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This paper presents in situ observational evidence from the Cluster Ion Spectrometer (CIS) on Cluster of injected solar wind "plasma clouds" protruding into the day-side high-latitude magnetopause. The plasma clouds, presumably injected by a transient process through the day-side magnetopause, show characteristics implying a generation mechanism denoted impulsive penetration (Lemaire and Roth, 1978). The injected plasma clouds, hereafter termed "plasma transfer events", (PTEs), (Woch and Lundin, 1991), are temporal in nature and relatively limited in size. They are initially moving inward with a high velocity and a magnetic signature that makes them essentially indistinguishable from regular magnetosheath encounters. Once inside the magnetosphere, however, PTEs are more easily distinguished from magnetopause encounters. The PTEs may still be moving while embedded in an isotropic background of energetic trapped particles but, once inside the magnetosphere, they expand along magnetic field lines. However, they frequently have a significant transverse drift component as well. The drift is localised, thus constituting an excess momentum/motional emf generating electric fields and currents. The induced emf also acts locally, accelerating a pre-existing cold plasma (e.g. Sauvaud et al., 2001). Observations of PTE-signatures range from "active" (strong transverse flow, magnetic turbulence, electric current, local plasma acceleration) to "evanescent" (weak flow, weak current signature). PTEs appear to occur independently of Interplanetary Magnetic Field (IMF) Bz in the vicinity of the polar cusp region, which is consistent with observations of transient plasma injections observed with mid- and high-altitude satellites (e.g. Woch and Lundin, 1992; Stenuit et al., 2001). However the characteristics of PTEs in the magnetosphere boundary layer differ for southward and northward IMF. The Cluster data available up to now indicate that PTEs penetrate deeper into the magnetosphere for northward IMF than for southward IMF. This may or may not mark a difference in nature between PTEs observed for southward and northward IMF. Considering that flux transfer events (FTEs), (Russell and Elphic, 1979), are observed for southward IMF or when the IMF is oriented such that antiparallel merging may occur, it seems likely that PTEs observed for southward IMF are related to FTEs.Key words. Magnetospheric physics (magnetopause, cusp, and boundary layers; magnetosphere-ionosphere interactions; solar-wind magnetosphere interactions)

ISHTAR or VENUS ORBITER is a mission to Venus similar to the mission Mars Express to Mars, with the use, possibly, of the same spacecraft and subsystems. This mission has been proposed to ESA for a feasibility study as a small mission (F2, F3). We shall describe here briefly the scientific objectives of the mission, and how could be implemented. The scientific objectives of the mission are evenly distributed in three broad areas: atmospheric science, planetary interior and surface, and interaction with the solar wind. Ishtar will study the surface and the atmosphere of the planet by making use of remote sensing instruments, and, if possible, by a descending meteorological balloon/lander. The model payload of the orbiter should include an Imaging Spectrometer, a Fourier Spectrometer, a Radar imaging/altimeter, a UV spectrometer and a Plasma Package studying also the Energetic Neutral Atoms population resulting from the plasma interaction with the planetary atmosphere.

In this paper we present some results concerning the Aristarchus plateau and Mounts Harbinger regions. The study was conducted using an imaging spectrometer in the 0.4–1.1 μm spectral range at the 1.5 meters Sierra Nevada Observatory telescope. We report the albedo, the UV-VIS continuum and 0.9–1 μm absorption band maps, showing the complex pattern of Aristarchus crater ejecta and the distribution of surrounding materials. The extension of the dark mantling deposits present on Aristarchus plateau is well shown from our data, appearing very red and with a uniform distribution of the 0.9–1 μm band.

We present here an evaluation of basic PFS sounding capabilities; results refer to the analysis of a single, calibrated spectrum. PFS is an infrared Fourier spectrometer on board of Mars Express mission. It covers the thermal infrared (250–1750 cm −1 ) with a resolution of 1.5 cm −1 , as well as the near infrared (2000–8200 cm −1 ), with a resolution of 1.5 cm −1 . It has been designed primarily to investigate the Martian atmosphere in its different cycles and interactions. This task can be achieved monitoring the fields of: (i) temperature (as a function of height) in the lower atmosphere, (ii) pressure, (iii) [H 2 O], and optical thickness at different wavenumbers related to (iv) water ice and (v) dust integrated contents. The above mentioned parameters can be retrieved from a single PFS measurement, taking advantage of the instrument’s wide spectral range and high resolution. Specific retrieval software is under development in IFSI – CNR. Its performance is evaluated using as a reference a wide dataset of simulated PFS observations. The synthetic spectra have been computed taking into account likely environmental conditions (extracted from the European Martian Climate Database) and the actual instrument noise. We present here the general structure of the code; realistic estimates of errors on retrieved quantities are also provided. Special attention is devoted to the evaluation of T(p) profile. These preliminary results demonstrate the capability of PFS to achieve its general scientific objectives.

