The use of multiple receivers to measure the wave characteristics of very-low-frequency noise in space

Until now most very-low-frequency (VLF) radio noise experiments in the ionosphere, magnetosphere and solar wind have been able to provide only the amplitude and spectral characteristics of VLF phenomena. Experiments using multiple receivers to measure the amplitudes and relative phases of the magnetic and electric wave components, however, can give the wave characteristics in addition. Knowledge of both the spectral and the wave characteristics are desirable in making deductions about the noise source location and mechanism and about the properties of the propagation path. Expressions are derived for obtaining the electromagnetic wave characteristics — wave normal vector, Poynting vector and wave polarization — and the electrostatic wave characteristics — wave normal direction and field magnitude — from the amplitudes and relative phases of the wave components. The antenna systems capable of measuring the necessary wave components on payloads which are not spinning, spinning, or spinning and precessing are described. Consideration is given to the experimental technique of reducing payload interference, of transferring the required data to the ground and of obtaining the desired spatial, frequency, amplitude and phase resolution.The data obtained with such an experiment may represent the superposition of signals from multiple sources and multiple paths and from interference signals. Interpretation of these results is discussed and the use of the results of obtaining information on the source location and mechanism and on the propagation path properties is described. Recently several sounding rocket and satellite experiments capable of measuring some of the wave characteristics have been flown. The results concerning the wave normal directions for several different types of VLF noise phenomena are summarized.     

Fluctuating magnetic fields in the magnetosphere

The study of ULF waves in space has been in progress for about 12 years. However, because of numerous observational difficulties the properties of the waves in this frequency band (10^-3 to 1 Hz) are poorly known. These difficulties include the nature of satellite orbits, telemetry limitations on magnetometer frequency response and compromises between dynamic range and resolution. Despite the paucity of information, there is increasing recognition of the importance of these measurements in magnetospheric processes. A number of recent theoretical papers point out the roles such waves play in the dynamic behavior of radiation belt particles.At the present time the existing satellite observations of ULF waves suggest that the level of geomagnetic activity controls the types of waves which occur within the magnetosphere. Consequently, we consider separately quiet times, times of magnetospheric substorms and times of magnetic storms. Within each of these categories there are distinctly different wave modes distinguished by their polarization: either transverse or parallel to the ambient field. In addition, these wave phenomena occur in distinct frequency bands. In terms of the standard nomenclature of ground micropulsation studies ULF wave types observed in the magnetosphere include quiet time transverse — Pc 1, Pc 3, Pc 4, Pc 5 quiet time compressional — Pc 1 and Pi 1; substorm compressional Pi 1 and Pi 2; storm transverse — Pc 1; storm compressional Pc 4, 5. The satellite observations are not yet sufficient to determine whether the various bands identified in the ground data are equally appropriate in space.Publication No. 982. Institute of Geophysics and Planetary Physics, University of California, Los Angeles, Calif. 90024.     

Radio plasma imager simulations and measurements

The Radio Plasma Imager (RPI) will be the first-of-its kind instrument designed to use radio wave sounding techniques to perform repetitive remote sensing measurements of electron number density (N _e) structures and the dynamics of the magnetosphere and plasmasphere. RPI will fly on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) mission to be launched early in the year 2000. The design of the RPI is based on recent advances in radio transmitter and receiver design and modern digital processing techniques perfected for ground-based ionospheric sounding over the last two decades. Free-space electromagnetic waves transmitted by the RPI located in the low-density magnetospheric cavity will be reflected at distant plasma cutoffs. The location and characteristics of the plasma at those remote reflection points can then be derived from measurements of the echo amplitude, phase, delay time, frequency, polarization, Doppler shift, and echo direction. The 500 m tip-to-tip X and Y (spin plane) antennas and 20 m Z axis antenna on RPI will be used to measures echoes coming from distances of several R _E. RPI will operate at frequencies between 3 kHz to 3 MHz and will provide quantitative N _e values from 10^–1 to 10⁵ cm^–3. Ray tracing calculations, combined with specific radio imager instrument characteristics, enables simulations of RPI measurements. These simulations have been performed throughout an IMAGE orbit and under different model magnetospheric conditions. They dramatically show that radio sounding can be used quite successfully to measure a wealth of magnetospheric phenomena such as magnetopause boundary motions and plasmapause dynamics. The radio imaging technique will provide a truly exciting opportunity to study global magnetospheric dynamics in a way that was never before possible.     

