents decrease rapidly down the tail; (3) the total Birkeland current at x ≈ −10 RE equals ≈500‐700 kA, which is ∼30% of the net region 1 current observed at ionospheric altitudes, in agreement with model mapping results; and (4) the Bz and By components of the interplanetary magnetic field influence the distribution of Birkeland currents in the tail.

A scenario for solar wind penetration of Earth's magnetic tail based on ion composition data from the ISEE 1 spacecraft

Energetic (0.1‐16 keV/e) ion data from the Plasma Composition Experiment on the ISEE 1 spacecraft show that Earth's plasma sheet (inside of 23 RE) always has a large population of H+ and He++ ions. This population is the largest, both in absolute numbers and relative to the terrestrial ions, during periods of extremely weak geomagnetic activity and is then also the most “solar wind like”, in the sense that the He++/H+ density ratio is at its peak (about 3% on average in 1978 and 79), and the H+ and He++ ions have mean (thermal) energies that are proportional to ionic mass and barely exceed the typical bulk flow energies in the solar wind. These quiet time conditions occur when the IMF is persistently northward and not excessively strong. At more active times the H+ and He++ ions are heated in the central plasma sheet, but they can still be found with lower solar wind like energies closer to the tail lobes, at least during plasma sheet thinning. In a number of cases examined here, such low‐energy H+ and He++ ions are found flowing antisunward close to the lobes, flowing roughly parallel (or antiparallel) to the tail magnetic field at speeds ranging from a few tens to a few hundred kilometers per second, as though they have recently entered the tail earthward of the ISEE 1. In some cases the flow vector appears to contain a significant E × B component directed away from the nearest tail flank. In order to explain these flows, as well as the solar wind like appearance of the plasma sheet as a whole during northward interplanetary magnetic field, it is argued that the solar plasma enters along slots between the tail lobes and the plasma sheet, even quite close to Earth, convected inward along the plasma sheet boundary layer, or adjacent to it, by the electric fringe field of the ever present low‐latitude magnetopause boundary layer (LLBL). The main point of the argument is that the required E × B drifts are produced by closing LLBL equipotential surfaces through the plasma sheet.

Approximate magnetotail equilibria with parallel flow

Approximate equilibrium solutions for the two‐dimensional magnetohydrodynamic equilibrium equations with flow along magnetic field lines applied to modeling the Earth's magnetic tail are constructed. An alternative version of the condition for whether a given magnetospheric configuration is open or closed is presented. Also developed are examples of open and closed magnetospheric configurations with flow occurring in a layer modeling the plasma sheet boundary layer. Examples of finite‐size compressible plasmoids traveling in the magnetotail are presented. The relation between the profile of the total pressure (thermal plus magnetic) and the shape of the plasmoid is also discussed.

U.S. and Japan launch Geotail satellite to probe Earth's magnetic tail

U.S. and Japan launch Geotail satellite to probe Earth's magnetic tail

New observations from ISEE‐3

On June 30, 1983, the International Sun‐Earth Explorer‐3 spacecraft (ISEE‐3) was at an apogee of 236 earth radii (RE) exploring particle and field phenomena in the earth's distant magnetic tail region. Most of what we know about the earth's magnetotail is based on spacecraft observations made inside of the lunar orbit distance (approximately 60 RE), and a few brief passes of several interplanetary spacecraft through the more distant tail. ISEE‐3 is greatly extending the exploration of the geotail by spending a large part of 1983 in the earth's magnetic tail between 60 and 240 RE.There will be a special session at the 1983 AGU Fall Meeting which will focus on the new geotail results. In December, ISEE‐3 will make a final lunar swing‐by, coming within 100 km of the surface of the moon. This maneuver will propel ISEE‐3 out into the solar system, where it will pass through the tail of comet Giacobini‐Zinner in early September 1985. Later, in March‐April 1986, ISEE‐3 will monitor the solar wind conditions near Halley's comet while other spacecraft from around the world get a close‐up view. ISEE‐3 celebrated the fifth anniversary of its launch on August 12.

