Plasma populations in a simple open model magnetosphere

The acceleration of charged particles in the magnetic current sheets downstream from magnetic neutral lines is discussed and related to the plasma populations expected to be formed in a simple open model magnetosphere. A simple model of plasma acceleration in the dayside current sheet is set up, and it is shown that magnetospheric particles may take up a considerable fraction of the electromagnetic energy dissipated in the sheet even though they may represent only a small fraction of the total particle influx. The process should result in energetic ring current and ionospheric particles being found in boundary layers on either side of the magnetopause, and accelerated ionospheric particles in the plasma mantle. Acceleration of magnetosheath plasma in the dayside current sheet should result in enhanced flow speeds in these boundary layers, but the process may amount to little more than a return to the sheath plasma of energy previously extracted from it during its inflow on the dayside and stored in the compressed sheath field, due to the appreciable energy take-up from the current sheet by magnetospheric particles. The energy separation between ionospheric plasma and magnetosheath plasma on cusp field lines is shown to result in a spatial separation of polar wind and plasma mantle populations in the tail, the polar wind ions usually reaching out to only a few tens of R _E down-tail such that they usually remain on closed field lines, forming a wedge-shaped region within the mantle shadow-zone. Polar wind ions are then convected back towards the Earth and thus their major sink is via the dayside current sheet rather than outflow into the tail. The major source for the plasmasheet depends upon the location of the neutral line, but mantle ions may usually be dominant. However, with a near-Earth neutral line during disturbed periods ionospheric plasma will be the sole ring-current source. Under usual conditions with a more distant neutral line the spatial separation of the two plasma sources in the tail may result in an energy separation in the inner ring current, with ionospheric particles dominant up to 2 to 20 keV and mantle ions dominant at higher energies. Formation of the plasmasheet is discussed, and it is shown that a layer of ions unidirectionally streaming towards the Earth should be formed on its outer boundary, due to current sheet acceleration of lobe particles and inward convection of the field lines. A similar process leads to earthward flows on the inner layer of the dayside cusp. Finally, the region tailward of the nightside neutral line is discussed and it is shown that a thin tailward flowing two-stream plasma band should be formed across the centre plane of the tail. The slow-speed stream corresponds to incoming lobe ions, the faster stream to the current sheet accelerated ions.     

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.     

Non-linear resonant excitation of whistler mode waves in the Earth's ionosphere

We examine the resonant non-linear interaction in the Earth's ionosphere of two powerful high frequency radio beams with frequencies f ₁ and f ₂ (both larger than the plasma frequency at F2_max) and wave numbers k ₁ and k ₂ such that a whistler mode wave can be excited with a frequency f ₃ = f ₁ — f ₂ and a wave number k ₃ = k ₁ – k ₂. The feasibility of an effective ground based installation, sited at low latitudes, is discussed and the field strength of the wave emerging from a 10 km wide ionospheric region illuminated by the beams is evaluated for a range of transmitted frequencies, beam orientations and plasma frequencies in the interaction region. It is suggested that the longitude dependence of the enhancement of VLF noise bands detected by the Ariel 3 satellite may be due to a non-linear interaction of this type between any two or more medium wavelength signals from areas where there is a high concentration of commercial broadcasting stations, such as the NE region of the U.S.A.     

Electric currents and voltage drops along auroral field lines

The current state of knowledge concerning Birkeland currents (j _∥) and parallel electric field (E _∥) is briefly reviewed. Four types of j _∥ are discussed-the primary ‘region 1’ sheets, the ‘region 2’ sheets which parallel them and which seem to close in the partial ring current, the cusp currents which appear to correlate with interplanetary B _y, and the ‘Harang filament’. The energy required by E _∥ and by the associated particle acceleration processes seems to be derived from j _∥. Much of the evidence for e _∥ comes from particles, from ‘inverted V’ spectra, rising ion beams and expanded loss cones, while ‘conies’ may signify acceleration by Electrostatic Ion Cyclotron (EIC) waves, associated with beams accelerated by E _∥. Different theoretical studies predict for E _∥ a smooth, disordered or abrupt structure, and evidence for all 3 types can be deduced from S3-3 electric field probe observations.