A marginal stability criterion for ion acoustic waves is applied to the analysis of five thin bow shock crossings observed by OGO-5; the observations began with the satellite in the magnetosheath. The assumption of marginally stable ion acoustic wave generation in the shock ramp provides the basis for determining the thickness of the magnetic gradients for subcritical, quasi-perpendicular shock features.

The location of the forward boundary of the upstream wave region ahead of the earth's bow shock is investigated statistically using plasma and magnetic-field data obtained by Heos 1. The analysis is conducted on the assumption that the waves are produced by protons reflected from the bow shock and traveling upstream with an effective velocity equal to the product of a parameter, p, and the solar-wind bulk velocity. The data-reduction methods are summarized, and uncertainties in the determination of the value of p are discussed. It is found that the overall average boundary corresponds to a velocity of approximately twice the solar-wind velocity for protons reflected from the daytime sector of the bow shock. More refined analysis of the data shows that the value of p is approximately 1.6 near the subsolar point and greater than 2 along the daytime flanks of the bow shock.

The electric field experiment on board ISEE-1 had a special mode of operation in which bursts of very high time resolution measurements were stored and slowly read-out. This mode of operation allows us to study in detail the waveforms of the large amplitude electrostatic waves observed at the Earth's bow shock in a frequency range (10--500 Hz) not well covered previously. These waves appear to be correlated with two streams in the ion angular distribution. Our observations indicate, therefore, that the ion--ion stream instability is the dissipation mechanism of a collisionless fast quasi-perpendicular shock wave.

Measurements from six OGO-5 particle and field experiments are used to examine the structure of the earth's bow shock during a period of extremely high beta (the ratio of plasma thermal to magnetic energy density), as determined from simultaneous measurements of the upstream plasma on board the HEOS satellite. Even though the interplanetary field is nearly perpendicular to the shock normal, the shock is extremely turbulent. Large field increases are observed up to a factor of 20 above the upstream values. Ahead of these large enhancements, smaller magnetic effects accompanied by electrostatic noise, electron heating, and ion deflection are observed for several minutes. These observations suggest that a steady-state shock may not be able to form at very high beta. Further, they show that while the magnetic energy density may be relatively unimportant in the upstream flow, it can become very significant within the shock structure, and hence the magnetic field should not be ignored in theoretical treatments of very high beta shocks.

HEOS-2 low energy electron data (10 eV–3.7 keV) from the LPS Frascati plasma experiment have been used to identify three different magnetospheric electron populations. Magnetosheathlike electron energy spectra (35–50 eV) are characteristic of the plasma mantle, entry layer and cusps from the magnetopause down to 2–3 R E Plasma sheet electrons (energy > 1 keV) are found at all local times, with strong intensities in the early morning quadrant and weaker intensities in the afternoon quadrant. The plasma sheet shows a well defined inner edge at all local times and latitudes, the inner edge coinciding probably with the plasmapause. The plasma sheet does not reach the magnetopause, but it is separated from it by a boundary layer electron population that is very distinct from the other two electron populations, most electrons having energies 100–300 eV. We map these three electron populations from the magnetopause down to the high latitude near earth regions, by making use of the HEOS-2 low latitude inbound passes and the high latitude outbound passes (in Solar Magnetic ( SM ) coordinates). The boundary layer extends along the magnetopause up to 5–7 R E above the equator; at higher latitudes it follows the magnetic lines of force and it is found closer and closer to the earth, so that it has the same invariant latitudes of the system 1 currents observed by Iijima and Potemra (1976) in their region 1. The plasma sheet can be mapped into their region 2 and the cusp-entry layer-plasma mantle can be mapped into their cusp currents region. The boundary layer is observed for any Interplanetary Magnetic Field ( IMF ) direction. We speculate that magnetosheath particles penetrate into the magnetosphere everywhere along the magnetopause. The electron energization, however, is observed only in the boundary layer, on both dawn and dusk side and could be due to the polarization electric field at magnetopause generated by the magnetosheath plasma bulk motion in the region where such motion is roughly perpendicular to the magnetospheric magnetic field. The electron energization is absent in the regions (entry layer and plasma mantle) where the sheath plasma motion is roughly parallel or antiparallel to the magnetospheric magnetic field.

