Sputtering processes: Erosion and chemical change

We review laboratory data and models on sputter-induced erosion and chemical alterations of ice films and apply the results to icy grains and satellites exposed to magnetospheric ion bombardment. We show that the source of the plasma in the inner magnetosphere of Saturn is likely to be the sputter erosion of the icy objects in this region and consider the sputter erosion and possible stabilization of the E-ring. Ion-induced polymerization is discussed as a source of the darkened rings of Uranus.     

Auroral signatures of transient processes in the outer magnetosphere

We present an analysis of sporadic and recurrent injections of magnetospheric ions in the midnight auroral oval during substorms and of the associated ionospheric ion outflows. The source of plasma sheet precipitating ions is determined using a simple method, based on the measured relation between the ion inverse velocity and time (l = v × t). This method is applied here to two typical passes of the Interball-Auroral (IA) satellite at distances of 3 R_E above the auroral regions. Substorm related ion injections are shown to be mainly due to time of flight effects. In contrast with particle trajectory computations (Sauvaud et al., 1999), the inverse velocity method does not require magnetic and electric field models and can thus be used systematically for the detection of time of flight dispersed ion structures (TDIS). This allowed us to build a large database of TDIS events and to perform a statistical analysis of their spatial distribution. For the cases presented here the source region of the injected ions is found at radial distances from 18 to 30 R_E near the equatorial magnetosphere. At Interball altitudes ( 3 R_E), ion injections detected at the poleward boundary of the nighside auroral oval are associated with shear Alfvén waves superimposed over large-scale quasi-static current structures. We show that the most poleward TDIS are collocated with a large outflow of ionospheric H⁺ and O⁺ displaying pitch-angle distributions peaked in the pitch-angle range 90°–120°. These ions are thus accelerated perpendicularly to the magnetic field not only in the main auroral acceleration region but also up to at least 3 R_E. The expanding auroral bulge thus constitutes a significant source of H⁺ and O⁺ ions for the mid-tail magnetosphere.     

Magnetospheric turbulence and properties of magnetospheric dynamics

High level of turbulence is one of the main peculiarities inherent to magnetospheric dynamics. Mechanisms for generation of magnetospheric turbulence are analyzed. The instabilities in the plasma pressure distribution are examined as source of large and medium scale modes in the turbulence spectra. Large-scale modes (which scales are comparable with scale of the magnetosphere) lead to convective transport of the magnetospheric particles. Excitation of such modes is analyzed being based on the suggestion of the existence of week instability in the distribution of plasma pressure.     

Circulation of energetic ions of terrestrial origin in the magnetosphere

Energetic ion composition measurements have now been performed from earth orbiting satellites for more than a decade. As early as 1972 we knew that energetic (keV) ions of terrestrial origin represented a non-negligible component of the storm time ring current. We have now assembled a significant body of knowledge concerning energetic ion composition throughout much of the earth's magnetosphere. We know that terrestrial ions are a common component of the hot equatorial magnetospheric plasma in the ring current and the plasma sheet out to ? 23 R_E. During periods of enhanced geomagnetic activity this component may become dominant. There is also clear evidence that the terrestrial component (specifically O⁺) is strongly dependent on solar cycle. Terrestrial ion source, transport, and acceleration regions have been identified in the polar auroral region, over the polar caps, in the magnetospheric boundary layers, and within the magnetotail lobes and plasma sheet boundary layer. Combining our present knowledge of these various magnetospheric ion populations, it is concluded that the primary terrestrial ion circulation pattern associated with enhanced geomagnetic activity involves direct injection from the auroral ion acceleration region into the plasma sheet boundary layer and central plasma sheet. The observed terrestrial component of the magnetospheric boundary layer and magnetotail lobes are inadequate to provide the required influx. They may, however, contribute significantly to the maintenence of the plasma sheet terrestrial ion population, particularly during periods of reduced geomagnetic activity. It is further concluded, on the basis of the relative energy distributions of H⁺ and O⁺ in the plasma sheet, that O⁺ probably contributes significantly to the ring current population at energies inaccessible to present ion composition instrumentation (? 30 keV).     

