Imaging Plasma Density Structures in the Soft X-Rays Generated by Solar Wind Charge Exchange with Neutrals

Both heliophysics and planetary physics seek to understand the complex nature of the solar wind’s interaction with solar system obstacles like Earth’s magnetosphere, the ionospheres of Venus and Mars, and comets. Studies with this objective are frequently conducted with the help of single or multipoint in situ electromagnetic field and particle observations, guided by the predictions of both local and global numerical simulations, and placed in context by observations from far and extreme ultraviolet (FUV, EUV), hard X-ray, and energetic neutral atom imagers (ENA). Each proposed interaction mechanism (e.g., steady or transient magnetic reconnection, local or global magnetic reconnection, ion pick-up, or the Kelvin-Helmholtz instability) generates diagnostic plasma density structures. The significance of each mechanism to the overall interaction (as measured in terms of atmospheric/ionospheric loss at comets, Venus, and Mars or global magnetospheric/ionospheric convection at Earth) remains to be determined but can be evaluated on the basis of how often the density signatures that it generates are observed as a function of solar wind conditions. This paper reviews efforts to image the diagnostic plasma density structures in the soft (low energy, 0.1–2.0 keV) X-rays produced when high charge state solar wind ions exchange electrons with the exospheric neutrals surrounding solar system obstacles.The introduction notes that theory, local, and global simulations predict the characteristics of plasma boundaries such the bow shock and magnetopause (including location, density gradient, and motion) and regions such as the magnetosheath (including density and width) as a function of location, solar wind conditions, and the particular mechanism operating. In situ measurements confirm the existence of time- and spatial-dependent plasma density structures like the bow shock, magnetosheath, and magnetopause/ionopause at Venus, Mars, comets, and the Earth. However, in situ measurements rarely suffice to determine the global extent of these density structures or their global variation as a function of solar wind conditions, except in the form of empirical studies based on observations from many different times and solar wind conditions. Remote sensing observations provide global information about auroral ovals (FUV and hard X-ray), the terrestrial plasmasphere (EUV), and the terrestrial ring current (ENA). ENA instruments with low energy thresholds (\(\sim1~\mbox{keV}\)) have recently been used to obtain important information concerning the magnetosheaths of Venus, Mars, and the Earth. Recent technological developments make these magnetosheaths valuable potential targets for high-cadence wide-field-of-view soft X-ray imagers.Section 2 describes proposed dayside interaction mechanisms, including reconnection, the Kelvin-Helmholtz instability, and other processes in greater detail with an emphasis on the plasma density structures that they generate. It focuses upon the questions that remain as yet unanswered, such as the significance of each proposed interaction mode, which can be determined from its occurrence pattern as a function of location and solar wind conditions. Section 3 outlines the physics underlying the charge exchange generation of soft X-rays. Section 4 lists the background sources (helium focusing cone, planetary, and cosmic) of soft X-rays from which the charge exchange emissions generated by solar wind exchange must be distinguished. With the help of simulations employing state-of-the-art magnetohydrodynamic models for the solar wind-magnetosphere interaction, models for Earth’s exosphere, and knowledge concerning these background emissions, Sect. 5 demonstrates that boundaries and regions such as the bow shock, magnetosheath, magnetopause, and cusps can readily be identified in images of charge exchange emissions. Section 6 reviews observations by (generally narrow) field of view (FOV) astrophysical telescopes that confirm the presence of these emissions at the intensities predicted by the simulations. Section 7 describes the design of a notional wide FOV “lobster-eye” telescope capable of imaging the global interactions and shows how it might be used to extract information concerning the global interaction of the solar wind with solar system obstacles. The conclusion outlines prospects for missions employing such wide FOV imagers.     