Magnetosphere Imaging Instrument (MIMI) on the Cassini Mission to Saturn/Titan

The magnetospheric imaging instrument (MIMI) is a neutral and charged particle detection system on the Cassini orbiter spacecraft designed to perform both global imaging and in-situ measurements to study the overall configuration and dynamics of Saturn’s magnetosphere and its interactions with the solar wind, Saturn’s atmosphere, Titan, and the icy satellites. The processes responsible for Saturn’s aurora will be investigated; a search will be performed for substorms at Saturn; and the origins of magnetospheric hot plasmas will be determined. Further, the Jovian magnetosphere and Io torus will be imaged during Jupiter flyby. The investigative approach is twofold. (1) Perform remote sensing of the magnetospheric energetic (E > 7 keV) ion plasmas by detecting and imaging charge-exchange neutrals, created when magnetospheric ions capture electrons from ambient neutral gas. Such escaping neutrals were detected by the Voyager l spacecraft outside Saturn’s magnetosphere and can be used like photons to form images of the emitting regions, as has been demonstrated at Earth. (2) Determine through in-situ measurements the 3-D particle distribution functions including ion composition and charge states (E > 3 keV/e). The combination of in-situ measurements with global images, together with analysis and interpretation techniques that include direct “forward modeling’’ and deconvolution by tomography, is expected to yield a global assessment of magnetospheric structure and dynamics, including (a) magnetospheric ring currents and hot plasma populations, (b) magnetic field distortions, (c) electric field configuration, (d) particle injection boundaries associated with magnetic storms and substorms, and (e) the connection of the magnetosphere to ionospheric altitudes. Titan and its torus will stand out in energetic neutral images throughout the Cassini orbit, and thus serve as a continuous remote probe of ion flux variations near 20R _S (e.g., magnetopause crossings and substorm plasma injections). The Titan exosphere and its cometary interaction with magnetospheric plasmas will be imaged in detail on each flyby. The three principal sensors of MIMI consists of an ion and neutral camera (INCA), a charge–energy–mass-spectrometer (CHEMS) essentially identical to our instrument flown on the ISTP/Geotail spacecraft, and the low energy magnetospheric measurements system (LEMMS), an advanced design of one of our sensors flown on the Galileo spacecraft. The INCA head is a large geometry factor (G ∼ 2.4 cm² sr) foil time-of-flight (TOF) camera that separately registers the incident direction of either energetic neutral atoms (ENA) or ion species (≥5^∘ full width half maximum) over the range 7 keV/nuc < E < 3 MeV/nuc. CHEMS uses electrostatic deflection, TOF, and energy measurement to determine ion energy, charge state, mass, and 3-D anisotropy in the range 3 ≤ E ≤ 220 keV/e with good (∼0.05 cm² sr) sensitivity. LEMMS is a two-ended telescope that measures ions in the range 0.03 ≤ E ≤ 18 MeV and electrons 0.015 ≤ E≤ 0.884 MeV in the forward direction (G ∼ 0.02 cm² sr), while high energy electrons (0.1–5 MeV) and ions (1.6–160 MeV) are measured from the back direction (G ∼ 0.4 cm² sr). The latter are relevant to inner magnetosphere studies of diffusion processes and satellite microsignatures as well as cosmic ray albedo neutron decay (CRAND). Our analyses of Voyager energetic neutral particle and Lyman-α measurements show that INCA will provide statistically significant global magnetospheric images from a distance of ∼60 R _S every 2–3 h (every ∼10 min from ∼20 R _S). Moreover, during Titan flybys, INCA will provide images of the interaction of the Titan exosphere with the Saturn magnetosphere every 1.5 min. Time resolution for charged particle measurements can be < 0.1 s, which is more than adequate for microsignature studies. Data obtained during Venus-2 flyby and Earth swingby in June and August 1999, respectively, and Jupiter flyby in December 2000 to January 2001 show that the instrument is performing well, has made important and heretofore unobtainable measurements in interplanetary space at Jupiter, and will likely obtain high-quality data throughout each orbit of the Cassini mission at Saturn. Sample data from each of the three sensors during the August 18 Earth swingby are shown, including the first ENA image of part of the ring current obtained by an instrument specifically designed for this purpose. Similarily, measurements in cis-Jovian space include the first detailed charge state determination of Iogenic ions and several ENA images of that planet’s magnetosphere.This revised version was published online in July 2005 with a corrected cover date.     