     

The Cassini Ion and Neutral Mass Spectrometer (INMS) Investigation

The Cassini Ion and Neutral Mass Spectrometer (INMS) investigation will determine the mass composition and number densities of neutral species and low-energy ions in key regions of the Saturn system. The primary focus of the INMS investigation is on the composition and structure of Titan’s upper atmosphere and its interaction with Saturn’s magnetospheric plasma. Of particular interest is the high-altitude region, between 900 and 1000 km, where the methane and nitrogen photochemistry is initiated that leads to the creation of complex hydrocarbons and nitriles that may eventually precipitate onto the moon’s surface to form hydrocarbon–nitrile lakes or oceans. The investigation is also focused on the neutral and plasma environments of Saturn’s ring system and icy moons and on the identification of positive ions and neutral species in Saturn’s inner magnetosphere. Measurement of material sputtered from the satellites and the rings by magnetospheric charged particle and micrometeorite bombardment is expected to provide information about the formation of the giant neutral cloud of water molecules and water products that surrounds Saturn out to a distance of ∼12 planetary radii and about the genesis and evolution of the rings.The INMS instrument consists of a closed ion source and an open ion source, various focusing lenses, an electrostatic quadrupole switching lens, a radio frequency quadrupole mass analyzer, two secondary electron multiplier detectors, and the associated supporting electronics and power supply systems. The INMS will be operated in three different modes: a closed source neutral mode, for the measurement of non-reactive neutrals such as N₂ and CH₄; an open source neutral mode, for reactive neutrals such as atomic nitrogen; and an open source ion mode, for positive ions with energies less than 100 eV. Instrument sensitivity is greatest in the first mode, because the ram pressure of the inflowing gas can be used to enhance the density of the sampled non-reactive neutrals in the closed source antechamber. In this mode, neutral species with concentrations on the order of ≥10⁴ cm⁻³ will be detected (compared with ≥10⁵ cm⁻³ in the open source neutral mode). For ions the detection threshold is on the order of 10⁻² cm⁻³ at Titan relative velocity (6 km sec⁻¹). The INMS instrument has a mass range of 1–99 Daltons and a mass resolutionM/ΔM of 100 at 10% of the mass peak height, which will allow detection of heavier hydrocarbon species and of possible cyclic hydrocarbons such as C₆H₆.The INMS instrument was built by a team of engineers and scientists working at NASA’s Goddard Space Flight Center (Planetary Atmospheres Laboratory) and the University of Michigan (Space Physics Research Laboratory). INMS development and fabrication were directed by Dr. Hasso B. Niemann (Goddard Space Flight Center). The instrument is operated by a Science Team, which is also responsible for data analysis and distribution. The INMS Science Team is led by Dr. J. Hunter Waite, Jr. (University of Michigan).This revised version was published online in July 2005 with a corrected cover date.     

Wave motions in the atmosphere and related ionospheric phenomena

We review important studies in the field of stratosphere-ionosphere coupling, including recent studies of wave motions of planetary waves, atmospheric tides and internal gravity waves in the atmosphere. The interrelation between stratospheric sudden warmings and winter anomaly of radio absorption, a dynamical model of stratospheric sudden warmings and some production mechanisms of intensified electron density in the D region are discussed. Other topics presented are atmospheric tides in the lower thermosphere including dynamo action, and internal gravity waves, by which we intend to explain travelling ionospheric disturbances in the F ₂ region and sporadic E layer at midlatitude (wave-enhanced sporadic E). Thermospheric winds are also reviewed and wind effects on the F ₂ layer are discussed. For each atmospheric event systematic observations of suitable physical quantities with proper time and spatial intervals are desirable.     