Galactic and Extragalactic Magnetic Fields

The current state of research of the Galactic magnetic field is reviewed critically. The average strength of the total field derived from radio synchrotron data, under the energy equipartition assumption, is 6±2 G locally and about 10±3 G at 3 kpc Galactic radius. These values agree well with the estimates using the locally measured cosmic-ray energy spectrum and the radial variation of protons derived from -rays. Optical and synchrotron polarization data yield a strength of the local regular field of 4±1 G, but this value is an upper limit if the field strength fluctuates within the beam or if anisotropic fields are present. Pulsar rotation measures, on the other hand, give only 1.4±0.2 G, a lower limit if fluctuations in regular field strength and thermal electron density are anticorrelated along the pathlength. The local regular field may be part of a `magnetic arm between the optical arms. However, the global structure of the regular Galactic field is not yet known. Several large-scale field reversals in the Galaxy were detected from rotation measure data, but a similar phenomenon was not observed in external galaxies. The Galactic field may be young in terms of dynamo action so that reversals from the chaotic seed field are preserved, or a mixture of dynamo modes causes the reversals, or the reversals are signatures of large-scale anisotropic field loops. The Galaxy is surrounded by a thick disk of radio continuum emission of similar extent as in edge-on spiral galaxies. While the local field in the thin disk is of even symmetry with respect to the plane (quadrupole), the global thick-disk field may be of dipole type. The Galactic center region hosts highly regular fields of up to milligauss strength which are oriented perpendicular to the plane. A major extension of the data base of pulsar rotation measures and Zeeman splitting measurements is required to determine the structure of the Galactic field. Further polarization surveys of the Galactic plane at wavelengths of 6 cm or shorter may directly reveal the fine structure of the local magnetic field.     

The effect of rotation on the hydrodynamics of stellar collapse

Summary Using values of d, _min, and _max that Van Riper (1978) has found most promising for a hydrodynamic envelope ejection, we have shown that even a small amount of rotation in the initial core can stop its collapse before nuclear densities are reached. We expected _i > 0.02 to produce significant deviations from a spherically symmetric collapse, but have found that _i as much as ten times smaller than this will not allow the core to reach densities as high as in the spherical collapse. In no case, however, does the core flatten very much, nor does the value of become very large. Low final 's preclude the formation of an axisymmetric torus. They also indicate that deformation of an iron core into a triaxial configuration or fragmentation of the core during its collapse is an extremely unlikely event. (Note: Classically, must exceed 0.27 before a dynamic instability to non-axisymmetric perturbations is encountered.)The small degree of flattening of the core also suggests that the reduced moment of inertia I of the core will always be relatively small in magnitude and hence that the third time derivative of I, which is proportional to the energy emitted in gravity wave radiation, will not be very significant. Numerically calculated estimates of I- during some of these model evolutions supports this suspicion. If the _min and used here are found to be realistic values after the detailed physics of the core collapse is well understood, it is clear that gravitational radiation from a core collapse will be difficult to measure.Finally, we should point out that it is the relatively large values of Y_min (near 4/3) combined with values of d near unity that (a) prevented the core from flattening significantly in these models and (b) prevented the core from reaching high configurations. If realistic values of either one (or both) of these parameters are found to be much smaller in more complete models of the core collapse, then the core will have to become flatter (and denser) before pressure gradients will support it along the rotation axis. All of the conclusions drawn here would be modified accordingly under those circumstances. It should also be noted that in general relativistic models, the critical for spherical collapse is somewhat larger than 4/3 (Van Riper, 1979). Therefore, we predict that when fully general relativistic core collapses are performed including rotation, a given choice of _min and _i will produce a slightly flatter and slightly denser core than the corresponding model that has been presented here.     

Intergalactic magnetic fields, and some connections with cosmic rays

An overview is presented of the methods of probing for the geometry, and strength of intergalactic magnetic fields. Recent results are briefly surveyed for galaxy halos, galaxy clusters, and the intergalactic medium on various scales, and some rele vant physical processes and radiation processes are mentioned, as well as the coupling between intergalactic magnetic fields and cosmic rays.The general trend of recent results indicates that, wherever we detect intergalactic hot gas and galaxies, we also find magnetic fields at levels of 10^–7 G, or higher. The hitherto undetected, weaker fields in the ratified i.g.m. and in large intergalactic voids could be probed by both Faraday rotation, and possibly using very energetic CR nuclei (> 10²⁰eV), and/or transient extragalactic ray bursts.     

