A brief review is presented of the concept of magnetic field reconnection or merging. This process occurs whenever an electric field is present along a separator line in the magnetic field. The basic properties of reconnection are discussed in the context of the classical MHD models by Sweet and Parker and by Petschek. Attention is then focussed on reconnection in collision-free plasmas. The energization of charged particles during their interaction with the current layers associated with the reconnection geometry is discussed and the nature of the processes occurring in the so-called diffusion region which surrounds the separator is considered. Finally, comments are made on the nonsteady aspects of reconnection at the earth's magnetopause.

Reconnection is clearly observed at the terrestrial magnetopause but seldom in the simple geometry originally proposed. Most often reconnection is patchy, forming tubes of twisted flux. The passage of one of these twisted tubes has been called a flux transfer event. Similar twisted tubes, or flux ropes, are formed at Venus by velocity shear. These tubes become so highly twisted that they become kink unstable. The presence of the kink instability suggests a way of creating compound flux ropes as have been postulated to be necessary to explain photospheric magnetic structure.

The magnetic flux ropes of Venus are small scale (ion gyroradius) cylindrically symmetric structures observed in situ by the Pioneer Venus orbiter in the largely magnetic field-free ionosphere of the planet. They are so named because of their helical magnetic structure, which in turn is due to primarily field-aligned currents within the rope. Empirical models can be used to examine the current structure in detail, and these models indicate that flux ropes may be unstable to the helical kink mode. Statistics of rope distribution and orientation also support this instability picture. The results of investigations into the direct measurements of Venus flux ropes may be relevant to certain astrophysical phenomena that must be observed remotely.

The role of laboratory experiments to the understanding of current systems in space plasmas is reviewed. It is shown that laboratory plasmas are uniquely suited to make detailed investigations of basic physical processes in current-carrying plasmas. Examples are given for double layers, current-driven instabilities, and the plasma dynamics at magnetic neutral points during reconnection. Observations of current sheet disruptions show the coupling between local plasma phenomena (double layers) and global circuit properties (magnetic energy storage).

The current theoretical understanding of the linear and nonlinear evolution of resistive tearing instabilities in sheared magnetic fields is reviewed. The physical mechanisms underlying this instability are emphasized. Some of the problems which are encountered in developing a model of magnetic energy dissipation in coronal loops are discussed and possible solutions are suggested.

Consideration of the many observed types of jets on scales ranging from parsecs to megaparsecs seen in radio, optical, infrared and X-ray wavebands with a variety of morphologies both in galactic and extragalactic systems leads to some constraints on their fundamental nature. Jet formation is introduced with the concept of the Laval nozzle and related points include the problem of maintaining the nozzle, Mach disk effects due to under and over-expansion and the potential importance of magnetic confinement and focussing. Current ideas on jet formation at the black hole and accretion disk are given with emphasis on the plasma physics associated with black-hole electrodynamics, thermal and magnetically driven winds and thick disks. Stability of jet propagation is reviewed with emphasis on magnetised and unmagnetised Kelvin-Helmholtz instabilities and the various dominant modes. The particle acceleration physics of shocks, wave-particle interactions and turbulence is summarised while noting some outstanding plasma physics problems. Jet equilibrium associated with the non-linear saturation of instabilities, the formation of cocoons, shock stabilisation and magnetic fields is discussed. Detailed plasma physics studies that could significantly clarify jet physics are indicated.

Present ideas concerning the electric heating of the solar corona are reviewed. We consider in some more detail the dissipation of MHD waves in strong horizontal gradients of the Alfvén velocity. Then we consider the evolution of DC currents in the solar corona. Some theories aiming at the evaluation of the net rate of energy dissipation by such mechanisms will be described. A short account will be given of a recent analytical study based on a generalization of Taylor's hypothesis concerning the evolution of magnetic helicity in plasma with a large magnetic Reynolds number.

Limitations of current knowledge of plasma double layers create difficulties in extrapolating double-layer concepts for application to astrophysical models. Some problems of this sort are described, and some central issues in structure and dynamics of double layers are identified, which must be addressed in astrophysical contexts. These include the determination of kinetic boundary conditions, and the relations of time and length scales of local dynamics and structure to those of the global circuit in which the double layer is contained.

Electrostatic turbulence develops in current carrying plasmas when the relative electron-ion drift exceeds the critical value for laminar current flow. Recent 2D computer experiments (Barnes, 1982) indicate that many weak ion acoustic double layers form in such turbulence when the plasma is strongly magnetized (ωce ≳ ωpe), the electron/ion temperature ratio is large (≳10), and the relative electron-ion drift is comparable to or less than the electron thermal speed. The double layers emerge from the incoherent spectrum of electrostatic ion cyclotron and ion acoustic waves as intense localized electric field structures propagating subsonically relative to the ion bulk flow. The occurrence of weak ion acoustic double layers, excited by field-aligned currents in the Earth's auroral regions, has also been reported from in situ spacecraft measurements (Temerin et al., 1982). An important question concerns the effect of these coherent electric fields on plasma transport properties such as bulk heating and acceleration. For example, one might expect nonlinear diffusion processes, manifested as distinct nonthermal features in the particle spectra, to accompany the quasilinear diffusion of ions as they traverse turbulent regions in space. This idea motivates the work presented here.