Acceleration of low-energy magnetospheric plasma

Low-energy plasma originates in the ionosphere and is accelerated and transported to the plasma sheet and ultimately to the ring current. Using observations and basic MHD concepts, it is argued that the acceleration results basically from entrainment in flows that are rapid compared with initial ion thermal speeds. Spatial or temporal variations of such flows launch impulsive waves of the appropriate variety (acoustic, shear Alfven, or magnetosonic) to effect readjustment to the imposed boundary conditions. The most violent transient events are the earthward inductive surges of plasma in the inner plasma sheet, which launch magnetosonic waves. A number of observations strongly suggest that the induction surge waves break as they reach the inner plasma sheet or outer plasmasphere, forming transient shock waves and dissipating their energy in turbulent flows, plasma heating, and acceleration of energetic particles, forming the substorm injection boundary. Preliminary work indicates that the magnetosphere is typically configured so as to produce wave breaking near synchronous orbit, and has other interesting optical properties for MHD wave propagation as well. Exploration of magnetospheric plasma wave optics will require a better empirical knowledge of the plasma distribution.     

Rapid development of corneal lesions in rats produced by heavy ions.

Scanning electron micrographs of heavy ion irradiated corneas demonstrate a significant correlation with the heavy ion beam: The average number of plasma membrane lesions per unit area of corneal surface is correlated with the particle fluence of the beam. This observation corroborates what has already been suggested theoretically about heavy ion tracks and what has been shown experimentally through etched plastics, developed emulsions, and bubble chambers. But the new data indicate that particle tracks occur in biological tissues as well, and that a single heavy ion is responsible for each membrane lesion.     

Mercury's magnetosphere

The Mariner 10 observations of Mercury's miniature magnetosphere collected during its close encounters in 1974 and 1975 are reviewed. Subsequent data analysis, re-interpretation and theoretical modeling, often influenced by new results obtained regarding the Earth's magnetosphere, have greatly expanded our impressions of the structure and dynamics of this small magnetosphere. Of special interest are the Earth-based telescopic images of this planet's tenuous atmosphere that show great variability on time scales of tens of hours to days. Our understanding of the implied close linkage between the sputtering of neutrals into the atmosphere due to solar wind and magnetospheric ions impacting the regolith and the resultant mass loading of the magnetosphere by heavy planetary ions is quite limited due to the dearth of experimental data. However, the influence of heavy ions of planetary origin (O⁺, Na⁺, K⁺, Ca⁺ and others as yet undetected) on such basic magnetospheric processes as wave propagation, convection, and reconnection remain to be discovered by future missions. The electrodynamic aspects of the coupling between the solar wind, magnetosphere and planet are also very poorly known due to the limited nature of the measurements returned by Mariner 10 and our lack of experience with a magnetosphere that is rooted in a regolith as opposed to an ionosphere. The review concludes with a brief summary of major unsolved questions concerning this very small, yet potentially complex magnetosphere.     

Origin and energetics of the Io plasma torus

The ionization of the gas ejections from the Io satellite into the Jovian magnetosphere by the corotating magnetospheric plasma flow is considered. It is shown that the plasma flow velocity at the Io orbit exceeds the critical velocity at which the anomalous electron ionization of the heavy gas components takes place due to collisionless energy transfer from ionized gas atoms to plasma electrons. The energy, number density and spatial distribution of suprathermal electrons is calculated using the quasilinear theory of newly ionized atoms instability in a moving plasma. Saturation of the plasma density build up in a plasma is considered in terms of the instability quenching by Coulomb collisions.     

Appearance of neutron stars in the state of subsonic propeller

The state of subsonic propeller is intermediate between the states of supersonic propeller and accretor in the evolutionary tracks of magnetized compact stars. The rotational rate of a star at this stage decelerates due to the interaction between its magnetosphere and the surrounding hot, quasi-static plasma envelope. The magnetospheric radius is smaller than the corotation radius and the boundary of the magnetosphere is stable with respect to interchange instabilities. The rate of the mass flux from the inner radius of the envelope to the stellar surface is limited by the rate of plasma diffusion into the magnetic field of the star. As a result, the subsonic propeller would appear as a low-luminosity accretion-powered pulsar with a soft X-ray spectrum.     

Advanced techniques for plasma composition and dynamics measurements

Magnetic and RF mass spectrometers have been used routinely in ionospheric research, while traditional ionospheric, magnetospheric, and interplanetary plasma measurements have been made with several types of electrostatic analyzers. Proper interpretation of these data is possible if the spectral peaks are well defined, although ambiguities between fast, light ions and slow, heavy ions cannot always be satisfactorily resolved. Recent and planned experiments involve the study of plasmas which are sufficiently energetic that the spectral peaks overlap. Furthermore, these studies of ionosphere/magnetosphere coupling and of the interaction of the solar wind with the atmospheres of Venus and comets require unambiguous identification of the ion masses with simultaneous mapping of the three-dimensional velocity distribution function of each ion species. This challenge has been partially met by several new types of instruments; the two most common types involve either (1) sequential electrostatic and magnetic analyses or (2) sequential electrostatic and time-of-flight analyses. Some new instruments have also incorporated measurements of total kinetic energy, electric charge, or secondary emission coefficients as diagnostic tools. This paper reviews these recent advances and points out areas where further development is expected and needed.     