The magnetogram inversion technique: Applications to the problem of magnetospheric substorms

The magnetogram inversion technique (MIT) is based upon recordings of geomagnetic variations at the worldwide network of ground-based magnetometers. MIT ensures a calculation of a global spatial distribution of the electric field, currents and Joule heating in the ionosphere. Variant MIT-2 provides, additionally, continuous monitoring of the following parameters: Poynting vector flux from the solar wind into the magnetosphere (); power, both dissipated and accumulated in the magnetosphere; magnetic flux in the open tail; and the magnetotail length (l _T) (distance between the dayside and nightside neutral points in the Dungey model). Using MIT-2 and data of direct measurements in the solar wind, an analysis is made of a number of substorms, and a new scenario of substorms is suggested. The scenario includes the convection model, the model with a neutral line and the model of magnetosphere-ionosphere coupling (outside the current sheet), i.e., the three known models. A brief review is given of these and some other substorms models. A new element in the scenario is the strong positive feedback in the primary generator circuit, which ensures growth of the ratio = / _Aby an order of magnitude or more during the substorms. Here _Ais the Pointing vector flux in the Akasofu-Perrault approximation, i.e., without the feedback taken into account. The growth of during the substorm is caused only by the feedback effect. It is assumed that the feedback arises due to an elongation of the magnetotail, i.e., a growth of l _Tby a factor of (23) during the substorm.In the active phase of substorm, a part (the first active phase) has been identified, where the principal role in the energetics is played by the feedback mechanism and the external energy source (although the internal source plus reconnection inside the plasma sheet make a marked contribution). In the second active phase (expansion) the external generator (solar wind) is switched off, and the main role is now played by the internal energy source (the tail magnetic field and ionospheric wind energy).Models of DP-2 DP-1 transitions are also considered, as well as the magnetospheric substorm-solar flare analogy.     

Tearing instability of reconnecting current sheets in space plasmas

Magnetic reconnection provides an efficient conversion of the so-called free magnetic energy to kinetic and thermal energies of cosmic plasmas, hard electromagnetic radiation, and accelerated particles. This phenomenon was found in laboratory and space, but it is especially well studied in the solar atmosphere where it manifests itself as flares and flare-like events. We review the works devoted to the tearing instability — the inalienable part of the reconnection process — in current sheets which have, inside of them, a transverse (perpendicular to the sheet plain) component of the magnetic field and a longitudinal (parallel to the electric current) component of the field. Such non-neutral current sheets are well known as the energy sources for flare-like processes in the solar corona. In particular, quasi-steady high-temperature turbulent current sheets are the energy sources during the main or hot phase of solar flares. These sheets are stabilized with respect to the collisionless tearing instability by a small transverse component of magnetic fiel, normally existing in the reconnecting and reconnected magnetic fluxes. The collision tearing mode plays, however, an important and perhaps dominant role for non-neutral current sheets in solar flares. In the MHD approximation, the theory shows that the tearing instability can be completely stabilized by the transverse fieldB _n if its value satisfies the conditionB _n /BS ^–3/4 B is the reconnecting component of the magnetic field just near the current sheet,S is the magnetic Reynolds number for the sheet. In this case, stable current sheets become sources of temporal spatial oscillations and usual MHD waves. The application of the theory to the solar atmosphere shows that the effect of the transverse field explains high stability of high-temperature turbulent current sheets in the solar corona. The stable current sheets can be sources of radiation in the radio band. If the sheet is destabilized (atB _n /BS ^–3/4) the compressibility of plasma leads to the arizing of the tearing instability in a long wave region, in which for an incompressible plasma the instability is absent. When a longitudinal magnetic field exists in the current sheet, the compressibility-induces instability can be dumped by the longitudinal field. These effects are significant in destabilization of reconnecting current sheets in solar flares: in particular, the instability with respect to disturbances comparable with the width of the sheet is determined by the effect of compressibility.     

Current dissipation within the chromosphere-corona transition

Studies of sporadic outbursts, ranging from flares to nano-flares, invariably endow the solar corona with steady plasma conditions, prior to seeking a current-flow (or the associated magnetic structure) which induces instability. Such an approach does not incorporate a crucial feature of the natural configuration, namely, that the material is of chromospheric origin, and only resides at coronal altitudes for as long as it can acquire adequate energy. There is clearly a feedback loop involved, which allows plasma to moderate the transfer of energy from the field while making use of this heat to permeate coronal altitudes. An examination of the whole procedure is necessary if the location and threshold-conditions for the energy-conversion mechanism are to be identified.A critical step in the feedback procedure mentioned involves the supply line which links the corona to the chromosphere. Because the solar atmosphere has such large vertical dimensions, even a modest change in average temperature and/or density can place heavy demands on this artery: the problem is that a conventional conduction-dominated transition layer cannot readily accommodate a rapid increase in current-density or plasma-flow. (Restructuring of the temperature gradient, to provide the carriers with extra heat, is a very slow process.) A transition layer of this type is unable to endure for long at the base of a sporadically-heated atmosphere in any case, since it becomes the target for plasma falling in the gravitational field during each intermediate cooling phase. As a result, the gap between the chromosphere and corona is more abrupt than is usually considered, endowing the region with thermo-electric characteristics which allow energy to be extracted when modest current-densities arise. Energy-conversion at this region fulfills two rôles: it supplies at least part of the heat required by the overlying corona, and maintains contact between the chromosphere and corona via non-thermal transport processes.     