The menagerie of geospace plasma waves

Sounding rockets and satellites have discovered a large variety of plasma waves within the Earth's magnetosphere—geospace. These waves are found over a frequency range of millihertz to megahertz. The frequency ranges are generally associated with characteristic frequencies such as the plasma frequency and gyrofrequency. Most waves are generated by hot or streaming magnetospheric plasma; some waves are due to lightning discharges, to intentional man-made transmitters or to incidental radiation from power transmission systems. Propagation of waves from the observation region back to a probable source region can be modelled using ray tracing techniques in a model magnetosphere where the electron number density, ion composition and magnetic field vector is specified. Information in addition to the common amplitude-frequency-time spectrograms can be obtained from the received waves using multiple antennas and receivers. Cross-correlation of the wave electric and magnetic components can provide information on the wave polarization and direction of propagation and on the wave distribution function.     

Stimulation of VLF amplification in the magnetosphere

Among the various plasma instabilities that exert influence on the dynamic equilibrium state of the magnetosphere, the cyclotron-resonance interaction appears to be the most accessible to artificial stimulation. The strength of the interaction is sensitive to both the background magnetoplasma parameters and the hot energetic particle distribution. Thus, proper modification of one or more conditions can induce significant wave amplification at the expense of hot plasma energy density. Several methods of hot and cold plasma injection have been investigated with the linear theory to assess their effectiveness as a means of stimulating amplification.Only the interaction of VLF waves (3–30 kHz) with hot electrons (0.1–100 keV) is treated here. The injection of a dense jet of barium that travels upward along the geomagnetic field causes appreciable amplification when the jet is within 30° of the geomagnetic equator. Injection of a geosynchronous lithium cloud stimulates amplification of both VLF and ULF waves, but the magnitude depends critically on the state of geomagnetic activity. Conventional hot electron beams may also amplify narrow frequency bands, but the net wave energy is severely limited by the beam energy.Although the cyclotron-resonance is recognized as a dominant interaction in magnetospheric dynamics, its properties have never been confirmed quantitatively by appropriate spacecraft experiments. Controlled injections would provide important insight into this fundamental process because the induced amplification has a well-defined signature.     

The Toroidal Imaging Mass-Angle Spectrograph (TIMAS) for the polar mission

The science objectives of the Toroidal Imaging Mass-Angle Spectrograph (TIMAS) are to investigate the transfer of solar wind energy and momentum to the magnetosphere, the interaction between the magnetosphere and the ionosphere, the transport processes that distribute plasma and energy throughout the magnetosphere, and the interactions that occur as plasma of different origins and histories mix and interact. In order to meet these objectives the TIMAS instrument measures virtually the full three-dimensional velocity distribution functions of all major magnetospheric ion species with one-half spin period time resolution. The TIMAS is a first-order double focusing (angle and energy), imaging spectrograph that simultaneously measures all mass per charge components from 1 AMU e^–1 to greater than 32 AMU e^–1 over a nearly 360° by 10° instantaneous field-of-view. Mass per charge is dispersed radially on an annular microchannel plate detector and the azimuthal position on the detector is a map of the instantaneous 360° field of view. With the rotation of the spacecraft, the TIMAS sweeps out very nearly a 4 solid angle image in a half spin period. The energy per charge range from 15 eV e^–1 to 32 keV e^–1 is covered in 28 non-contiguous steps spaced approximately logarithmically with adjacent steps separated by about 30%. Each energy step is sampled for approximately 20 ms;14 step (odd or even) energy sweeps are completed 16 times per spin. In order to handle the large volume of data within the telemetry limitations the distributions are compressed to varying degrees in angle and energy, log-count compressed and then further compressed by a lossless technique. This data processing task is supported by two SA3300 microprocessors. The voltages (up to 5 kV) for the tandem toroidal electrostatic analyzers and preacceleration sections are supplied from fixed high voltage supplies using optically controlled series-shunt regulators.     