Bursts of energetic electrons (from >40keV up to 2MeV) as distinct from the magnetopause electron layer observed by Domingo et al. (1977) have been observed in the magnetosheath and in the solar wind by HEOS-2 at high-latitudes. Although these electrons are occasionally found close to the bow shock and simultaneously with low frequency (magnetosonic) upstream waves our observations strongly indicate that these electrons are of exterior cusp origin. Indeed, the flux intensity is highest in the exterior cusp region and decreases as the spacecraft moves away from it both tailward or upward. The energy spectrum becomes harder with increasing radial distance from the exterior cusp. The measured anisotropy indicates that the particles are propagating away from the exterior cusp. The magnetic field points to the exterior cusp region when these electrons are observed, being, for solar wind observations, centred at longitude 0° or 180° rather than along the spiral and in the magnetosheath, being usually different from the 90° or 270° orientation typical of that region. We exclude, therefore, that acceleration in the bow shock is the source of these particles because B is not tangent to the shock when bursts are observed. We have also found a one to one correlation between geomagnetic storms' recovery phases and intense, continuous observations of >40 keV electrons in the magnetosheath, while, on the other hand, during geomagnetically quiet (Dst) periods bursts are observed only if AE is much larger than average.

Nearly 2500 shock crossings from HEOS-1, HEOS-2 and 5 IMP spacecraft, covering most of the northern and part of the southern bow shock surface for X values X > − 20 RE, have been used to carry out a detailed study of the three-dimensional shape and location of the bow shock. The influence of the different solar wind conditions has been reduced by normalising the observed crossings to an average solar wind dynamical pressure (N0 = 9.4 cm−3, V0 = 450 km s−1). It has been shown that the shock surface is symmetric with respect to the ecliptic plane and intersects the coordinate axes at 11.9 RE (X), + 27.0 and − 22.9 RE (Y), + 23.9 and − 24.5 RE (Z) for the average dynamical pressure (N0 = 9.4 cm−3, V0 = 450km s−1, with M A = 9.3 , M MS = 6.1) . The observed aberration of the shock surface is 8.9° ± 1°, i.e. 5.1° larger than the aberration predicted from the Earth's motion. This asymmetry around the solar wind apparent direction is described by equation (6) for different Mach numbers MA and confirms the predictions of Walters [J. geophys. Res. 71, 1319 (1964)] and Michel [J. geophys. Res. 70, 1 (1965)]. The magnetosheath thickness is 3.3 RE along the X-axis, 11.4 RE (+ Y), 8.7 RE (− Y), 9.9 RE (+Z) and 10.9 RE along the negative Z axis.

Preliminary results from the JPA instrument on Giotto indicate that Comet Halley, even on the flanks, has a bow shock which moves backwards and forwards over the spacecraft. To understand the structure properly will require more detailed investigation of the relationships between three particle populations, cometary ions, solar wind ions and electrons.

A statistical study of correlated observations of plasma, energetic (>40 keV) electrons and magnetic fields on HEOS-2 in the outer dayside magnetosphere and magnetosheath has been performed. We find that on the dayside magnetosphere the occurrence of plasma inside the magnetopause is anti-correlated with the appearance of energetic electrons in the magnetosheath. A mechanism is derived that attempts to explain how plasma may penetrate across the magnetopause into the magnetosphere and how energetic electrons may escape from the (pseudo) trapping region in the dayside magnetosphere into the magnetosheath. The mechanism is an E × B drift across the magnetopause. The drift is directed inward or outward depending on the direction of the electric field E at the magnetopause.