Hybrid control of orbit normal and along-track two-craft Coulomb tethers

The dynamics and stability of a charged two craft formation with nominal fixed separation distance (Coulomb tethers) is studied where the cluster is aligned with either the along-track or orbit normal direction. Unlike the charged two-craft formation scenario aligned along the orbit radial direction, a feedback control law using inter-spacecraft electrostatic Coulomb forces and the differential gravitational accelerations is not sufficient to stabilize the Coulomb tether length and the formation attitude. Therefore, a hybrid feedback control law is presented which combines conventional thrusters and Coulomb forces. The Coulomb force feedback requires measurements of separation distance error and error rate, while the thruster feedback is in terms of Euler angles and their rates. This hybrid feedback control is designed to asymptotically stabilize the satellite formation shape and attitude while avoiding plume impingement issues. The effects of differential solar drag on the formation and the ability of the controller to withstand this disturbance is also studied.     

Hydra — A 3-dimensional electron and ion hot plasma instrument for the POLAR spacecraft of the GGS mission

HYDRA is an experimental hot plasma investigation for the POLAR spacecraft of the GGS program. A consortium of institutions has designed a suite of particle analyzers that sample the velocity space of electron and ions between 2 keV/q – 35 keV/q in three dimensions, with a routine time resolution of 0.5 s. Routine coverage of velocity space will be accomplished with an angular homogeneity assumption of 16°, appropriate for subsonic plasmas, but with special 1.5° resolution for electrons with energies between 100 eV and 10 keV along and opposed to the local magnetic field. This instrument produces 4.9 kilobits s^–1 to the telemetry, consumes on average 14 W and requires 18.7 kg for deployment including its internal shielding. The scientific objectives for the polar magnetosphere fall into four broad categories: (1) those to define the ambient kinetic regimes of ions and electrons; (2) those to elucidate the magnetohydrodynamic responses in these regimes; (3) those to assess the particle populations with high time resolution; and (4) those to determine the global topology of the magnetic field. In thefirst group are issues of identifying the origins of particles at high magnetic latitudes, their energization, the altitude dependence of the forces, including parallel electric fields they have traversed. In thesecond group are the physics of the fluid flows, regimes of current, and plasma depletion zones during quiescent and disturbed magnetic conditions. In thethird group is the exploration of the processes that accompany the rapid time variations known to occur in the auroral zone, cusp and entry layers as they affect the flow of mass, momentum and energy in the auroral region. In thefourth class of objectives are studies in conjunction with the SWE measurements of the Strahl in the solar wind that exploit the small gyroradius of thermal electrons to detect those magnetic field lines that penetrate the auroral region that are directly open to interplanetary space where, for example, the Polar Rain is observed.     