Radiation-driven diskons: An overview

The current status of the theory of a new astrophysical phenomenon, aradiation-driven diskon, is outlined.The cyclotron radiation pressure around sufficiently hot, strongly magnetized white dwarfs and neutron stars is shown to be able to drive a wind from the photosphere and support a plasma envelope in the closed part of the magnetosphere. The magnetohydrostatic configuration of an optically thin, radiatively supported plasma envelope is determined. It consists of an equatorial disk in the region where the cyclotron radiation force exceeds the local force of gravity and a closed shell near the equilibrium surface where the radiation pressure equals gravity. The effects of finite optical depth on the behaviour of the magnetospheric plasma and the influence of the envelope on the observed radiation are discussed.Classes of magnetic degenerate stars are pointed out in which radiation-driven diskons may be found. The best candidates are two individual stars, the strongly magnetized white dwarfs GD 229 and PG 1031+234. Both exhibit broad and deep depressions in the ultraviolet which are explained as a result of cyclotron scattering by an optically thick radiation-driven envelope in the inhomogeneous magnetic field of the star. We predict a temporal and spectral variability of these features due to non-stationary plasma motions in the envelope.     

Wave particle interactions as an energy transfer mechanism between different particle species

Recent improvements in experimental techniques and cooperative data analysis efforts have brought a lot of information on the basic mechanisms by which energy can be exchanged between different particle species in the collisionless magnetospheric or solar wind plasmas. Some of these mechanisms are reviewed. A particular emphasis is put on interactions which occur in the equatorial magnetosphere between energetic protons and electromagnetic ultra low frequency (ULF) waves and which are linked with He⁺ ion trapping and heating as well as with field-aligned suprathermal electron beam generation. The process by which ion conic distributions are produced by electrostatic ion cyclotron waves generated at high altitude along auroral field lines by drifting electrons is also discussed.     

Three-dimensional plasma measurements within the earth's magnetosphere

First magnetospheric measurements of the three-dimensional velocity distributions for positive ions and electrons within the energy range 1 eV E/Q 45keV are reported. These velocity distributions are gained with quadrispherical Lepedeas on board the spacecraft ISEE-1 and -2. Three-dimensional bulk flows of protons in the vicinity of the magnetopause and within the dayside magnetosphere and dawn sector of the magnetotail are presented. Proton drift velocities within the magnetosphere and magnetotail can be directly determined and employed to calculate the corresponding quasi-static perpendicular electric fields and to provide quantitative analyses of kinematical models for plasma motions. Nonmonotonic features in the electron velocity distributions are found simultaneously with the presence of electron cyclotron harmonic electrostatic waves in the dayside magnetosphere. The relationship of the observed electron velocity distributions to expectations for resonant pitch-angle and energy diffusion is discussed, as well as the possibility of the existence of proton cyclotron harmonic instabilities. Examples of the signature of field-aligned acceleration of protons into the magnetosphere and the presence of low-energy ionospheric ions in the near-earth magnetotail are also presented. Perpendicular electrostatic fields can be calculated from the observed three-dimensional velocity distributions and are found to have typical magnitudes of 1 mV m^-1.     

Substorm aspects of magnetic pulsations

The different types of magnetic pulsations occurring during magnetospheric substorms are analysed into the concept of polar substorms recently described in detail by Akasofu (1968). Special attention is thus paid, to the simultaneous occurrence of different types of micropulsations at different places around the earth, during the development of a substorm. Time lags between the appearance of micropulsations and other geophysical effects of the substorm are of fundamental importance in this respect. Relationships between the occurrence or spectral shape of micropulsations and the state of the magnetosphere, as determined by satellite measurements are also of interest. Recent theoretical studies about the origin of these micropulsations are reviewed: natural h.m. emissions are directly linked to the thermal plasma density, the high energetic particle fluxes and pitch angle distributions in the far magnetosphere (L 5–8). We can thus expect to be able to deduce some information about the changes of these quantities during substorms. New semi-quantitative work is reported, which tries to interpret the repetitive structure of SIP events in terms of thickness of the magnetospheric tail, and the frequency drift of IPDP's in terms of magnetospheric electric fields. The present knowledge about absorption and dispersion of hydromagnetic waves through the ionosphere or inside the submagneto-ionospheric guide is also stated, because not taking these effects into account could lead to misinterpretation of the data.     