Transient phenomena in the magnetotail and their relation to substorms

Recent observations of magnetic field, plasma flow and energetic electron anisotropies in the magnetotail plasma sheet during substorms have provided strong support for the idea that a magnetospheric substorm involves the formation of a magnetic neutral line (the substorm neutral line) within the plasma sheet at X _SM — 10R _E to -25R _E. An initial effect, in the tail, of the neutral line's formation is the severance of plasma sheet field lines to form a plasmoid, i.e., a closed magnetic loop structure, that is quickly (within 5–10 min) ejected from the tail into the downstream solar wind. The plasmoid's escape leaves a thin downstream plasma sheet through which plasma and energetic particles stream continuously into the solar wind, often throughout the duration of the substorm's expansive phase. Southward oriented magnetic field threads this tailward-flowing plasma but its detection, as an identifier of the occurrence of magnetic reconnection, is made difficult by the thinness and turbulence of the downstream plasma sheet. The thinning of the plasma sheet downstream of the neutral line is observed, by satellites located anywhere but very close to the tail's midplane, as a plasma dropout. Multiple satellite observations of plasma droputs suggest that the substorm neutral line often extends across a large fraction (> ) of the tail's breadth. Near the time of substorm recovery the substorm neutral line moves quickly tailward to a more distant location, progressively inflating the closed field lines earthward of it, to reform the plasma sheet.Proceedings of the Symposium on Solar Terrestrial Physics held in Innsbruck, May–June 1978.     

Dynamics of the geomagnetic storm

This paper is intended as a critical review of current ideas concerning the mechanisms responsible for the geomagnetic storm.The dynamical theory of the geomagnetic storm phenomenon is formulated as a problem in elasticity. The observed variations in the field are the strains produced by particle stresses exerted by gases in interplanetary space, by gases enmeshed in the field, and by the gases in the ionosphere. The stresses exerted by interplanetary gases are principally inward, resulting in the initial phase increase of the horizontal component. The stresses exerted by gases enmeshed in the field are principally outward, resulting in the main phase decrease of the horizontal component. The transient sudden commencement is a hydromagnetic wave phenomenon.The main phase is most simply explained by the shock heating of the ions to kev energies at 3 – 5 R _E during the active phase of the storm. The recovery follows then from charge exchange with the ambient neutral hydrogen. The predicted more rapid recovery at sunspot minimum has been verified observationally.This work was supported by the National Aeronautics and Space Administration under grant NASA-NsG-96-60.     

Review of the CIV phenomenon

Alfvén's Critical Ionization Velocity (CIV) phenomenon is reviewed, with the main emphasis on comparisons between experimental and theoretical results. The review covers (1) the velocity measurements in laboratory experiments, (2) the effect of wall interaction, (3) the experimental and theoretical limits to the magnetic field strength and the neutral density, (4) ionospheric release experiments, (5) theoretical models for electron energization in comparison to experimental results, and (6) CIV models. All laboratory investigations of the CIV are found to obey the three following simple rules of thumb: (1) if the magnetic field is so strong that V _A > 3V ₀, and if there is enough neutral gas that the Townsend condition is fulfilled, then the CIV effect occurs, (2) when it occurs, the threshold velocity (or E/B value) is within ± 50% of Alfvén's proposed value V _c , and (3) for weaker magnetic fields, the effect gradually becomes irreproducible or weak and disappears altogether for V _A < V ₀. The theoretical understanding of the process has grown rapidly during the last decade, mainly due to the introduction of computer simulation models which have to a large degree confirmed and extended earlier analytical theories. The CIV mechanism is not due to one single plasma process, but to several different mechanisms which are applicable in different parameter regimes and geometries. The computer simulations have shown that in order to understand the mechanism properly it is necessary to consider a large number of interlocking collisional and plasma processes. The theoretical development has reached the stage where it should be possible to adapt computer simulation models to specific experiments and predict ionization rates, plasma flow velocities, E/B values, particle distributions, and wave spectra. Such models should for the first time provide a really firm basis for extrapolations of the CIV process to space applications.     