The normal component of the dc electric field measured on ISEE-1 ordinarily exhibits an enhancement of a few mV/m over both upstream and downstream values at the earth's bow shock. Using the measured relative velocity between the shock and the spacecraft (from the ISEE-1/2 time delay in the magnetometer data), it is possible to transform the observed E enhancement to a potential drop (delta phi). For a subcritical shock the potential drop is found to be very close to the measured change of particle kinetic energy (delta phi, approximately 280 V on day 330, 1977), whereas for a supercritical shock the potential drop is only a fraction of the measured change of kinetic energy (delta phi, approximately 140 V on day 324, 1977).

The EGD Plasma experiment on board of ISEE-2 spacecraft has been able to observe, in the Earth's magnetotail, positive ions flowing tailward. These ions are observed mostly in the evening sector of the magnetosphere during the recovery phase of the main geomagnetic storms and are identified to be O+ ions. Our observations are made above the plasma sheet, in the lobes of the geomagnetic tail or in the boundary layer. The energy of the oxygen ions is observed to vary from 200 to 10000 eV. When the oxygen energy is high enough, also protons are observed at similar flow speeds. The O+ pitch angle distribution is not centered around the magnetic field direction, and often has a conical pattern. The energy of the O+ is also strongly correlated with the magnetic field component Bz, and is a function of local time.

Observations made by HEOS-2 of low energy electrons and protons in the high latitude magnetosphere are presented. Plasma in the magnetosphere is observed in the cusp (which extend down to low altitudes) and over large areas adjacent to the high latitude magnetopause both on the dayside and on the nightside (the entry layer and the plasma mantle respectively). A comparative study of the plasma properties in the various parts of the magnetosphere is performed. An ion bulk motion directed tailward along the geomagnetic field lines is observed both in the entry layer and in the plasma mantle; in the cusp, on the contrary, the bulk motion is practically absent. Moreover the electron thermal anisotropy is parallel to the magnetic field in the magnetosheath, and perpendicular to it in the plasma mantle. One possible explanation (suggested by Rosenbauer et al. , 1975) of the origin of these populations is that plasma, penetrated from the magnetosheath in the entry layer, flows tailward along the field lines, is then reflected in the cusp region and convected in the plasma mantle.

We use ISEE data together with previously known results to identify magnetospheric plasma regimes and their boundaries. The bow shock and its asymmetric foreshock is the first magnetospheric boundary: the entire magnetosphere is tilted by 5°–10° (close to the earth) due to its intrinsic asymmetry. The magnetopause limits the magnetosheath, but magnetospheric phenomena (energetic electron layer) are observed outside it. Studies of the shape of the magnetopause have shown that it cannot be described by a single second order surface. Inside the magnetopause at low latitudes we have the boundary layer that (following the magnetic lines of force) mappes into the ionospheric region of system 1 current. At high latitudes along the magnetopause we find the entry layer and the plasma mantle whose properties are somewhat influenced by the IMF. In the tail a flowing plasma is observed which originates from the polar ionospheric regions and makes an important contribution to the magnetospheric plasma from a source other than the solar wind. The plasma sheet is then found with its hot plasma, inside the boundary layer, extending from the distant geomagnetic tail to the plasmapause at low latitudes and to the ionosphere at high latitudes and to the dayside. The plasmasphere is encompassed by the plasmapause, the source of many interesting wave phenomena presently under study.

The existence of a layer of energetic (≳20 keV) electrons along the magnetopause, extending from the polar cusp region to the distant magnetotail, has been established by measurements on various spacecraft. We have performed a statistical study of electron flux and spectral measurements above 0.5 MeV observed in different regions of the magnetopause accessible to HEOS 2, with the aim of identifying the source for this magnetopause electron layer. The measurements along the polar magnetotail (i.e., adjacent to the plasma mantle), along the mid-latitude dayside magnetopause (i.e., adjacent to the dayside plasma boundary layer), and on both sides of the magnetosheath/polar cusp interface (i.e., in the exterior polar cusp and in the entry layer) have been separately analyzed and are statistically compared. We conclude that escape of electrons from the magnetosphere can be excluded as a significant source for the tail magnetopause layer on the basis of the observed intensities, of the spectra, and of the frequency of appearance of electrons outside those magnetopause regions which lie adjacent to the outer trapping zone. The electron intensities in the exterior polar cusp region are found to be correlated with the north/south component of the interplanetary magnetic field (IMF). High intensity levels are observed when the IMF is directed southward (BZ 0). The electron flux in the exterior cusp region at times of southward IMF and at the near-earth polar magnetotail at times of northward IMF is well correlated with the number of field lines carried by the solar wind towards the magnetosphere (expressed by the product of solar wind speed and interplanetary field magnitude). We interpret this as strong evidence that acceleration processes associated with magnetic reconnection occur at the magnetopause at those times. Local acceleration near the high-latitude magnetopause appears therefore to be a prime source for the magnetopause electron layer.