The structure of Titan's wake from plasma wave observations

During the Voyager 1 flyby of Saturn's moon Titan, the plasma wave instrument detected several types of plasma wave emissions. On the inbound leg a broad region of intense low‐frequency noise was detected on the side of Titan facing away from Saturn. This noise has characteristics similar to the electric field turbulence observed in the magnetosheath at the earth and the ionosheath at Venus and is believed to be generated by newly created ions that are being accelerated in the vicinity of Titan by the corotational electric field. During the pass through the induced magnetic tail of Titan, a series of upper hybrid resonance emissions were observed. The electron density profile inferred from these emissions shows three distinct peaks with densities of ∼40 cm−3, the first peak corresponding to the entry into the magnetic tail, the second peak corresponding to the neutral sheet crossing from the northern to the southern tail lobe, and the third, somewhat smaller, peak corresponding to the outbound exit from the tail. Large depressions in the magnetic field strength are observed coincident with each of the density peaks. These effects indicate that a dense plume of plasma is being carried downstream of Titan by the interaction with the rapidly rotating magnetosphere of Saturn. By equating the magnetic field pressure in the tail lobe to the plasma pressure in the neutral sheet, the temperature of the plasma is estimated to be about 8600°K. This low temperature suggests that the plasma originates from the ionosphere of Titan, probably forming a plume of plasma with a θ or H cross section extending downstream from Titan. Within the tail lobes, a second type of low‐frequency electric field noise was observed, with characteristics very similar to a type of noise called broadband electrostatic noise, which is found in the earth's magnetic tail. As in the case of the earth, this noise is most intense near the outer boundary of the plasma sheet and is almost completely absent in the high‐density region near the neutral sheet.

Anomalous resistivity in the geomagnetic tail region

The beam cyclotron instability and electron acoustic instability, driven by cross-tail current and inhomogeneity in density and magnetic field, are found to be unstable in the earth's magnetic tail region. The anomalous resistivities due to these instabilities are found to be of the order of (10 −1−10 −3)Ω e −1 (Ω e being the electron gyro frequency). It is also suggested that the non-linear saturation of the beam cyclotron instability may lead to conditions favourable for exciting ion acoustic instability.

Plasma flow pulsations in Earth's magnetic tail

During a 5‐hour interval on November 9, 1972, Imp 7 detected strong earthward plasma flows within the plasma sheet. The plasma flow exhibited a sequence of nine clear temporal pulsations in which the bulk speed varied from zero up to 1000–1500 km/s. The pulsations occurred at 15‐ to 30‐min intervals. These speed variations are shown to be positively correlated with changes in the standard deviation of the magnetic field (15‐s accumulations). During the flow pulsations, ground magnetograms exhibited only weak high‐latitude substorm activity, which is consistent with the Heos 2 detection of low values for the interplanetary field, with only a moderate southward component. Closer to earth, ATS 5 and Imp 6 detected some plasma heating during the period of Imp 7 flow pulsations. However, there was no evident periodicity corresponding to the Imp 7 flow pulsations in any of the other magnetospheric or solar wind data. Integrated over the 5‐hour interval, the strong plasma flows injected roughly 1022 ergs into the inner magnetosphere, yet both the solar wind and the ground magnetogram data are consistent with the magnetosphere having been in a relatively quiet state with only weak substorm activity along a contracted auroral oval. We suggest that the pulsations may be evidence that tail reconnection is inherently unsteady, with 10‐ to 30‐min periods.

A model of the magnetospheric substorm

IT is generally recognised that the Earth's magnetic tail and the plasma sheet play a vital role in the processes responsible for the observational phenomenon known as the magnetospheric substorm. Many different models1–5 of substorm behaviour have been described with differing degrees of success, and to propose another requires some justification. A new model, if it is to be useful, must do two things. It must be specific enough to make quantitative theoretical analysis possible, and it must fit the morphological features more completely or in a more elegant way than other models. In addition, it should be distinctive enough from other models to be separated from them by observational tests. I believe the model presented here has those properties.