Magnetospheric energetic ions from the Earth's ionosphere

In the decade and a half since the initial discovery that the Earth's own ionosphere could at times contribute measurably to the hot plasma in the magnetosphere we have made significant progress in both our knowledge and understanding of this connection. We now know that ions of ionospheric origin are found in all major regions of the magnetosphere and at its boundaries. The source region in the ionosphere and the acceleration and transport processes involved in coupling the cold ionospheric plasma to the hot magnetospheric plasma are complex and variable. We now have a good understanding of the large scale morphology of the ionospheric outflow and its distribution throughout the magnetosphere and progress is being made in the understanding of the fundamental physical processes involved. In this paper we concentrate on the large scale morphology and our understanding of the sources for ionospheric ions found in various regions of the magnetosphere and their transport.     

The thermosphere as a sink of magnetospheric energy: A review of recent observations of dynamics

The study of the dynamics and thermodynamics of the earth's upper atmosphere has made significant progress over the past few years owing to the availability of new global-scale data sets from the Dynamics Explorer satellites. The thermospheric wind and temperature fields at high latitude have been observed to depend strongly on forcing processes of magnetospheric origin. A key momentum source is due to the drag effect of ions convecting in response to electric fields mapped down on the ionosphere from magnetospheric boundary regions. Likewise, an important heat source derives from Joule or frictional dissipation due to ion/neutral difference velocities governed, in turn, by magnetospheric forcing. In this paper we discuss the progress made over the last 2–3 years initiated by the new satellite measurements and we review published data on ion and neutral motions in the context of the energy and momentum coupling between the magnetosphere and the ionosphere/neutral upper atmosphere. The observations indicate the existence of a “flywheel effect” which implies direct feedback from the neutral thermosphere to the magnetosphere via the release of energy and momentum previously “stored” in the neutral thermosphere.     

Plasma effects of active ion beam injections in the ionosphere at rocket altitudes

Data from ARCS rocket ion beam injection experiments will be primarily discussed in this paper. There are three results from this series of active experiments that are of particular interest in space plasma physics. These are the transverse acceleration of ambient ions in the large beam volume, the scattering of beam ions near the release payload, and the possible acceleration of electrons very close to the plasma generator which produce intense high frequency waves. The ability of 100 ma ion beam injections into the upper E and F regions of the ionosphere to produce these phenomena appear to be related solely to the process by which the plasma release payload and the ion beam are neutralized. Since the electrons in the plasma release do not convect with the plasma ions, the neutralization of both the payload and beam must be accomplished by large field-aligned currents (milliamperes/square meter) which are very unstable to wave growth of various modes. Future work will concentrate on the wave production and wave-particle interactions that produce the plasma/energetic particle effects discussed in this paper and which have direct application to natural phenomena in the upper ionosphere and magnetosphere.     

Bow shock: Power aspects

It is clear that the primary energy source for magnetospheric processes is the solar wind, but the process of energy transfer from the solar wind into the magnetosphere, or rather, to convecting magnetospheric plasma, appears to be rather complicated. Bow shock is a powerful transformer of the solar wind kinetic energy into the gas dynamic and electromagnetic energy. A jump of the magnetic field tangential component at front crossing means that the front carries an electric current. The solar wind kinetic energy partly transforms to gas kinetic and electromagnetic energy during its passage through the bow shock front. The transition layer (magnetosheath) can use part of this energy for accelerating of plasma, but can conversely spend part its kinetic energy on the electric power generation, which afterwards may be used by the magnetosphere. Thereby, transition layer can be both consumer (sink) and generator (source) of electric power depending upon special conditions. The direction of the current behind the bow shock front depends on the sign of the IMF Bz-component. It is this electric current which sets convection of plasma in motion.     

