This review highlights conceptual issues that have both governed and reflected the direction of collisionless shock research in the past quarter century. These include MHD waves and their steepening, the MHD Rankine-Hugoniot relations, the super-critical shock transition, nonlinear oscillatory wave trains, ion sound anomalous resistivity and the resistive-dispersive transition for subcritical shocks, ion reflection and the structure of supercritical quasi-perpendicular shocks, the earth's foreshock, quasi-parallel shocks, and finally, shock acceleration processes.

A quarter century of collisionless shock research

The linear theory of electromagnetic instabilities driven by an energetic ion beam streaming parallel to a magnetic field in a homogeneous Vlasov plasma is considered. Numerical solutions of the full dispersion equation are presented. At propagation parallel to the magnetic field, there are four distinct instabilities. A sufficiently energetic beam gives rise to two unstable modes with right-hand polarization, one resonant with the beam, the other nonresonant. A beam with sufficiently large T (perpendicular to B)/T (parallel to B) gives rise to the left-hand ion cyclotron anisotropy instability at relatively small beam velocities, and a sufficiently hot beam drives unstable a left-hand beam resonant mode. The parametric dependences of the growth rates for the three high beam velocity instabilities are presented here. In addition, some properties at oblique propagation are examined. It is demonstrated that, as the beam drift velocity is increased, relative maxima in growth rates can arise at harmonics of the ion cyclotron resonance for both right and left elliptically polarized modes.

Electromagnetic ion beam instabilities

A fast Fermi process which may explain how solar wind electrons are energized in the vicinity of the point of tangency of the IMF to the earth's bow shock is discussed. In the solar wind frame, the nearly perpendicular bow shock behaves as a rapidly moving magnetic mirror in such a manner that electrons with sufficiently large pitch angles can be reflected and accelerated. If the seed electrons have energies of several hundred electron volts, they can attain energies of several keV through the acceleration process.

A fast Fermi process: Energetic electrons accelerated by a nearly perpendicular bow shock

The distinction between two types of upstream ion populations is made on the basis of pronounced differences in their distribution functions. The reflected ions represent a fast beam with temperatures typically one-million to five-million K and speeds up to five times the solar wind speed. An important feature of the reflected ion distributions is their strong temperature anisotropy, with perpendicular temperature exceeding parallel temperature by a factor of two or three. In contrast, the diffuse ions occupy a much larger region of phase space, both in energy and angle; their distribution function generally has the form of a circular ridge in two dimensions and a spherical shell in three dimensions. Accordingly, their temperature is much larger (not less than about 10-million K) and their bulk speed is typically smaller than the solar wind speed.

Characteristics of reflected and diffuse ions upstream from the earth's bow shock

The evolution of the electromagnetic ion beam instability driven by the reflected ion component backstreaming away from the earth's bow shock into the foreshock region is studied by means of computer simulation. The linear and quasi-linear stages of the instability are found to be in good agreement with known results for the resonant mode propagating parallel to the beam along the magnetic field and with theory developed in this paper for the nonresonant mode, which propagates antiparallel to the beam direction. The quasi-linear stage, which produces large amplitude delta B approximately B, sinusoidal transverse waves and 'intermediate' ion distributions, is terminated by a nonlinear phase in which strongly nonlinear, compressive waves and 'diffuse' ion distributions are produced. Additional processes by which the diffuse ions are accelerated to observed high energies are not addressed. The results are discussed in terms of the ion distributions and hydromagnetic waves observed in the foreshock of the earth's bow shock and of interplanetary shocks.

Diffuse ions produced by electromagnetic ion beam instabilities

The deceleration of the solar wind in the region of the interplanetary space filled by ions backstreaming from the Earth bow shock was studied using a two spacecraft technique. This deceleration, which is correlated with the "diffuse" but not with the "reflected" ion population, depends on the solar wind bulk velocity: at low velocities (below 300 km/sec) the velocity decrease is about 5 km/sec, while at higher velocities (above 400 km/sec) the decrease may be as large as 30 km/sec. Along with this deceleration, the solar wind undergoes a deflection of about 1 deg away from the direction of the Earth bow shock. The energy balance shows that the kinetic energy loss far exceeds the thermal energy which is possibly gained by the solar wind, therefore, at least part of this energy must go into waves and/or into the backstreaming ions.

