High-energy particle acceleration is observed to proceed in a diverse variety of astrophysical sites ranging from the terrestrial aurorae to the most distant quasars. Particle acceleration is a fairly common channel for the release of large-scale kinetic, rotational, and magnetic energy. Physical mechanisms include electrostatic acceleration, stochastic processes and diffusive shock energization. Cosmic-ray energy spectra have shapes which reflect escape, collisional, and radiative losses. The overall acceleration efficiency is controlled by the low-energy particle injection which may, in turn, feed back into the energization. Recent observational developments, which illustrate these general principles and raise fresh questions, are briefly summarized.

Subject heading: acceleration of particles

Energetic particles in the magnetospheres of the solar system originate from various different sources, such as the solar wind, the planetary ionospheres as well as the moons and rings of the planetary systems. Important acceleration sites are the auroral regions, the magnetotail, and the equatorial regions of the magnetospheres where electric fields, wave-particle interactions and magnetic pumping are among the major acceleration mechanisms proposed. Over the last decade mass- and charge-sensitive particle spectrometers on satellites and space probes have collected a wealth of information about the relative contribution of the various particle sources and the major acceleration processes to the energetic particle populations. Emphasis will be put on recent studies of the source populations and the acceleration processes in the Earth’s auroral zones and magnetotail. Furthermore, the Jovian system with the largest magnetosphere and its unique mixture of particle sources with strong contributions from moons will be highlighted in some results from Ulysses.

Subject headings: acceleration of particles — planets and satellites: general

Magnetospheric substorms represent the episodic dissipation of energy stored in the geomagnetic tail that was previously extracted from the solar wind. This energy release produces activity throughout the entire magnetosphere-ionosphere system, and it results in a wide variety of phenomena such as auroral intensifications and the generation of new current systems. All of these phenomena involve the acceleration of particles, sometimes up to several MeV. In this paper we present a brief overview of substorm phenomenology. We then review some of the evidence for particle acceleration in Earth’s magnetosphere during substorms. Such in situ observations in this most accessible of all cosmic plasma domains may hold important clues to understanding acceleration processes in more distant astrophysical systems.

Subject headings: acceleration of particles — Earth — solar wind

Particle acceleration processes are important in understanding many of the Jovian radio and plasma wave emissions. However, except for the high-energy electrons that generate synchrotron emission following inward diffusion from the outer magnetosphere, acceleration processes in Jupiter’s magnetosphere and between Jupiter and Io are poorly understood. We discuss very recent observations from the Ulysses spacecraft of two new Jovian radio and plasma wave emissions in which particle acceleration processes are important and have been addressed directly by complementary investigations. First, radio bursts known as quasi-periodic bursts have been observed in close association with a population of highly energetic electrons. Second, a population of much lower energy (keV range) electrons on auroral field lines can be shown to be responsible for the first observation of a Jovian plasma wave emission known as auroral hiss.

Subject headings: acceleration of particles — planets and satellites: individual (Jupiter) — radio continuum: solar system

The Fermi shock acceleration mechanism may be responsible for the production of high-energy cosmic rays in a wide variety of environments. Modeling of this phenomenon has largely focused on plane-parallel shocks, and one of the most promising techniques for its study is the Monte Carlo simulation of particle transport in shocked fluid flows. One of the principal problems in shock acceleration theory is the mechanism and efficiency of injection of particles from the thermal gas into the accelerated population. The Monte Carlo technique is ideally suited to addressing the injection problem directly, and previous applications of it to the quasi-parallel Earth bow shock led to very successful modeling of proton and heavy ion spectra, as well as other observed quantities. Recently this technique has been extended to oblique shock geometries, in which the upstream magnetic field makes a significant angle ΘB1 to the shock normal. In this paper, spectral results from test particle Monte Carlo simulations of cosmic-ray acceleration at oblique, nonrelativistic shocks are presented. The results show that low Mach number shocks have injection efficiencies that are relatively insensitive to (though not independent of) the shock obliquity, but that there is a dramatic drop in efficiency for shocks of Mach number 30 or more as the obliquity increases above 15°. Cosmic-ray distributions just upstream of the shock reveal prominent bumps at energies below the thermal peak; these disappear far upstream but might be observable features close to astrophysical shocks.