Magnetic field measurements in space

Conclusions The magnetosphere boundary has been penetrated in several places, conflicting evidence about the ring current location has been found, and the field exterior to the boundary has revealed some unexpected features. Pronouncements about the structure of the geomagnetic and interplanetary magnetic fields are still based on scanty evidence but the experimental basis of such estimates is more adequate than in 1958.The boundary between the geomagnetic field and the interplanetary medium has been found, by Explorer XII, to be located at approximately 10 R _E on the sunlit side of the earth near the equator. It has been observed to fluctuate between 8 and 12 R _E during August, September and October of 1961. During several days in March, 1961, the boundary, on the dark side of the earth, was penetrated repeatedly by Explorer X on an outbound pass near 135° from the earth-sun line. Several interpretations are possible; the most reasonable one at present is that the boundary was fluctuating in this period, placing the satellite alternately inside the geomagnetic field and outside in a region of turbulent magnetic fields and plasma flow.A region of turbulent magnetic fields was also observed by Pioneer I, Pioneer V, and Explorer XII between 10 and 15 R _E on the sunlit side of the earth. Pioneer V observed also a steady field 2 to 5 gammas in magnitude beyond 20 R _E. It appears that there exists a region of turbulent magnetic fields between the geomagnetic field boundary near 10 R _E, and another boundary, located near 14–15 R _E near the earth-sun line. This second boundary was seen only by Pioneer I and Pioneer V; Explorer XII and Explorer X apparently did not reach it. This boundary has been tentatively identified as a shock front in the flow of solar plasma about the magnetosphere (see Figure 5).^{41, 42} The geomagnetic field inside the boundary is relatively quiet. An abrupt transition in the magnitude of fluctuations occurs at the boundary surface. The ratio of fluctuation amplitude, B, to average field, B, decreases from 1 to 0.1 on a passage through the boundary on 13 September 1961.⁴³ The boundary is not unstable in the solar wind but fluctuations in solar wind pressure do cause changes in boundary location.^42,43 The ring current location appears to be above 1.4 R _E and below 5 R _E on the basis of Pioneer I, Vanguard III, and Explorer XII data. Lunik I and II records indicate that it is located between 3 and 4 R _E. Explorer VI data indicates that it must be at distances greater than 4 R _E on the dark side of the earth. Some variation in altitude of a ring current with time appears likely, but the bulk of present evidence limits a possible ring current to a distance of 3 to 5 R _E.The interplanetary field during quiet times is of the order of 2 to 5 gammas. The direction indicated for this field, with a significant component perpendicular to the earth-sun line, is puzzling in view of solar cosmic ray transit times. Solar disturbances with resultant plasma flow past the satellite produce increases in the field magnitude. Field increases at the satellite are sometimes correlated with disturbances observed at the earth.Further investigations are needed to map the magnetosphere and boundary more completely, to investigate the postulated shock front and the turbulent region inside, to refute or confirm the ring current theory, and to measure the interplanetary field direction and magnitude more completely. Theoretical studies are needed to support these experiments and to suggest new avenues of investigations. Particularly needed are theoretical investigations of collisionless shock fronts in plasma flow and of characteristics of the flow between the shock front and the obstacle.     

An overview by pioneers observations of the distant geomagnetic tail

Pioneer 7 and Pioneer 8 spacecraft provided the only direct observations of the geomagnetic tail at geocentric distances as large as 1000R _e and 500R _e respectively. The presence of a low density plasma flow in the region of expected tail and the intermittent and short duration character of the tail encounters suggested in the past a distant tail structure remarkably different from its near-earth and cislunar shape. However the recent discovery of the plasma mantle allows to interpret the Pioneer observations in terms of a distant tail that possibly is still preserving most of its near-earth characteristics. In particular, the region of tail encounters and the magnitude and direction of the observed magnetic field might be consistent with a cylindrical tail with a modestly increased cross-section. Neutral sheet observations also appear to be consistent with the most recent bidimensional tail models. Finally, as in the cislunar region, the double peaked proton energy spectra can be interpreted in terms of a partial intermingling of plasma sheet and plasma mantle populations.Also at Laboratorio Plasma nello Spazio, CNR, Frascati.     