The process of mass loading of the solar wind by cometary ions, which forms comet tails, has been observed throughout the coma of comet Halley. Three distinct regimes were found where the nature of the energy and momentum coupling between solar wind and cometary ions is different. Outside the bow shock, where there is little angular scattering of the freshly ionised particles, the coupling is described by the simple pickup trajectory and the energy is controlled by the angle between the flow and the magnetic field. Just inside the bow shock, there is considerable scattering accompanied by another acceleration process which raises some particle energies well above the straightforward pickup value. Finally, closer to the nucleus, the amount of scattering decreases and the coupling is once more controlled by the magnetic field direction.

A statistical study of the cusp plasma has been performed using mainly electron data from the LPS, Rome, plasma experiment flown onboard HEOS-2. We have located the cusp by means of 35–50 eV electrons, from 1.5 to 2.5 R E (south pole) and from 3 R E up to 11 R E (north pole) at 60–70° SM latitude within ±60° of SM longitude from the noon meridan plane. The average cusp thickness is 4.2° of invariant latitude. The location of the cusp in invariant latitude around the noon meridian plane depends on the IMF component B zGSM according to the linear best fit: Λ = 78.7° + 0.48 B z GSM( γ ). Away from the noon meridian plane the invariant latitude of the cusp decreases from 79–84° to 70–74° (at ±50° SM Longitude). At the equatorward edge of the north pole cusp, at all radial distances and at all SM longitudes, we have found a population of electrons with a harder energy spectrum than in the cusp itself. These electrons show a peak at 170–280 eV in our data. They are not the cusp (35–50 eV) electrons and are easily distinguishable from the 1 keV magnetospheric electrons. In the south pole auroral oval they are found at any SM longitude mainly poleward of the 1 keV electrons. The cusp electrons (35–50 eV) and protons have anisotropies that vary with radial distance and SM latitude, both flowing earthward more or less along the magnetic field.

It was found that low solar wind parameters M (less than or around 2.5) and beta (much less than 1) and high angles to the local shock normal, theta (greater than or around 65 deg), produced oblique laminar shock profiles as expected from theory, with marginal or vanishing upstream standing whistlers probably damped by acoustic or other plasma wave instabilities. The whistler mode appeared to dominate the electromagnetic spectrum. The laminar shock ramp thickness was several hundred kilometers and equal to (2-4)c/omega-pi. Composition of the shock as an accumulation of near-standing waves and an evidently reproducible varying flux pattern was discernible. Electron thermalization occurred early in, or just before, the magnetic ramp, while proton thermalization appeared to occur later in the ramp. Instantaneous shock velocities derived from the standing whistler wavelength were consistent with average velocities derived from the elapsed time estimates and were as high as 200 km/sec.

A thick, quasi-parallel bow shock structure was observed with field and particle detectors of both HEOS 1 and OGO 5. The typical magnetic pulsation structure was at least 1 to 2 earth radii thick radially and was accompanied by irregular but distinct plasma distributions characteristic of neither the solar wind nor the magnetosheath. Waves constituting the large pulsations were polarized principally in the plane of the nominal shock, therefore also in the plane perpendicular to the average interplanetary field. A separate interpulsation regime detected between bursts of large amplitude oscillations was similar to the upstream wave region magnetically, but was characterized by disturbed plasma flux and enhanced noise around the ion plasma frequency. The shock structure appeared to be largely of an oblique, whistler type, probably complicated by counterstreaming high energy protons. Evidence for firehose instability-based structure was weak at best and probably negative.

The interaction of the solar wind with ions backstreaming from the earth’s bow shock is studied at high time resolution. It turns out that the bulk velocity of the solar wind oscillates, both in magnitude and direction, with typical periods of ∼1 minute in presence of the ‘diffuse’ ion population. Oscillations of comparable periods are also observed in the angular distribution and energy spectrum of the diffuse ions.