Subject headings: acceleration of particles — cosmic rays — shock waves

Scatter-free acceleration of energetic particles by quasi-perpendicular interplanetary shocks is investigated. A brief review is given on the predictions of the gradient drift acceleration model concerning the energy, time, and angular dependence of the particle flux caused by a single shock encounter interaction. The angular distribution of ions in the energy range 35 keV to 1 MeV has been determined by the low-energy ion spectrometer aboard the ISEE 3 spacecraft at several shock associated events. Reflections of particles from the shock were clearly identifiable by the loss cone in the upstream pitch angle distributions. The measurements were compared to the predictions of the gradient drift acceleration model, showing a qualitative agreement in many respects. However, bidirectional distributions observed at nearly perpendicular shocks cannot be explained in the framework of the single shock encounter mechanism. It is suggested that multiple intersections of the field lines with the surface of the shock, forming magnetic traps on the upstream side, are responsible for the observed bidirectional distributions. Results obtained from numerical test particle simulations are discussed and compared to observations. A qualitative agreement between model calculations and measurements confirms that the energetic particles are trapped and accelerated, due to special field line topology, on the upstream side of the shock. It is also argued that the collapse of the trap by the convection of the field lines through the shock is accompanied by a considerable increase of the particle flux, which may be responsible for the shock spikes.

Subject headings: acceleration of particles — interplanetary medium

We discuss the fundamental ideas of particle acceleration in plasma shocks with emphasis on those features that are required to produce the “universal” power-law spectrum. We compare shock acceleration with the more familiar second-order or stochastic acceleration and see that they are not too different in many respects. We discuss the features of shock acceleration that make it appealing and some of its problems as well.

Subject headings: acceleration of particles — MHD — plasmas — shock waves

New developments in the understanding of the interplanetary transport of solar cosmic rays are reviewed. Based on carefully analyzed solar particle events observed on the Helios and ISEE 3 spacecraft, the relation of transport parameters to the structure of the interplanetary magnetic field is discussed. Special emphasis is given to a comparison of particle mean free paths determined from fits to intensity and anisotropy profiles with theoretical predictions derived from magnetic field spectra measured at the time of the solar particle event. Different aspects of the turbulence and wave models for the magnetic fluctuations are considered, including the effects resulting from the finite temperature of the plasma and of resonance broadening. It is found that a modified quasi-linear theory of particle scattering taking into account the effects of plasma waves propagating with respect to the average solar wind flow and the proper treatment of the dispersion relation at high wavenumber gives results which are in several cases in good agreement with particle observations in the interplanetary medium between 0.3 and 1 AU, indicating that quasi-linear theory is probably a good approximation to a full theory of solar particle transport. This has important implications for other astrophysical problems where quasi-linear theory is often used, such as the propagation and acceleration of Galactic cosmic rays and particle acceleration at shock waves.

Subject headings: acceleration of particles — cosmic rays — interplanetary medium — MHD — solar wind — Sun: particle emission

Recently, strong large amplitude magnetic field structures (SLAMS) have been observed as a common phenomenon in the vicinity of the quasi-parallel region of Earth’s bow shock. A quasi-parallel shock transition can be considered as a patchwork of SLAMS. Using the data of the AMPTE/IRM magnetometer the properties of SLAMS are studied. Within SLAMS the magnetic field is strongly deformed and, thus, the magnetic field geometry is locally swung into a quasi-perpendicular regime. Therefore, electrons can locally be accelerated to high energies within SLAMS. Assuming that SLAMS also exist in the vicinity of supercritical, quasi-parallel shocks in the solar corona, they are able to generate radio radiation via the enhanced Langmuir turbulence excited by the accelerated electrons. Since SLAMS are connected with strong density enhancements, the aforementioned mechanism can explain the multiple-lane structure often occurred in solar Type II radio bursts.

Subject headings: acceleration of particles — Earth — shock waves — Sun: corona — Sun: radio radiation

The paper deals with the acceleration of electrons at a nearly perpendicular curved shock wave and results are compared with the case of a plane shock wave. We formulate the equations of motion of an electron in adiabatic approximation and solve them in a special case. The results show that the energy gain of reflected electrons at a curved shock depends on the ratio of the radius of curvature to the shock thickness; for a low ratio the energy gain is significantly lower than at a plane shock, while the energy gain of transmitted electrons is approximately the same as at a plane shock. The analytical solution provides a tool for direct calculations of fluxes of accelerated electrons near the curved shock of given parameters.

Subject headings: acceleration of particles — shock waves

We present some conditions for the stability of a strong double layer in a warm, current-carrying plasma, which can be extended into the relativistic regime. We apply a model for plasma heating by the electron beam emitted from the double layer and show that this leads to a finite life time of the double layer. We also show that the radio emission accompanying this process can well describe the observed phenomena in Type I radio bursts using a direct emission mechanism, not involving Langmuir waves.