Magnetic field variations in the polar region during magnetically quiet periods and interplanetary magnetic fields

The concepts of near-pole magnetic field variations during magnetically quiet periods are explored, with special emphasis on the relationships of these variations to interplanetary magnetic field components. Methods are proposed for relating the variations which have been observed to the fields from the various sources, based on a thorough selection of reference levels. We assume that the field variations in the summer polar cap during magnetically quiet periods consist of the following components: (i) the middle-latitude S _qvariation extended to the polar region; (ii) the DPC(B _y) single-cell current system with a polar electrojet in day-side cusp latitudes; (iii) the DMC(B _z) two-cell current system of magnetospheric convection, in the form of a homogeneous current sheet in the polar cap towards the sun, with return currents through lower latitudes; (iv) the DPC(B _z) single-cell counterclockwise current system with a focus in the day-side cusp region. Quantitative relations between the near-pole variation intensities and the value and sign of the IMF azimuthal component, with a 1 hr time resolution, have been obtained and used to suggest ways of diagnosing the interplanetary magnetic field on the basis of ground observations.     

Observations of the Martian Subsolar ENA Jet Oscillations

The Neutral Particle Detector (NPD) of the ASPERA-3 experiment (Analyser of Space Plasmas and Energetic Atoms) on board the Mars Express (MEX) spacecraft observed an intense flux of H ENAs (energetic neutral atoms) with average energy of about 1.5 keV emitted anisotropically from the subsolar region of Mars. The NPD detected the ENA jet near the bow shock at radial distances of about 1 R _M from the Martian surface as the spacecraft moved outbound, while the NPD continuously pointed towards the subsolar region. The jet intensity shows oscillative behavior. These intensity variations occur on two clearly distinguishable time scales. The majority of the identified events have an average oscillation period of about 50 sec. The second group consists of events with long-scale variations with a time scale of approximately 300 sec. The fast oscillations of the first group exhibit a periodic structure and are detected in every orbit, while the slow variations of the second group are identified in ∼40% of orbits. The intensity of the fast oscillations have a peak-to-valley ratio about 20 to 30% of the peak intensity. One of the possible mechanisms to explain fast oscillations is the formation of the low frequency ion waves at the subsolar region of Mars. Slow variations may be explained by either temporal variations in the ENA generation source or by a specific structure of the ENA generation source, in which hair-like ENA subjets can be present.     

Magnetic storms and associated phenomena

This review will not merely be a précis of the literature in this field though a partial survey is attempted. A critical stand will be taken and a point of view put forward. Experiments to test this point of view and others will be suggested. Several new ideas are introduced.Two broad conditions of the magnetosphere are discussed, the quiet and the disturbed. During the quiet condition, the polar cap F region either glows red or is filled with a family of red auroral arcs parallel roughly to L-contours. Auroras near the auroral zone have an increasing amount of green (5577) coloration. The ionospheric F region exists even in winter over the polar caps despite the absence of solar ionizing radiation or obvious corpuscular bombardment. The red polar glow and the maintenance of the quiet polar winter F region are suggested to be accounted for by the cooling of plasma in the geomagnetic tail. These phenomena consume less than 0.01 of the energy and flux of the solar wind impinging on the magnetosphere. The relevance of dynamo theory to this quiet condition is discussed.During the disturbed condition, many phenomena such as polar magnetic substorms, auroral substorms, the sudden appearance of islands of energetic particles in the magnetosphere, and the rapid acceleration of auroral particles appear to call for the operation of an instability deep in the magnetosphere.The energetics of various facets of geomagnetic disturbance are discussed, and joule dissipation of ionospheric current is found to be a major sink of energy during storms. This causes significant heating of the ionosphere particularly at the site of auroral electrojets. Corpuscular bombardment may consume as much energy, but its heating effect is likely to be less.The stable auroral red arc (SAR-arc) observed equatorwards of normal active aurora during magnetic storms is a major sink of energy of a magnetospheric ring current. It is contended that the ring current generally consists of particles of energy of less than a few keV. It is suggested that the ring current is caused by the irreversible pumping and energisation of plasma from the outer to the inner magnetosphere. This pumping is achieved by the random electrostatic fields associated with the noisy component of geomagnetic disturbance. The SAR-arc must be a major feature of ring current theory.The consumption of energy in polar magnetic and auroral substorms, during a complete storm, is tentatively concluded to be far greater than that of the ring current. The ring current is considered to be a byproduct of magnetic disturbance on higher L-shells.The main phase of a storm should be considered, in storm analysis, as a separate entity from the initial phase, for physically they bear a tenuous and unpredictable relationship to one another. A new system of analysis is proposed in which the onset of geomagnetic noise rather than sudden commencement is taken as the origin of time, both for magnetic and ionospheric storms. This will enable analysis of storms with both gradual and sudden commencements to be made on a common basis.No reliable evidence is found to support the contention that magnetic storms are caused dominantly by neutral H-atoms ejected from the sun. In fact much evidence can be amassed to deny this hypothesis.     