The consequences of the interaction of incoming solar wind protons with particles reflected at the bow shock have been studied with Heos 1 plasma and magnetic field data. The proton bulk speed has been found to decrease by 30 km/s in the region where upstream waves are observed. The wave amplitude has been found to increase with increasing bulk speed, on average, by the relation ..delta..B=0.28+8.5 x 10/sup -3/V/sub s//sub w/, where B is in gammas and V/sub s//sub w/ is in kilometers per second. The wave amplitude has also been found to decrease with increasing proton number density. Agreement with Barnes' (1970) theoretical prediction of the ..delta..B-N/sub p/ relation is achieved only if the assumption of a constant number density of reflected protons is made. (AIP)

A general survey of the average properties of interplanetary plasma from December 11, 1968, to April 13, 1969, and from August 27, 1969, to January 10, 1970, is presented, as observed by the European satellite Heos 1. The average values of the proton parameters are: number density, 4.3 p/cm³; bulk velocity, 409 km/sec; and most probable thermal speed, 33 km/sec. A map is presented of the observed magnetosheath traversals. A parabolic best fit of bow shock and magnetopause crossings gives, at the subsolar point, distances of 15.2 and 12.4 RE from the earth, respectively. The average plasma parameters and the positions of the discontinuity surfaces are compared with the results obtained by other satellites and space probes during the last period of low solar activity.

Four anomalous Earth's bow shock locations observed during 1969 are studied, using plasma and magnetic field data from the European satellite HEOS-1. The results show that when no magnetic storms are present, the shock position is predicted with a good approximation by magnetohydrodynamic equations, making use of the magnetosonic Mach number of the solar wind. When magnetic storms are present, symmetric and asymmetric ring currents should be taken into account. For low magnetosonic Mach numbers (1 M MS ≲ 3), the magnetosheath magnetic field is characterized by the unusual absence of low frequency turbulence. This experimental result seems to indicate that under such conditions the bow shock is a laminar shock.

The MIT plasma experiment on Pioneer 6 has been occasionally able to detect the electron component of the solar wind. Because of the interest in the interplanetary electrons, we report in this note the preliminary results of our analysis. Our observations support conclusions of recently analyzed data from other experiments [see, e.g., Montgomery et al, 1968], and they agree with the suggestion from proton data analysis that the electron temperature is greater than the temperature of the proton component [Burlaga, 1967]. The results do not agree with the larger electron-to-proton temperature ratio predicted by the two-fluid model discussed by Sturrock and Hartle [1966].

A few short bow shock crossings have been studied (Days 44, 53, 255, and 48, 1969). The first three events are shown to be incomplete shock crossings; i.e., the satellite spent a few minutes each time within the shock layer. Large-amplitude waves are observed in |B| with frequency close to Ωi. Proton velocity distribution shows a second peak corresponding to an energy 2–4 times the upstream proton energy. This second peak is observed both with cold solar wind (first part of the shock layer) and with heated solar wind (second part of the shock layer). The possibility of a substructure is discussed. The acceleration of the bow shock is estimated to be ≈5 km/sec².

The earth's bow shock has been investigated as a magnetohydrodynamic discontinuity using the plasma and magnetic data supplied by the European satellite Heos 1. The jumps of the fluid parameters through the shock have been studied as a function of the Mach number and of the geometry of the shock surface. The solar wind specific heat ratio has been found to be γ1 = 1.75 or 2.25 depending on the presence or absence of upstream waves. Computations of the shock velocity performed for 23 crossings gave an average speed of ∼85 km/sec.

A classification of earth's bow shock structures on the basis of upstream MA, β, Δ = ∠ Bn (angle between shock normal and magnetic field direction) has been found. Typical properties for each case are studied. In particular, the downstream turbulence present in has been positively correlated with upstream β. It is suggested that the shock precursor, as far as low-frequency waves are concerned, is always due to waves generated upstream and carried by the solar wind. The use of the shock velocity allows estimation of the shock thickness: if MA > 3, the shock is a few ion gyroradii thick.