Galaxien vom Magellanschen Typ

In the first part of this paper the morphological structure of Magellanic type galaxies (Irr I) is investigated. The galaxies of Magellanic type present a basic pattern consisting of a disk, a bar, stellar arms, rudimentary or well developed, spiral filaments and condensations in the disk. With the help of this pattern a well-defined classification scheme is set up. The subgroup of Irr II-systems consists of normal galaxies which are more or less tidally disturbed. Bursts of star formation have a great influence on structure and colour of irregular galaxies. Using the ESO-B Atlas, 580 galaxies of Magellanic type (out of a sample of 3187 galaxies) were classified; 57 are new SB(s)m systems (prototype Large Magellanic Cloud). The sample shows dominant bar structures at the classification stages d-, dm-, and m. A striking feature is the asymmetric position of bar and disk. This asymmetry is a general characteristic of galaxies of types SBd-SBm IB. The asymmetry can be discribed by a relative displacement parameter \(\tilde \Lambda \) = 0.78 ±0.15, defined as the quotient of small and great distance of the bar center to the optical edge of the disk. The displacement cannot be explained by tidal interaction with neighbouring galaxies. In the second part of the paper the kinematics and dynamics of the Large Magellanic Cloud (LMC) as the nearest and best-known example of a galaxy of Magellanic type is investigated. The main structural features of the LMC are disk, bar, rudimentary and well developed stellar arms as well as spiral filaments (not necessarily connected with density waves); the γ-structure is a broken up ring structure. Embedded into these features are young, asymmetrically located spiral arm filaments. As an explanation for these structures stochastic start formation in an ordered chain reaction is proposed. The pattern of the spiral arm filaments is determined by the rotation curve. The morphological peculiarities of the LMC can also be detected in other galaxies of that type. The mean absolute displacement of the centers of bar and disk, determined from 18 galaxies, is Λ = 800 pc. The displacement between the bar center and the symmetry center of the rotation curve is of the same order. The presently known radial velocities of planetary nebulae, star clusters, Hi and Hii regions and stars belonging to the LMC have been collected in a catalogue as the basis of a discussion of the kinematics and dynamics of the LMC. Contrary to earlier work, we have used, for the first time, the radial velocities of objects of all subgroups together by a proper weighting scheme. Thus the basic kinematics and dynamics of the LMC has been deduced. The radial velocity field shows no central symmetry; it is characterized by large scale (2–3 kpc) disturbances. By comparison with the velocity field of other galaxies three main disturbances are identified: an oval distortion of the velocity field in the bar region, a radial velocity field around 30 Doradus, and disturbances connected with a warp or material above the disk in the southern quadrants. The results of a detailed numerical analysis of these three facts can be summed up as follows:

1. The rotation curve is determined over 10° diameter; it shows differential rotation, an asymmetric behavior in the south and a double structure in its Hi component. The rotation center is displaced by 0°.7 from the bar center. The orientation of the kinematic line of nodes and the systemic velocity vary as functions of the distance from the center. Therefore, it is possible to show definitely that large scale disturbances (warping, z-structure and streaming motions) are existent.
2. By variation of the kinematical parameters (systemic velocity, inclination, orientation of the line of nodes, rotation center) the dispersion of the measured radial velocities was minimized and the basic rotation curve determined. The rotation curves for the north and south side of the LMC are significantly different. The south side is either warped or there is material above the main plane. There seems to be a connection between this structure, the Panmagellanic Gas and the Magellanic Stream. The north side appears to be free of distorsion.
3. The residual velocity field (observed minus model) deduced from a basic rotation curve shows that the displacement between the rotation center and the bar center is not caused by local streaming motions. The rotation center must be the mass center. The bar shows a radial velocity field; in the 30 Doradus region inward and outward motions are found.

The mean velocity dispersion of population I objects is 10.5 km s^-1 of population II objects 16.0 km s^-1. Red and blue globular clusters show different kinematical behavior. By comparison of eight mass models, taking into consideration thickness effects and controlled by surface photometric data, the mass of the LMC is found to be (0.5 ± 0.1) × 10¹⁰ \(\mathfrak{M}_ \odot \) (assuming the inclination 33°, the systemic velocity 46.9 km s^-1, and the distance 56 kpc). Dynamically, the LMC can be described by a dominating disk potential with an additional bar potential as a disturbance. The mass of the bar is 0.6 × 10⁹ \(\mathfrak{M}_ \odot \) . The stable neutral point of such a configuration can be found in the residual velocity field. The absorption feature crossing the bar coincides with the maximum velocity gradient of the computed radial velocity field in the plane of the system.     

Mass distribution in our Galaxy

This article summarizes recent work on the luminosity and mass distribution of the galactic bulge and disk, and on the mass of the Milky Way's dark halo. A new luminosity model consistent with the COBE NIR data and the apparent magnitude distributions of bulge clump giant stars has bulge/bar length of 3.5 kpc, axis ratios of 1:(0.3–0.4):0.3, and short disk scale-length (2.1 kpc). Gas-dynamical flows in the potential of this model with constant M/L fit the terminal velocities in 10° le|l|le50° very well. The luminous mass distribution with this M/L is consistent with the surface density of known matter near the Sun, but still underpredicts the microlensing optical depth towards the bulge. Together, these facts argue strongly for a massive, near-maximal disk in our L ^*, Sbc spiral galaxy. While the outer rotation curve and global mass distribution are not as readily measured as in similar spiral galaxies, the dark halo mass estimated from satellite velocities is consistent with a flat rotation curve continuing on from the luminous mass distribution.     