An updated theory of the polar wind

The ‘classical’ polar wind is an ambipolar outflow of thermal plasma from the terrestrial ionosphere at high latitudes. As the plasma escapes along diverging geomagnetic flux tubes, it undergoes four major transitions, including a transition from chemical to diffusion dominance, a transition from subsonic to supersonic flow, a transition from collision-dominated to collisionless regimes, and a transition from a heavy to a light ion. A further complication arises because of horizontal convection of the flux tubes owing to magnetospheric electric fields. Recent modelling predictions indicate that the polar wind has the following characteristics: (1) The ion and electron distributions are anisotropic and asymmetric in the collisionless regime; (2) Elevated electron temperatures ( ∼ 10,000 K) act to produce significant escape fluxes of suprathermal O⁺ ions; (3) The interaction of the hot magnetospheric and cold ionospheric electron populations leads to a localized (double layer) electric field which accelerates the polar wind ions; (4) A time-dependent expansion produces suprathermal ions; and (5) Large perturbations lead to the formation of forward and reverse shocks. These and other results are reviewed.     

Entry of dust particles into planetary magnetospheres

While interplanetary dust constitutes a primary source of cosmic particulate matter in planetary magnetospheres, the debris produced by its impact with small satellites and ring material provides an important secondary source. Internal processes, such as volcanic activity, particularly in the smaller satellites, could result in a third source. In the case of the terrestrial magnetosphere there are also artificial (internal) sources: 1–10μ sized A?₂O₃ particles injected by solid rocket mortar burns between near earth and geosynchronous orbit constitute one such source, while the fragments of larger bodies (artificial satellites) due to explosions (e.g., “killer satellites”) and collisions constitute another. Finally, if we include the purely induced cometary magnetosphere among planetary magnetospheres, the injection of cometary dust into it due to entrainment by the outflowing gases constitutes another source.As a result of being immersed in a radiative and plasma environment these dust grains get electrically charged up to some potential (positive or negative). Particularly in those regions where the magnetospheric plasma is hot and dense and their own spatial density is low, the dust grains could get charged to numerically large negative potentials.While this charging may have physical consequences for the larger grains, such as electrostatic erosion (“chipping”) and disruption, it also can effect the dynamics of the smaller grains. Indeed, the small but finite capacitance of these grains, which leads to a phase lag in the gyrophase oscillation of the grain potential, could even lead to the permanent magneto-gravitational capture of interplanetary grains within planetary magnetospheres in certain situations. Here we will review the sources of dust in planetary magnetospheres and discuss their physics and their dynamics under the combined action of both planetary gravitational and magnetospheric electromagnetic forces.     

Geomagnetically trapped heavy ions resulting from the anomalous cosmic rays

Blake and Freisen painted out that if the anomalous cosmic rays are only singly charged they can penetrate deeply into the Earth's magnetosphere well below the cutoff for stripped ions. Ions which reach low altitude can be stripped by the residual atmosphere and become stably trapped. An experiment has been developed which will be able to detect the intensities of such stably trapped heavy ions as a function of magnetospheric position. In anticipation of the flight of this experiment, the fluxes and spectra of the trapped heavy ions have been calculated based upon the anomalous component observed in the IPM.     

Plasmaphere and plasmapause region characteristics as measured by DE-1

Thermal ion composition measurements by the Retarding Ion Mass Spectrometer (RIMS) on Dynamics Explorer-1 have revealed new and intriguing features of the thermal ion distributions in the plasmasphere and plasmapause regions. Some of the interesting new findings include: the presence of intense fluxes of heated and equatorially-trapped light ions within the plasmapause region; the existence of a heavy ion (0+, 0++, N+) ‘torus’ or ‘shell’ in the outer plasmasphere; and the relatively stable nature of the He+/H+ concentration ratio (∼0.2–0.3) within the plasmasphere. The relatively short (∼7.5 hours) orbital period of DE-1 has also allowed improved observations on the formation of the new outer plasmasphere during the recovery of geomagnetic storms. Statistical studies of plasmaspheric density structure and boundaries are beginning to reveal a picture of their relation to other magnetospheric boundaries, such as the inner edge of the electron plasma sheet, and trends in the internal density structure of the plasmasphere.     

Interaction of Titan's atmosphere with Saturn's magnetosphere

The Voyager 1 measurements made during the Titan flyby reveal that Saturn's rotating magnetospheric plasma interacts directly with Titan's neutral atmosphere and ionosphere. This results from the lack of an intrinsic magnetic field at Titan. The interaction induces a magnetosphere which deflects the flowing plasma around Titan and forms a plasma wake downstream. Within the tail of the induced magnetosphere, ions of ionospheric origin flow away from Titan. Just outside Titan's magnetosphere, a substantial ion-exosphere forms from an extensive hydrogen-nitrogen exosphere. The exospheric ions are picked up and carried downstream into the wake by the plasma flowing around Titan. Mass loading produced by the addition of exospheric ions slows the wake plasma down considerably in the vicinity of the magnetopause.