Subject headings: plasmas — Sun: flares — Sun: radio radiation

An analysis based on a fluid type equation for the spatial diffusion of the anomalous component of cosmic rays in the solar wind is presented. The source distributions of the high-energy particles are related to the pick-up ion flux intensities at the position of the heliospheric shock. Due to the strong asymmetric distributions of the different species of pick-up ions in the heliosphere and a possible aspherical termination shock of the solar wind, the source distributions are expected to exhibit element specific upwind-downwind asymmetries. An analytical treatment of the problem, using boundary conditions derived from observations by the Pioneer and Voyager satellites, leads to an estimate of the asymmetries of the anomalous component in the heliosphere. The investigation is performed in two stages: the problem is solved for an axially symmetric heliosphere in the first instance, and a latitudinal variation of the solar wind velocity as well as in the hydrodynamic diffusion coefficient is incorporated to model the effects of a heliospheric magnetic field in the second.

Subject headings: acceleration of particles — cosmic rays — shock waves — solar wind

We present comparisons of multiwavelength data for a number of solar flares observed during the major campaign of 1991 June. The different wavelengths are diagnostics of energetic electrons in different energy ranges: soft X-rays are produced by electrons with energies typically below 10 keV, hard X-rays by electrons with energies in the range 10-200 keV, microwaves by electrons in the range 100 keV-1 MeV, and millimeter-wavelength emission by electrons with energies of 0.5 MeV and above. The flares in the 1991 June active period were remarkable in two ways: all have very high turnover frequencies in their microwave spectra, and very soft hard X-ray spectra. The sensitivity of the microwave and millimeter data permit us to study the more energetic (>0.3 MeV) electrons even in small flares, where their high-energy bremsstrahlung is too weak for present detectors. The millimeter data show delays in the onset of emission with respect to the emissions associated with lower energy electrons and differences in time profiles, energy spectral indices incompatible with those implied by the hard X-ray data, and a range of variability of the peak flux in the impulsive phase when compared with the peak hard X-ray flux which is two orders of magnitude larger than the corresponding variability in the peak microwave flux. All these results suggest that the hard X-ray-emitting electrons and those at higher energies which produce millimeter emission must be regarded as separate populations. This has implications for the well-known “number problem” found previously when comparing the numbers of nonthermal electrons required to produce the hard X-ray and radio emissions.

Subject headings: Sun: flares — Sun: radio radiation — Sun: X-rays, gamma rays

Hard X-rays and gamma-rays are the most direct signature of the energetic electrons and ions which are accelerated during solar flares. Since the beginning of 1990 the PHEBUS instrument and the SIGMA anticoincidence shield aboard GRANAT have provided hard X-ray and gamma-ray observations of solar bursts in the energy range 0.075-124 and 0.200-15 MeV, respectively. After a brief description of the experiments, we present some results obtained on solar bursts recorded in 1990 and 1991 June. Special emphasis is given to the results related with particle acceleration during solar flares.

The first part of the review is devoted to the constraints obtained on the electron acceleration timescale through the analysis of the temporal characteristics of the bursts. Combined studies of hard X-ray and gamma-ray emissions from PHEBUS and radio emissions from the Nançay Multifrequency Radioheliograph are used to infer constraints on the coronal magnetic topology involved in flares. The characteristics (location, spectrum) of the radio-emitting sources are found to vary within a flare from one hard X-ray peak to the other. Hard X-ray and gamma-ray burst onsets and rapid increases of the > 10 MeV emission are coincident with changes in the associated radio emission pattern. These results will be discussed in the context of the flare energy release.

The second part of the paper concerns the heliocentric angle distribution of > 10 MeV events and presents more detailed observations of some of the largest flares in the gamma-ray line and the high-energy domains produced by ultrarelativistic electrons and > 100 MeV nucleon−1 ions. The PHEBUS observations of the gamma-ray line flare of 11 June 1991 have been used to deduce the hardness of the accelerated ion spectrum. The link between the main part of the flare and the late long-lasting >50 MeV emission detected by EGRET/COMPTON is discussed. Finally some observations of the large 1990 May 24 flare which produced a large neutron event at ground level are presented.

Subject headings: acceleration of particles — Sun: flares — Sun: radio radiation — Sun: X-rays, gamma rays

Turbulent acceleration in the impulsive phase of solar flares is reviewed, with the emphasis on bulk energization of nonrelativistic electrons and prompt acceleration of the gamma-ray emitting nonrelativistic ions and relativistic electrons. The primary energy release in a flare cannot be due to collisional dissipation. Anomalous resistivity requires that the current flows in many narrow channels with the current density above threshold for current instability. The bulk energization of the electrons is due to the damping of low-frequency electrostatic turbulence generated by the current instability. This turbulence also limits the rate a nonthermal electron tail forms due to runaway acceleration. Stochastic and gyroresonant acceleration by MHD turbulence are discussed briefly, emphasizing the need for preacceleration. Stochastic acceleration is favorable for the gamma-ray emitting particles only if an adequated source of the MHD turbulence can be identified.

Subject headings: acceleration of particles — MHD — Sun: flares — turbulence