Ballooning Instability as a Mechanism of the Near-Earth Onset of Substorms

After introducing a mathematical definition of the tail-like equilibrium and the dipole-like equilibrium in the magnetosphere, it is shown by using physical intuition based on the Energy Principle that the incompressible assumption for the ballooning instability is more valid for the tail-like configuration when the unstable ballooning mode is strongly localized near the equator. Therefore, before the substorm onset, the near-Earth plasma sheet becomes more tail-like and more likely to be subject to the ballooning instability without the stabilizing influence of the compressibility, when the critical plasma due to the stabilizing tension force is exceeded. The onset of the ballooning instability in the near-Earth plasma sheet seems promisingly relevant to the substorm onset phenomena. Also, the effect of the stochastic plasma dynamics on the ballooning and interchange instabilities is clearly shown.     

A numerical study of the fission hypothesis for rotating polytropes

Conclusions Our results show that n = 3/2 and n = 1/2 polytropes are dynamically unstable to non-axisymmetric perturbations if t 0.30, in rough agreement with linear theory. Instead of fission as the direct end product of dynamic instability in rapidly rotating, centrally condensed stars, it appears that a rapidly rotating star can, through gravitational torques, eject some high angular momentum material in its equatorial plane and settle down into a dynamically stable (lower t) configuration. The central object, at least in the detailed evolution described above, ends up as a stable traixial star! The assumption of reflection symmetry through the rotation axis needs to be relaxed before this evolutionary picture can be considered realistic and before meaningful comparisons can be made with the results of Lucy (1977) and Gingold and Monaghan (1978, 1979), who found odd modes to be of crucial importance.We cannot escape mentioning that the ejected disk/ring of material in our models may have some connection with the formation of planetary systems.This research was supported by National Science Foundation Grant No. AST-7821449.     

X-Ray and extreme ultraviolet emissions from comets

There is significant progress in the observations, theory, and understanding of the x-ray and EUV emissions from comets since their discovery in 1996. That discovery was so puzzling because comets appear to be more efficient emitters of x-rays than the Moon by a factor of 80000. The detected emissions are general properties of comets and have been currently detected and analyzed in thirteen comets from five orbiting observatories. The observational studies before 2000 were based on x-ray cameras and low resolution (E/E1.5–3) instruments and focused on the morphology of x-rays, their correlations with gas and dust productions in comets and with the solar x-rays and the solar wind. Even those observations made it possible to choose uniquely charge exchange between the solar wind heavy ions and cometary neutrals as the main excitation process. The recently published spectra are of much better quality and result in the identification of the emissions of the multiply charged ions of O, C, Ne, Mg, and Si which are brought to comets by the solar wind. The observed spectra have been used to study the solar wind composition and its variations. Theoretical analyses of x-ray and EUV photon excitation in comets by charge exchange, scattering of the solar photons by attogram dust particles, energetic electron impact and bremsstrahlung, collisions between cometary and interplanetary dust, and solar x-ray scattering and fluorescence in comets have been made. These analyses confirm charge exchange as the main excitation mechanism, which is responsible for more than 90% of the observed emission, while each of the other processes is limited to a few percent or less. The theory of charge exchange and different methods of calculation for charge exchange are considered. Laboratory studies of charge exchange relevant to the conditions in comets are reviewed. Total and state-selective cross sections of charge exchange measured in the laboratory are tabulated. Simulations of synthetic spectra of charge exchange in comets are discussed. X-ray and EUV emissions from comets are related to different disciplines and fields such as cometary physics, fundamental physics, x-rays spectroscopy, and space physics.     