Application of the restricted problem of three bodies to space mechanics

The most celebrated problem of dynamics, the problem of three bodies, is defined in the first chapter and it is shown how the problème restreint is obtained from the general formulation. This is followed by a historical review of the restricted problem from the point of view of space dynamics applications and the present state of the art is described in some detail. The third and final chapter reviews the major activities in the field of space mechanics and their relation to the restricted problem are pointed out.The purpose of this compte rendu is to demonstrate two theses. Firstly, based on the history of the restricted problem of three bodies, it will be shown that combining recent advances in computer technology and in analytical dynamics with modern topological techniques, a terra firma exists for a new attack on this classical problem.The second thesis is that the restricted problem of three bodies plays a central role in celestial mechanics and in space dynamics. From the second thesis it follows that progress in the solution of this problem will further the case of the entire field of space dynamics.An outline of a combined theoretical and experimental program, leading to possible significant contributions to this pièce de résistance of dynamics, completes the paper.     

Absolute and Convective Instabilities of Circularly Polarized Alfvén Waves

We discuss the recent progress in studying the absolute and convective instabilities of circularly polarized Alfvén waves (pump waves) propagating along an ambient magnetic field in the approximation of ideal magnetohydrodynamics (MHD). We present analytical results obtained for pump waves with small dimensionless amplitude a, and compare them with numerical results valid for arbitrary a. The type of instability, absolute or convective, depends on the velocity U of the reference frame where the pump wave is observed with respect to the rest plasma. One of the main results of our analysis is that the instability is absolute when U _l < U < U _r and convective otherwise. We study the dependences of U _l and U _r on a and the ratio of the sound speed to the Alfvén speed b. We also present the results of calculation of the increment of the absolute instability on U for different values of a and b. When the instability is convective (U < U _l or U > U _r) we consider the signalling problem, and show that spatially amplifying waves exist only when the signalling frequency is in two symmetric frequency bands. Then, we write down the analytical expressions determining the boundaries of these frequency bands and discuss how they agree with numerically calculated values. We also present the dependences of the maximum spatial amplification rate on U calculated both analytically and numerically. The implication of the obtained results on the interpretation of observational data from space missions is discussed. In particular, it is shown that circularly polarized Alfvén waves propagating in the solar wind are convectively unstable in a reference frame of any realistic spacecraft.     

Initial results from the ISEE-1 and -2 plasma wave investigation

In this paper we present an initial survey of results from the plasma wave experiments on the ISEE-1 and -2 spacecraft which are in nearly identical orbits passing through the Earth's magnetosphere at radial distances out to about 22.5R _e . Essentially every crossing of the Earth's bow shock can be associated with an intense burst of electrostatic and whistler-mode turbulence at the shock, with substantial wave intensities in both the upstream and downstream regions. Usually the electric and magnetic field spectrum at the shock are quite similar for both spacecraft, although small differences in the detailed structure are sometimes apparent upstream and downstream of the shock, probably due to changes in the motion of the shock or propagation effects. Upstream of the shock emissions are often observed at both the fundamental, f _- ^p , and second harmonic, 2f _p ^- , of the electron plasma frequency. In the magnetosphere high resolution spectrograms of the electric field show an extremely complex distribution of plasma and radio emissions, with numerous resonance and cutoff effects. Electron density profiles can be obtained from emissions near the local electron plasma frequency. Comparisons of high resolution spectrograms of whistler-mode emissions such as chorus detected by the two spacecraft usually show a good overall similarity but marked differences in detailed structure on time scales less than one minute. Other types of locally generated waves, such as the (n+1/2)f _- ^g electron cyclotron waves, show a better correspondence between the two spacecraft. High resolution spectrograms of kilometric radio emissions are also presented which show an extremely complex frequency-time structure with many closely spaced narrow-band emissions.     