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Deceleration of the solar wind in the Earth's foreshock region: Isee 2 and Imp 8 observations

The deceleration of the solar wind, in the region of the interplanetary space filled by ions backstreaming from the earth's bow shock and associated waves, is studied using a two-spacecraft technique. This deceleration, which is correlated with the ‘diffuse’ but not with the ‘reflected’ ion population, depends on the solar wind bulk velocity: at low velocities (below 300 km/s) the velocity decrease is ∼5 km/s, while at higher velocities (above 400 km/s) the decrease may be as large as 30 km/s. Along with this deceleration the solar wind undergoes a deflection by ∼1° away from the direction of the earth's bow shock. The energy balance shows that the kinetic energy loss far exceeds the thermal energy which is possibly gained by the solar wind; therefore at least part of this energy must go into waves and/or into the backstreaming ions.

Deceleration of the solar wind in the Earth foreshock region: ISEE 2 and IMP 8 observations

The interaction of the solar wind with ions backstreaming from the earth's bow shock is investigated using plasma and magnetic field measurements on ISEE 1 and 2 and IMP 8 at widely separated positions in the earth's foreshock. This technique separates temporal and spatial variations within the foreshock. It is found that the solar wind acceleration associated with backstreaming ions is correlated with the amplitude of the MHD turbulence, and that the largest decelerations are seen close to the bow shock. The density of the backstreaming ion beam is strongly correlated with distance from the shock, and decreases by about a factor of three in a distance of about 3R(e).

Solar wind deceleration and MHD turbulence in the earth's foreshock region: ISEE 1 and 2 and IMP 8 observations

Magnetic field and plasma data obtained by ISEE 2 experiments in the earth's foreshock region have been used, in conjunction with a shock model, to correlate with geometrical parameters the intensity of MHD turbulence and some moments of the distributions of back-streaming protons. The geometrical parameters are the distance of the observation point from the proton foreshock boundary and the distance from the shock surface. The results obtained allow a preliminary discussion of two basic problem of the physics of the foreshock. These are (1) the relative role of shock structure and/or wave particle interaction in determining the form of the backstreaming ion distributions and (2) the physical mechanisms determining the observed saturation of the MHD turbulence.

Distribution of MHD wave activity in the foreshock region and properties of backstreaming protons

The energies of the field-aligned proton beams observed upstream of the earth's bow shock are tested, on a statistical basis, against a simple reflection model. The comparison is carried out using both plasma and magnetic field data collected by the ISEE 2 spacecraft. The observations refer to the period from November 5 to December 20, 1977. According to this model, some of the solar wind protons incident upon the earth's shock front when reflected upstream gain energy by displacement parallel to the interplanetary electric field. The energy gained in the reflection can be described as a function of the angles between the interplanetary magnetic field, the solar wind bulk velocity, and the local shock normal. The task of finding these angles, i.e., the expected source point of the reflected ions at the earth's shock front, has been resolved using both the measured magnetic field direction and the actual beam trajectory. The latter method, which takes into account the ion drift velocity, leads to a better agreement between theory and observations when far from the shock. In particular, it allows us to check the energies of the field-aligned beams even when they are observed far from the earth's bow shock (at distances up to 10–15 RE). We confirm, on a statistical basis, the test of the model recently carried out using the Los Alamos National Laboratory/Max-Planck-extraterrestrische observations on ISEE 1 and 2. We infer that reflected beams can sometimes propagate far upstream of the earth's bow shock without changing their energy properties.

G. Moreno

Papers

Deceleration of the solar wind in the Earth's foreshock region: Isee 2 and Imp 8 observations

Deceleration of the solar wind in the Earth foreshock region: ISEE 2 and IMP 8 observations

Solar wind deceleration and MHD turbulence in the earth's foreshock region: ISEE 1 and 2 and IMP 8 observations

Distribution of MHD wave activity in the foreshock region and properties of backstreaming protons

Reflection of the solar wind ions at the Earth's bow shock: Energization