Geomagnetic field sudden impulses relation to solar wind pressure changes using Pioneer 6 measurements

Summary. The interplanetary shock of March 8, 1970, has been studied by using Heos 1 plasma data. Plasma density and velocity jump from 4 to 13 particles/cc and from 430 to 790 km/sec. The piston of the shock is observed 5–6 hours later and compresses the earth's magnetosphere so much that it makes the solar wind detectable at geosynchronous orbit. Maximum solar wind speed observed in this period is 880 km/sec.

Properties of plasma and magnetic field within the magnetosheath in the period December 1968 to December 1969 are presented as observed by the European satellite Heos 1. Three states of the magnetosheath plasma have been distinguished on the basis of different upstream conditions and are characterized by low magnetosonic Mach number (M1 < 3) and low β, high values of M1 and β and absence of upstream waves, and presence of upstream waves. The distribution function of proton velocities and the magnetic turbulence in the magnetosheath have been studied in connection with these three upstream situations.

The position of the bow shock far back in the magnetotail (68 R/sub E/) was studied for extremely low Mach numbers. (WDM)

This paper summarizes our knowledge of the Earth’s bow shock fine structure. Review papers (Sagdeev, 1967; Paul, 1968, and Friedman et al., 1971) and books (Tidman and Krall, 1971) on collisionless shocks in plasmas, which have appeared so far, deal especially with laboratory produced collisionless shocks and theories to describe the corresponding observations, but only marginally with the Earth’s bow shock. This is probably due to the great complexity of the bow shock structure (see Figure 1), the intermixing of the many different phenomena occurring there and the difficulties of observation. Furthermore, although the available literature on bow shock observations is now very extensive, systematic investigations were not possible until recently since in general the different parameters observed were rather uncorrected. On many occasions for example magnetic field data on the bow shock crossings were reported without a corresponding knowledge of the plasma data and vice versa. In such cases unambiguous checking of the measurements against theories of collisionless shocks in plasmas was clearly practically impossible. Open image in new window Fig. 1. Sketch in the ecliptic plane of the interaction between the solar wind and the Earth’s magnetic field. The magnetopause and the bow shock are indicated together with the region where the particles reflected at the bow shock interact with the incoming plasma generating four kinds of wave phenomena (see Section 3).

Ions produced by ionization of the cometary neutrals interact with the solar wind protons to produce large amplitude oscillations of the ambient magnetic field. Such oscillations are convected towards the comet at the unperturbed solar wind speed far from the shock and at a lower speed closer to the shock (due to the solar wind mass loading); hence, they can energize the incoming ions by Fermi acceleration. The spatial extension of the acceleration region is of the order of 10 6 km and the resulting energy spectrum is harder than in the Earth's bow shock case. The energization of cometary ions produces an additional deceleration of the solar wind. It is suggested that Comet Halley may be the most efficient “cosmic ray shock” in the solar system.

The ISEE 2 plasma experiment data are used to investigate the properties of the O+ ions streams flowing in the earth's magnetotail low latitude boundary layer and northern plasma lobe. The E × B filter is seen to be the organizer of the flow of these ions, after their acceleration in the topside ionosphere. Between XSE=−15 RE and −23 RE the velocity of the O+ ions is a function of the distance from the center magnetotail axis, the slower flowing closer to this and the faster closer to the magnetopause. The O+ stream density is observed to vary with this distance as well. The temperature of the streaming ions is shown to be anisotropic with respect to the ambient magnetic field; a generally unresolved narrow conic pitch angle distribution of the O+ ions is suggested to be the cause of this anisotropy. The properties of the O+ ions are shown to be consistent with an acceleration mechanism, or a combination of mechanisms, that act orthogonally and parallel to the magnetic field.

The magnetopause crossings of November 10, 1977, which are considered to be representative of a class of structures, are investigated, and a procedure is developed to derive approximate surface currents near the magnetopause from ISEE 1 magnetic field data by assuming a constant speed of the magnetopause along its normal for each crossing of the magnetopause layer. Changes in the magnetic field indicate that current sheets exist on each side of the magnetopause layer, which is characterized by an irregular magnetic field and a magnetosheath-like energy distribution. No boundary layer plasma is observed on the magnetospheric field lines inside the magnetopause, and electromagnetic energy is found to be dissipated at the two edges of the magnetopause layer where the current layers are observed.