At the head of a jet the confining medium of plasma frequency νp is compressed, so that streaming instabilities between relativistic electrons and this plasma produce waves at νp′ > νp. Considerable power can be lodged in these electrostatic waves, and conversion to electromagnetic waves allows them to propagate far beyond the jet. Emission at ν ≈ νp′ or Compton boosted radiation at ν ≲ γ2 νp′ yields a cone of radiation of angle ~ 1/γ, which illuminates the region directly in front of the jet. This emission is not absorbed by the surrounding plasma unless a cloud blocks the jet. Absorption in a cloud can lead to tunneling through large clouds, or propelling of smaller clouds out of the jet path. In this fashion jets may clear their way through an inhomogeneous medium, avoiding lateral disturbances and preheating their path.

This presentation focuses upon the coronal heating problem and reports the results of Ionson's (1984) unified theory of electrodynamic heating. This generalized theory, which is based upon Ionson's (1982) LRC approach, unveils a variety of new heating mechanisms and links together previously proposed processes. Specifically, Ionson (1984) has derived a standing wave equation for the global current, I, driven by emfs that are generated by the β≳1 convection. This global electrodynamics equation has the same form as a driven LRC equation where the equivalent inductance, L=4ℓ/πc2, scales with the coronal loop length and where the equivalent capacitance, C=c2 ℓ/4πv2 A, is essentially the product of the free space capacitance, ℓ/4π, and the low frequency dielectric constant, c2/v2 A. The driving emf, ∊=vBa/c, is a formal integration constant associated with the convective stressing of β≳1 magnetic fields. Since the transition from the β≳1 driver to the β<1 coronal loop is typically small compared to the “wavelength” of the associated magnetic fluctuation, this integration constant is not sensitive to details of the transition zone. The total resistance, Rtot = L(1/tdiss+1/tphase+1/tleak), represents electrodynamic energy “loss” from dissipation, magnetic stress leakage out of the loop and phase-mixing. These three processes have been parameterized by appropriate timescales. Note that Rleak=L/tleak and Rphase=L/tphase do not result in resistive heating but do participate in limiting the amplitude of the global current, I. This is fairly obvious with regard to magnetic stress leakage but not for phase-mixing. The phase-mixing resistance, Rphase, represents coupling between the global current and the local current density. Since the global current is essentially an integration of the local currents, the degree of coherency between the local currents can play an important role in determining the ultimate amplitude of I. The rate at which coherency between the local currents is lost is given by the phase-mixing time, tphase. A loss of coherency implies a corresponding reduction in the amplitude of I. In this sense, Rphase measures the phase-mixing contribution to the global current limitation process.

Laboratory reconnection experiments dedicated to problems of space and astrophysics are briefly reviewed with the purpose of demonstrating that such experiments can provide important insights of considerable value to the development of reconnection theory. Moreover, many of these insights are of a kind not likely to be perceived either when working directly with space observations or while pursuing a course of pure theoretical reasoning without reference to laboratory results.

The earth's magnetosphere provides an ideal opportunity to model reconnection in well known geometries that are close enough to the idealized analytic models to make a comparison of the computer models with analytic theory meaningful. In addition more detailed, even three-dimensional, models can be used for a comparison with extended data from in situ observations. The computer studies have basically confirmed the reconnection picture that was based on two-dimensional steady state models and linear analytic theory. The three-dimensional models in particular have also added a lot more information on the reconnection process and the structure of flow, magnetic fields, and currents including many features that are consistent with observations and empirical models of geomagnetic substorms.

In this paper, we first discuss a set of 6 cm observations made with the NRAO Very Large Array (VLA) (spatial resolution ~ 2″) that pertain to changes in the coronal magentic field configurations that took place before the onset of an impulsive burst observed on 14 May 1980. We also discuss a second set of 6 cm VLA observations (spatial resolution 18″ arc) where several interacting loops were involved in triggering the onset of an impulsive burst observed on June 24, 1980, 19:57:00 UT. Both sets of observations are examples of magnetic reconnection process being involved in accelerating microwave emitting electrons.

D.C. electric fields provide the simplest and most direct means of accelerating electrons out of a thermal plasma. Most solar flare models result in the production of D.C. electric fields. On the other hand, microwave and hard X–ray observations of flares provide specific requirements for the number and energy of energetic electrons produced during a flare, and the timescales involved in accelerating them. The microwave emission from flares is understood to be gyrosynchrotron radiation from electrons with energies of 100 keV or greater. The hard X–ray emission (≳ 25 keV) can be interpreted as being either thick–target bremsstrahlung from non–thermal electrons (thin–target radiation may also contribute to the X–ray emission, but the process is less efficient), or thermal bremsstrahlung from hot, impulsively heated plasma. Hence, it is of interest to study the electric field acceleration of “runaway” electrons and the simultaneous Joule heating of the thermal plasma in light of these results from flare observations, without recourse to a specific flare model. Some of the results of such a study are summarized here.

The non-linear coalescence instability of current carrying solar loops can explain many of the characteristics of the solar flares such as their impulsive nature, heating and high energy particle acceleration, amplitude oscillations of electromagnetic and emission as well as the characteristics of 2–D microwave images obtained during a flare. The plasma compressibility leads to the explosive phase of loop coalescence and its overshoot results in amplitude oscillations in temperatures by adiabatic compression and decompression. We note that the presence of strong electric fields and super–Alfvenic flows during the course of the instability play an important role in the production of non–thermal particles. A qualitative explanation on the physical processes taking place during the non–linear stages of the instability is given.

A 3D magnetohydrodynamic simulation is presented. The essential conclusions obtained by the previous 2D model, such as the slow shock formation and the strong plasma jet generation, are reconfirmed. In addition, several new findings pertinent to three dimensionality are obtained. Among them, particularly interesting and important is the generation of field aligned currents at the slow shock associated with a local interruption of the neutral sheet current. It is also interesting to observe the generation of super–magnetosonic flows with the Mach number of 2.