The plasma mantle: Composition and other characteristics observed by means of the PROGNOZ-7 satellite

A brief summary of the main results of magnetospheric ion composition measurements in general is first presented. PROGNOZ-7 measurements in the nightside plasma mantle are then described and analyzed. Some of the results are the following: In the nightside mantle not too far from midnight the properties of the mantle are sometimes consistent with the open magnetosphere model. However during most magnetic storm situations O⁺ ions appear in the mantle in large proportions and with high energies. The acceleration process affecting the ions has been found in several cases to give equal amounts of energy to all ions independent of mass. Along the flanks of the magnetosphere the flow of the plasma is often low or absent. The O⁺ content is high (up to 20%) and the energy spectrum of both ions and electrons may be very hot, even up to the level of the ring current plasma in the keV range.The O⁺ content in the plasma mantle is positively correlated with the magnetospheric activity level. The mantle, however, does not appear to be the dominating source for the storm time ring current. Direct acceleration of ionospheric ions onto the closed field lines of the plasma sheet and ring current is most likely the main source. The magnetopause on the nightside and along the flanks of the magnetosphere appears to be a fairly solid boundary for mantle ions of ionospheric origin. This is especially evident during periods with high geomagnetic activity, when the mantle is associated with fairly strong fluxes of O⁺ ions.An interesting observation in most of the mantle passages during geomagnetically disturbed periods is the occurrence of intense, magnetosheath like, regions deep inside the mantle. In some cases these regions with strong antisunward flow and with predominant magnetosheath ion composition was observed in the innermost part of the mantle, i.e. marking a boundary region between the lobe and the mantle. These magnetosheath penetration events are usually associated with strong fluxes of accelerated ionospheric ions in nearby parts of the mantle. Evanescent penetration regions with much reduced flow properties are frequently observed in the flank mantle.     

Energetics of the magnetosphere

Energy flow in various large-scale processes of the Earth's magnetosphere is examined. This energy comes from the solar wind, via the dawn-to-dusk convection electric field, a field established primarily by magnetic merging but with viscous-like boundary interaction as a possible contributor. The convection field passes about 5 × 10¹¹ W to the near-Earth part of the plasma sheet, and also moves the plasma earthward. In addition, 1–3 × 10¹¹ W are given to the complex system of the Birkeland currents: about 4 × 10¹⁰ of this, on the average, goes to parallel acceleration, chiefly of auroral electrons, about 2–3 times that amount to joule heating of the ionosphere, and the rest heats the ring current. The ring current stores energy (mainly as kinetic energy of particles) of the order of 2 × 10¹⁵ J, and this value rises and decays during magnetic storms, on time scales ranging from a fraction of a day to several days. The tail can store comparable amounts as magnetic energy, and appreciable fractions of its energy may be released in substorms, on time scales of tens of minutes. The sporadic power level of such events reaches the order of 3 × 10¹² W. The role of magnetic merging in such releases of magnetic energy is briefly discussed, as is the correlation between properties of the solar wind and magnetospheric power levels.     

The Time-of-Flight Energy,Angle, Mass Spectrograph (Teams) Experiment for Fast

The Time-of-flight Energy Angle Mass Spectrograph (TEAMS) is being flown on the FAST Small Explorer mission to measure the 3-dimensional distribution function of the major ion species present in the lower magnetosphere. The instrument is similar to time-of-flight plasma analyzer systems that have been designed and planned for flight as CODIF (COmposition and DIstribution Function analyzer) on the four European Space Agency Cluster-II spacecraft and, as ESIC (Equator-S Ion Composition instrument) on Equator-S. This instrument allows the 3-dimensional distribution functions of individual ion species to be determined within spin period (2.5 s). Two-dimensional distributions are measured in 80 ms. These capabilities are crucial for the study of selective energization processes in the auroral regions of the magnetosphere. The design, operational characteristics, and test and calibration results for this instrument are presented. The sensor consists of a toroidal top-hat electrostatic analyzer with instantaneous acceptance of ions over 360° in polar angle. After post-acceleration of the incoming ions by up to 25 kV, a time-of-flight mass spectrograph discriminates the individual species. It has been demonstrated through calibration that the instrument can easily separate H⁺, He²⁺, He⁺, O⁺ and, for energies after post-acceleration of > 20 keV, even O₂ ⁺ molecules. On-board mass discrimination and the internal accumulation of several distinct data quantities combined with the spacecraft's flexible telemetry formatting allow for instrument data rates from 7.8 kb s^–1 to 315 kb s^–1 to be telemetered to ground through the FAST centralized Instrument Data Processor.     

