Particle acceleration at astrophysical shocks: A theory of cosmic ray origin

Among the various acceleration mechanisms which have been suggested as responsible for the nonthermal particle spectra and associated radiation observed in many astrophysical and space physics environments, diffusive shock acceleration appears to be the most successful. We review the current theoretical understanding of this process, from the basic ideas of how a shock energizes a few reactionless particles to the advanced nonlinear approaches treating the shock and accelerated particles as a symbiotic self-organizing system. By means of direct solution of the nonlinear problem we set the limit to the test-particle approximation and demonstrate the fundamental role of nonlinearity in shocks of astrophysical size and lifetime. We study the bifurcation of this system, proceeding from the hydrodynamic to kinetic description under a realistic condition of Bohm diffusivity. We emphasize the importance of collective plasma phenomena for the global flow structure and acceleration efficiency by considering the injection process, an initial stage of acceleration and, the related aspects of the physics of collisionless shocks. We calculate the injection rate for different shock parameters and different species. This, together with differential acceleration resulting from nonlinear large-scale modification, determines the chemical composition of accelerated particles. The review concentrates on theoretical and analytical aspects but our strategic goal is to link the fundamental theoretical ideas with the rapidly growing wealth of observational data.

http://iopscience.iop.org/article/10.1088/0034-4885/64/4/201/pdf

Nonlinear theory of diffusive acceleration of particles by shock waves

The notion that plasma shocks in astrophysical settings can and do accelerate charged particles to high energies is not a new one However, in recent years considerable progress has been achieved in understanding the role particle acceleration plays both in astrophysics and in the shock process itself In this paper we briefly review the history and theory of shock acceleration, paying particular attention to theories of parallel shocks which include the backreaction of accelerated particles on the shock structure We discuss in detail the work that computer simulations, both plasma and Monte Carlo, are playing in revealing how thermal ions interact with shocks and how particle acceleration appears to be an inevitable and necessary part of the basic plasma physics that governs collisionless shocks We briefly describe some of the outstanding problems that still confront theorists and observers in this field

The plasma physics of shock acceleration

The plasma physics of shock acceleration.

A theory for the self-consistent configuration of upstream hydromagnetic waves, upstream energetic storm particle (ESP) events, and downstream postshock ion enhancements at interplanetary traveling shocks is presented. The observations of upstream ultralow frequency waves and those ESP events and postshock enhancements which exhibit approximately isotropic ion distributions in the solar wind or shock frame are briefly reviewed. The theory of Lee (1982) for application to interplanetary traveling shocks is modified and analytical solutions for the wave spectrum as a function of wavenumber and z are presented along with the ion omnidirectional distribution functions as functions of energy and z for all ion species. The theory quantitatively explaines the observed features of the shock-associted energetic ions and predicts the configuration of upstream hydromagnetic waves.

Coupled hydromagnetic wave excitation and ion acceleration at interplanetary traveling shocks

Regions of plasma turbulence extending several tenths of an astronomical unit upstream or downstream of interplanetary shocks have been detected by the plasma wave instrument on ISEE 3. Highly impulsive electric field bursts at 1-10 kHz were found (hours upstream of quasi-parallel interplanetary shocks) whose average and peak amplitudes occasionally increased until the shock crossing, when they were suppressed. A 0.1-1 kHz electric field component was enhanced at nearly all shocks, and persisted downstream. A smooth, high-frequency continuum near and above the local electron plasma frequency was enhanced at, and persisted downstream of, every interplanetary shock studied. While no single interplanetary shock showed every effect, the ensemble of shocks contained at least one example of each type of plasma wave found upstream of the earth's bow shock.

Nonlocal plasma turbulence associated with interplanetary shocks

The jump in plasma parameters exhibited by the intense interplanetary shock event of Nov. 12, 1978 is analyzed using ISEE 1, 2 and 3 data. Magnetic and electric field measurements indicated that the shock magnetic field profile was similar to the earth bow shock profile. Data on the electron and proton densities, temperatures, bulk velocities and alpha particles showed a steady electron temperature increase across the shock on a 12 earth radii scale. The upstream and downstream flow parameters are found to be within 10 percent of Rankin-Hugoniot jump conditions. The shock moved at 614 km/sec and had three dissipative scales, one a few Larmor radii determined by the magnetic field jump, a second 10 earth radii correlated with the electron equilibrium and the other 30 earth radii connected to the energetic proton foreshock.

Structure of the November 12, 1978, quasi‐parallel interplanetary shock

The two Phobos spacecraft, which will start to orbit Mars early in 1989, will be capable of investigating in detail the microphysics of the Mars-solar wind interaction. Simple scaling arguments and analogies with other planetary bow shocks indicate that the sub-solar shock standoff distance should be small compared with plasma scalelengths, giving the shocked solar wind insufficient space in which to thermalize downstream before encountering the magnetospheric obstacle. Both the magnetosphere and ionosphere can be affected by particles and waves from the solar wind interaction.

Expectations for the microphysics of the Mars‐solar wind interaction

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.

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Structure of a quasi-parallel, quasi-laminar bow shock

The ISEE-3 plasma wave instrument detects associated bursts of electron plasma oscillations and whistler mode waves at an average rate of event one every two days. The plasma wave measurements give the electron number density, and simultaneously measured E and B amplitudes are used to deduce an index of refraction consistent with whistler mode propagation for the measured number density and magnetic field. Burst durations are a few minutes, with some trains of bursts lasting up to an hour. Individual spectral scans (two per second) reveal that the whistler and plasma wave amplitude-time profiles differ within a burst. Peak plasma wave amplitudes are near one mV/m, and the peak whistler mode energy density exceeds that of the plasma oscillations by about a factor 100. The frequency of the whistler mode wave observed in one well diagnosed event agrees with the predictions of the heat flux whistler instability theory. The associated plasma wave instability probably requires a bump-on-tail feature in the heat flux electron component, possibly due to impulsive heating elsewhere on the field-line connecting to ISEE-3.

F. L. Scarf

Papers

Nonlocal plasma turbulence associated with interplanetary shocks

Structure of the November 12, 1978, quasi‐parallel interplanetary shock

Expectations for the microphysics of the Mars‐solar wind interaction

Structure of a quasi-parallel, quasi-laminar bow shock

Correlated whistler and electron plasma oscillation bursts detected on ISEE‐3