Modelling of the electromagnetic field in the interplanetary space and in the Earth's magnetosphere

A magnetohydrodynamic model of the solar wind flow is constructed using a kinematic approach. It is shown that a phenomenological conductivity of the solar wind plasma plays a key role in the forming of the interplanetary magnetic field (IMF) component normal to the ecliptic plane. This component is mostly important for the magnetospheric dynamics which is controlled by the solar wind electric field. A simple analytical solution for the problem of the solar wind flow past the magnetosphere is presented. In this approach the magnetopause and the Earth's bow shock are approximated by the paraboloids of revolution. Superposition of the effects of the bulk solar wind plasma motion and the magnetic field diffusion results in an incomplete screening of the IMF by the magnetopause. It is shown that the normal to the magnetopause component of the solar wind magnetic field and the tangential component of the electric field penetrated into the magnetosphere are determined by the quarter square of the magnetic Reynolds number. In final, a dynamic model of the magnetospheric magnetic field is constructed. This model can describe the magnetosphere in the course of the severe magnetic storm. The conditions under which the magnetospheric magnetic flux structure is unstable and can drive the magnetospheric substorm are discussed. The model calculations are compared with the observational data for September 24–26, 1998 magnetic storm (Dst _min=−205 nT) and substorm occurred at 02:30 UT on January 10, 1997. This revised version was published online in August 2006 with corrections to the Cover Date.     

Role of ionospheric effects and plasma sheet dynamics in the formation of auroral arcs

At the ionospheric level, the substorm onset (expansion phase) is marked by the initial brightening and subsequent breakup of a pre-existing auroral arc. According to the field line resonance (FLR) wave model, the substorm-related auroral arc is caused by the field-aligned current carried by FLRs. The FLRs are standing shear Alfvén wave structures that are excited along the dipole/quasi-dipole lines of the geomagnetic field. The FLRs (that can cause auroral arc) thread from the Earthward edge of the plasma sheet and link the auroral arc to the plasma sheet region of 6–15 R _E. The region is associated with magnetic fluctuations that result from the nonlinear wave-wave interactions of the cross-field current-instability. The instability (excited at the substorm onset) disrupts the cross-tail current which is built up during the growth phase of the substorms and results in magnetic fluctuations. The diversion of the current to polar regions can lead to auroral arc intensification. The current FLR model is based on the amplitude equations that describe the nonlinear space-time evolution of FLRs in the presence of ponderomotive forces exerted by large amplitude FLRs (excited during substorms). The present work will modify the FLR wave model to include the effects arising from magnetic fluctuations that result from current disruption near the plasma sheet (6–15 R _E). The nonlinear evolution of FLRs is coupled with the dynamics of plasma sheet through a momentum exchange term (resulting from magnetic fluctuations due to current disruption) in the generalized Ohm's law. The resulting amplitude equations including the effects arising from magnetic fluctuations can be used to study the structure of the auroral arcs formed during substorms. We have also studied the role of feedback mechanism (in a dipole geometry of the geomagnetic field) in the formation of the discrete auroral arc observed on the nightside magnetosphere. The present nonlinear dispersive model (NDM) is extended to include effects arising from the low energy electrons originating from the plasma sheet boundary layer. These electrons increase the ionospheric conductivity in a localized patch and enhance the field-aligned current through a feedback mechanism. The feedback effects were studied numerically in a dipole geometry using the the NDM. The numerical studies yield the magnitude of the field-aligned current that is large enough to form a discrete auroral arc. Our studies provide theoretical support to the observational work of Newell et al. that the feedback instability plays a major role in the formation of the discrete auroral arcs observed on the nightside magnetosphere.