Comparative analyses of observations of lunar transient phenomena

From the author's collection of more than 900 reports of Lunar Transient Phenomena (LTP) covering the period 1540–1970, 771 positive plus 112 negative observations (several times more than any previously published analyses) with sufficient ancillary data were analyzed for five hypotheses of causes. Approximately one third were of Aristarchus, divided almost equally between one observer, Bartlett, and all others. Treated as two groups they were divided into four categories: (1) gaseous, (2) reddish, (3) bluish, and (4) brightenings, and analyzed separately and combined with respect to the hypotheses. Six other individual LTP sites also were analyzed. The five hypotheses involved effects of tides, sunrise, low-angle illumination, Earth's magnetic tail, and solar particles. Novel analyses were made by [1] histograms with respect to (a) phase (φ) of anomalistic period, (b) Moon's age; [2] plot on a representative cycle of anomalistic orbits; [3] ratios of observed to expected percentages ( O E ); [4] plot of distributions of (a) LTP sites (<100), (b) dark, flat-floored craters, and (c) the author's (1967) ringdike candidates. The histograms show: [1] caprice in analytical behavior between (a) the sites individually, even between near neighbors, and collectively, (b) the different categories, and (c) Bartlett and All Others, especially in φ: [2] any tidal correlations found were almost exclusively with perigee; [3] in lunar age, rather persistent correlations were found with feature sunrise and with the magnetic tail; [4] differences between Bartlett and All Others for the reddish phenomena, and similarities between the Bartlett bluish and All Others' brightenings suggest variant optical physiological responses, indicating higher blue-sensitivity and and lower(?) red-sensitivity for Bartlett; [5] inverse relationships between Bartlett's gaseous and reddish phenomena in both tidal and age diagrams, and the same inverse relation between his reddish and All Others' suggest that Bartlett glimpsed true ground color in the absence of obscuring matter; [6] the sharp perigee and apogee correlations found by Burley and Middlehurst (1966) smeared out or disappeared in the much larger number of observations of all sites analyzed here. Most perigee or apogee correlations clustered more at times of greater orbital eccentricity than at lesser eccentricity, as shown by the orbital plot; [7] absent and present (especially the onset) phenomena correlated similarly, instead of oppositely as expected, and both were found to advance around the orbits on the orbital plot, which weakens any positive correlations found; [8] phenomena have been reported for all ages, from O d to 28 d with >50% within ±4 d of full Moon ( < 1 4 near perigee). The ratios ( O E ) revealed: [1] universely high correlations with sunrise, not all accountable by observational selection; [2] rather high correlation with Earth's magnetopause, but not the bow-shock front turbulence; [3] decline of perigee correlation with increasing number of observations, from ( O E ) = 1.9 to 1.1 ; [4] little correlation with direct bombardment of solar flare particles but apparent enhancement by the magnetic tail when the Moon was within it at time of arrival of the particles in the Earth-Moon system. Some facts indicate that some of the brightenings result from unknown atmospheric, instrumental, and geometric effects, but many puzzling aspects still remain. Comparison of the distributions (nonrandom) of the LTP sites, dark craters, and the author's proposed ringdikes shows strong affinities for the mare edges for each, implying internal activity (probably degassing) as the source of most LTP. Considering everything the data show, the author is skeptical that there are any decisive external (even tidal) influences on this internal activity released at LTP sites.

Plasma trapped at the end of a sheet current in a dipole field

A two-dimensional model is considered in which a pocket of infinitely conducting plasma, free of magnetic field, is trapped near the end of a current sheet in the presence of a dipole magnetic field. The configuration somewhat resembles that of Earth's magnetic tail. The problem is solved by the use of complex variable methods involving conformal transformations.

Relationship between Thunderstorm Frequency and Lunar Phase and Declination

Using the superposed-epoch method, we statistically analyzed thunderstorm frequencies for 108 stations in eastern and central United States in relation to lunar positions for the years 1930–1933 and 1942–1965. The results are as follows. With full moon as key day, a peak in thunderstorm frequency occurs for 1953–1963 two days after full moon, the mean being 2.7σ above average level. This mean frequency is 4.8σ above the mean when computed only for the full moons as key days that have a declination of 17° north or more. We suggest that these increases near full moon may be related to the earth's magnetic tail and the neutral sheet.

Propagation of low-frequency MHD waves through the earth's magnetic tail

Es wird die Problematik der Ausbreitung von MHD-Wellen sehr niedriger Frequenzen durch den magnetischen Erdschweif studiert. Man stellt fest, dass sich fur solche Wellen die geometrische Konfiguration und die physikalischen Bedingungen im Schweife und an seiner Grenze den Eigenschaften der hydromagnetischen Wellenleitung nahern. In Berucksichtigung der Bewegung des Plasmas im Schweife, die radial von der Erde vorgeht, ist ein einfaches Modell rechtwinkliger, homogener, hydromagnetischer Wellenleitung mit magnetischem Langsfeld konstruiert worden, das cine Approximation fur jede von den Halften der ϑ-Form des Schweifes darstellt. An diesem Modell werden in den Grundzugen einige Eigenschaften der Leitung von MHD-Wellen niedriger Frequenzen durch den magnetischen Erdschweif studiert.