Heavy ions in the magnetosphere

For purposes of this review heavy ions include all species of ions having a mass per unit charge of 2 AMU or greater. The discussion is limited primarily to ions in the energy range between 100 eV and 100 keV. Prior to the discovery in 1972 of large fluxes of energetic O⁺ ions precipitating into the auroral zone during geomagnetic storms, the only reported magnetosphere ion species observed in this energy range were helium and hydrogen. More recently O⁺ and He⁺ have been identified as significant components of the storm time ring current, suggesting that an ionosphere source may be involved in the generation of the fluxes responsible for this current. Mass spectrometer measurements on board the S3-3 satellite have shown that ionospheric ions in the auroral zone are frequently accelerated upward along geomagnetic field lines to several keV energy in the altitude region from 5000 km to greater than 8000 km. These observations also show evidence for acceleration perpendicular to the magnetic field and thus cannot be explained by a parallel electric field alone. This auroral acceleration region is most likely the source for the magnetospheric heavy ions of ionospheric origin, but further acceleration would probably be required to bring them to characteristic ring current energies. Recent observations from the GEOS-1 spacecraft combined with earlier results suggest comparable contributions to the hot magnetopheric plasma from the solar wind and the ionosphere.Proceedings of the Symposium on Solar Terrestrial Physics held in Innsbruck, May–June 1978.     

The Thermal Ion Dynamics Experiment and Plasma Source Instrument

The Thermal Ion Dynamics Experiment (TIDE) and the Plasma Source Instrument (PSI) have been developed in response to the requirements of the ISTP Program for three-dimensional (3D) plasma composition measurements capable of tracking the circulation of low-energy (0–500 eV) plasma through the polar magnetosphere. This plasma is composed of penetrating magnetosheath and escaping ionospheric components. It is in part lost to the downstream solar wind and in part recirculated within the magnetosphere, participating in the formation of the diamagnetic hot plasma sheet and ring current plasma populations. Significant obstacles which have previously made this task impossible include the low density and energy of the outflowing ionospheric plasma plume and the positive spacecraft floating potentials which exclude the lowest-energy plasma from detection on ordinary spacecraft. Based on a unique combination of focusing electrostatic ion optics and time of flight detection and mass analysis, TIDE provides the sensitivity (seven apertures of 1 cm² effective area each) and angular resolution (6°×18°) required for this purpose. PSI produces a low energy plasma locally at the POLAR spacecraft that provides the ion current required to balance the photoelectron current, along with a low temperature electron population, regulating the spacecraft potential slightly positive relative to the space plasma. TIDE/PSI will: (a) measure the density and flow fields of the solar and terrestrial plasmas within the high polar cap and magnetospheric lobes; (b) quantify the extent to which ionospheric and solar ions are recirculated within the distant magnetotail neutral sheet or lost to the distant tail and solar wind; (c) investigate the mass-dependent degree energization of these plasmas by measuring their thermodynamic properties; (d) investigate the relative roles of ionosphere and solar wind as sources of plasma to the plasma sheet and ring current.Deceased.     

Skylab measurements of low energy cosmic rays

In this review the present state of our knowledge on the properties of heavy ions in low energy cosmic rays measured in the Skylab mission and in other spacecrafts is summarised and the possible mechanisms of their origin are discussed. A brief review of the general features of the galactic and solar cosmic rays is given in order to understand the special features of the low energy heavy ions of cosmic rays. The results of the cosmic ray experiment in the Skylab show that in the low energy interval of 8–30 MeV/N, the abundances of oxygen, nitrogen, and neon ions, relative to carbon are enhanced by a factor of 5 to 2 as compared to high energy cosmic rays; while Mg, Si, S, and A are depleted. In 50–150 MeV/N energy interval the abundance of nuclei of Ca-Cr relative to iron-group (Z = 25–28) is found to be highly enhanced, as compared to high energy cosmic rays. Furthermore the observations of the energy spectra of O, N, and Ne ions and their fairly large fluences in the energy interval of 8–30 MeV/N below the geomagnetic cut off energy of 50 MeV/N for fully stripped nuclei at the Skylab orbit indicate that these heavy ions are probably in partly ionised states. Thus, it is found that the Skylab results represent a new type of heavy ion population of low energy cosmic rays below 50 MeV/N, in the near Earth space and their properties are distinctly different from those of high energy cosmic rays and are similar to those of the anomalous component in the interplanetary space. The available data from the Skylab can be understood at present on the hypothesis that low energy interplanetary cosmic ray ions of oxygen etc. occur in partly ionised state such as O⁺¹,O⁺², etc. and these reach the inner magnetosphere at high latitudes where stripping process occurs near mirror points and this leads to temporarily trapped ions such as O⁺³, O⁺⁴, etc. It is noted that the origin of these low energy heavy cosmic ray ions in the magnetosphere and in interplanetary space is not yet fully understood and new type of sources or processes are responsible for their origin and these need further studies.     