Why space physics needs to go beyond the MHD box

Magnetohydrodynamic (MHD) theory has been used in space physics for more than forty years, yet many important questions about space plasmas remain unanswered. We still do not understand how the solar wind is accelerated, how mass, momentum and energy are transported into the magnetosphere and what mechanisms initiate substorms. Questions have been raised from the beginning of the space era whether MHD theory can describe correctly space plasmas that are collisionless and rarely in thermal equilibrium. Ideal MHD fluids do not induce electromotive force, hence they lose the capability to interact electromagnetically. No currents and magnetic fields are generated, rendering ideal MHD theory not very useful for space plasmas. Observations from the plasma sheet are used as examples to show how collisionless plasmas behave. Interpreting these observations using MHD and ideal MHD concepts can lead to misleading conclusions. Notably, the bursty bulk flows (BBF) with large mean velocities left(〈 v 〉≥400 km s right) that have been interpreted previously as E×B flows are shown to involve much more complicated physics. The sources of these nonvanishing 〈 v 〉 events, while still not known, are intimately related to mechanisms that create large phase space gradients that include beams and acceleration of ions to MeV energies. The distributions of these nonvanishing 〈 v 〉 events are associated with large amplitude variations of the magnetic field at frequencies up to and exceeding the local Larmor frequency where MHD theory is not valid. Understanding collisionless plasma dynamics such as substorms in the plasma sheet requires the self-consistency that only kinetic theory can provide. Kinetic modeling is still undergoing continual development with many studies limited to one and two dimensions, but there is urgent need to improve these models as more and more data show kinetic physics is fundamentally important. Only then will we be able to make progress and obtain a correct picture of how collisionless plasmas work in space.     

Basic Physical Mechanism of Fast Reconnection Evolution in Space Plasmas

Large dissipative events, such as solar flares and geomagnetic substorms, may result from sudden onset of fast (explosive) magnetic reconnection. Hence, it is a long-standing problem to find the physical mechanism that makes magnetic reconnection explosive; in particular, how can the fast magnetic reconnection explosively evolve in space plasmas? In this respect, we have proposed the spontaneous fast reconnection model as a nonlinear instability that grows by the positive feedback between plasma microphysics (anomalous resistivity) and macrophysics (global reconnection flow). On the basis of MHD simulations, we demonstrate for a variety of physical situations that the fast reconnection mechanism involving slow shocks in fact evolves explosively as a nonlinear instability and is sustained quasi-steadily on the nonlinear saturation phase. Also, distinct plasma processes, such as large-scale plasmoid propagation, magnetic loop development and loop-top heating, and asymmetric fast reconnection evolution, directly result from the spontaneous fast reconnection model. Obviously, MHD simulations are very useful in understanding the basic physics of explosive fast reconnection evolution in space plasmas. However, they cannot treat the details of microphysics near an X neutral point, which should be precisely studied in the coming 21st century.     

The spacelab Lyman alpha and white light coronagraphs program

The Harvard-Smithsonian Center for Astrophysics and the High Altitude Observatory have defined a joint coronagraphs experiment for a future Spacelab mission. The instrumentation package would include an ultraviolet light coronagraph to measure the intensity and profiles of spectral lines formed between 1.2 and 8 solar radii from Sun center and a white light coronagraph to measure the intensity and polarization of visible light. The overall goals of the joint program are to use new coronal plasma diagnostic techniques to understand the physical processes and mechanisms operating in the solar corona, to understand the acceleration of high-speed and low-speed solar wind streams and to extrapolate this knowledge to other stars in order to help understand the physics of stellar coronae and stellar mass loss.Proceedings of the Conference Solar Physics from Space, held at the Swiss Federal Institute of Technology Zurich (ETHZ), 11–14 November 1980.     

The application of dimensional analysis to cosmology

Cosmology as it is usually studied suffers from the problem that no criterion is known which isolates from the large class of models allowed by the equations of physics those few which are realized in Nature. To provide such a criterion, it is proposed that cosmology should be based on the study of models which are free of arbitrary scales or units, this condition being compatible with (but not identical with) the Cosmological Principle. Formally, the basis for scale-free cosmology can be expressed in a dimensional Conspiracy Hypothesis: The material parameters of a system (mass, density, pressure etc.), the constants of physics and the coordinates have realizable physical meanings only when they occur together in dimensionless combinations (-numbers) in which the components may vary with time or place but in such a manner that the variations conspire to keep the -numbers constant. The Conspiracy Hypothesis (CH) streamlines cosmology, simplifying it to the finding of a few dimensionless numbers. Applied to Einstein's general relativity, the CH yields a simple cosmological model consisting of static clusters of galaxies with inverse-square density profiles embedded in an expanding, homogeneous background. This model agrees well with the observed Universe insofar as the latter can be described by general relativity. The CH can also be applied to other theories of gravity, especially those in which the gravitational parameter G is variable, and can also in itself be taken as a basis for gravitational theory.