High-Latitude Particle Precipitation and its Relationship to Magnetospheric Source Regions

Two central issues in magnetospheric research are understanding the mapping of the low-altitude ionosphere to the distant regions of the magnetsphere, and understanding the relationship between the small-scale features detected in the various regions of the ionosphere and the global properties of the magnetosphere. The high-latitude ionosphere, through its magnetic connection to the outer magnetosphere, provides an important view of magnetospheric boundaries and the physical processes occurring there. All physical manifestations of this magnetic connectivity (waves, particle precipitation, etc.), however, have non-zero propagation times during which they are convected by the large-scale magnetospheric electric field, with phenomena undergoing different convection distances depending on their propagation times. Identification of the ionospheric signatures of magnetospheric regions and phenomena, therefore, can be difficult. Considerable progress has recently been made in identifying these convection signatures in data from low- and high-altitude satellites. This work has allowed us to learn much about issues such as: the rates of magnetic reconnection, both at the dayside magnetopause and in the magnetotail; particle transport across the open magnetopause; and particle acceleration at the magnetopause and the magnetotail current sheets.     

Mapping Magnetospheric Equatorial Regions at Saturn from Cassini Prime Mission Observations

Saturn??s rich magnetospheric environment is unique in the solar system, with a large number of active magnetospheric processes and phenomena. Observations of this environment from the Cassini spacecraft has enabled the study of a magnetospheric system which strongly interacts with other components of the saturnian system: the planet, its rings, numerous satellites (icy moons and Titan) and various dust, neutral and plasma populations. Understanding these regions, their dynamics and equilibria, and how they interact with the rest of the system via the exchange of mass, momentum and energy is important in understanding the system as a whole. Such an understanding represents a challenge to theorists, modellers and observers. Studies of Saturn??s magnetosphere based on Cassini data have revealed a system which is highly variable which has made understanding the physics of Saturn??s magnetosphere all the more difficult. Cassini??s combination of a comprehensive suite of magnetospheric fields and particles instruments with excellent orbital coverage of the saturnian system offers a unique opportunity for an in-depth study of the saturnian plasma and fields environment. In this paper knowledge of Saturn??s equatorial magnetosphere will be presented and synthesised into a global picture. Data from the Cassini magnetometer, low-energy plasma spectrometers, energetic particle detectors, radio and plasma wave instrumentation, cosmic dust detectors, and the results of theory and modelling are combined to provide a multi-instrumental identification and characterisation of equatorial magnetospheric regions at Saturn. This work emphasises the physical processes at work in each region and at their boundaries. The result of this study is a map of Saturn??s near equatorial magnetosphere, which represents a synthesis of our current understanding at the end of the Cassini Prime Mission of the global configuration of the equatorial magnetosphere.     

Ring Currents and Internal Plasma Sources

The discovery of terrestrial O⁺ and other heavy ions in magnetospheric hot plasmas, combined with the association of energetic ionospheric outflows with geomagnetic activity, led to the conclusion that increasing geomagnetic activity is responsible for filling the magnetosphere with ionospheric plasma. Recently it has been discovered that a major source of ionospheric heavy ion plasma outflow is responsive to the earliest impact of coronal mass ejecta upon the dayside ionosphere. Thus a large increase in ionospheric outflows begins promptly during the initial phase of geomagnetic storms, and is already present during the main phase development of such storms. We hypothesize that enhancement of the internal source of plasma actually supports the transition from substorm enhancements of aurora to storm-time ring current development in the inner magnetosphere. Other planets known to have ring current-like plasmas also have substantial internal sources of plasma, notably Jupiter and Saturn. One planet having a small magnetosphere, but very little internal source of plasma, is Mercury. Observations suggest that Mercury has substorms, but are ambiguous with regard to the possibility of magnetic storms of the planet. The Messenger mission to Mercury should provide an interesting test of our hypothesis. Mercury should support at most a modest ring current if its internal plasma source is as small as is currently believed. If substantiated, this hypothesis would support a general conclusion that the magnetospheric inflationary response is a characteristic of magnetospheres with substantial internal plasma sources. We quantitatively define this hypothesis and pose it as a problem in comparative magnetospheres.