Plasma Astrophysics

Plasma astrophysics studies electromagnetic processes and phenomena in space, mainly the role of forces of an electromagnetic nature in the dynamics of cosmic matter. Two factors are specific to the latter: its gaseous state and high conductivity. Such a combination is unlikely to be found under natural conditions on Earth; the matter is either a non-conducting gas (the case of gas dynamics or hydrodynamics) or a liquid or a solid conductor. By contrast, plasma is the main state of cosmic matter. It is precisely the poor knowledge of cosmic phenomena and cosmic plasma properties that explains the retarded development of plasma astrophysics. It has been distinguished as an independent branch of physics in the pioneering works of Alfvén (see Alfvén, 1950).

There exist two different ways to describe exactly the behaviour of a system of charged particles in electromagnetic and gravitational fields. The first description, the Newton set of motion equations, is convenient for a small number of interacting particles. For systems of large numbers of particles, it is more advantageous to deal with the single Liouville equation for an exact distribution function.

In a system which consists of many interacting particles, the statistical mechanism of ‘mixing’ in phase space works and makes the system’s behaviour on average more simple.

In a system which consists of many interacting particles, the weakcoupling assumption allows us to introduce a well controlled approximation to consider the chain of the equations for correlation functions. This leads to a very significant simplification of the original collisional integral to describe collisional relaxation and transport in astrophysical plasma but not in self-gravitating systems.

Among a variety of kinetic phenomena related to fast particles in astrophysical plasma, the simplest effect is Coulomb collisions under propagation of the particles in a plasma. An important role of the reverse-current electric field in this situation is demonstrated.

Astrophysical plasma is often an extremely tenuous gas of charged particles, without net charge on average. If there are very few encounters between particles, we need only to consider the responses of a particle to the force fields in which it moves. The simplest situation, a single particle in given fields, allows us to understand the drift motions of different origin and electric currents in such collisionless plasma.

Adiabatic invariants are useful to understand many interesting properties of collisionless plasma in cosmic magnetic fields: trapping and acceleration of charged particles in collapsing magnetic traps, the Fermi acceleration, “cosmic rays” origin.

The growth or damping of the waves, the emission of radiation, the scattering and acceleration of particles — all these phenomena may result from wave-particle interaction, a process in which a wave exchanges energy with the particles in astrophysical plasma.

Binary collisions of particles with the Coulomb potential of interaction are typical for physics of collisional plasmas in space and especially for gravitational systems. Coulomb collisions of fast particles with plasma particles determine momentum and energy losses of fast particles, the relaxation processes in astrophysical plasma.

In this Chapter we are not concerned with individual particles but we will treat individual kinds of particles as continuous media interacting between themselves and with an electromagnetic field. This approach gives us the multi-fluid models of plasma, which are useful to consider many properties of astrophysical plasma.

The multi-fluid models of the astrophysical plasma in magnetic field allow us to derive the generalized Ohm’s law and to consider important physical approximations as well as many interesting applications.

Single-fluid models are the simplest but sufficient approximation to describe many large-scale low-frequency phenomena in astrophysical plasma: regular and turbulent dynamo, plasma motions driven by strong magnetic fields, accreation disks, and relativistic jets.

Magnetohydrodynamics (MHD) is the simplest but sufficient approximation to describe many large-scale low-frequency phenomena in astrophysical plasma: regular and turbulent dynamo, plasma motions driven by strong magnetic fields, accreation disks, and relativistic jets.

A sufficiently strong magnetic field easily moves a comparatively rarified plasma in many non-stationary phenomena in space, for example in solar flares and coronal mass ejections which strongly influence the interplanetary and terrestrial space.

There are four different modes of magnetohydrodynamic waves in an ideal plasma with magnetic field. They can create turbulence, nonlinearly cascade in a wide range of wavenumbers, accelerate particles and produce a lot of interesting effects under astrophysical conditions.

The phenomena related to shock waves and other dicontinuous flows in astrophysical plasma are so numerous that the study of MHD discontinuities on their own is of independent interest for space science.

A discontinuity cannot exist in astrophysical plasma with magnetic field if small perturbations disintegrate it into other discontinuities or transform it to a more general nonsteady flow.

Sir Charles Darwin (1949) presumably thought that shock waves are responsible for accelerating cosmic rays. Nowadays shocks are widely recognized as a key to understanding high-energy particle acceleration in a variety of astrophysical environments.

The concept of equilibrium is fundamental to any discussion of the energy contained in an astrophysical object or phenomenon. The MHD non-equilibrium is often related to the onset of dynamic phenomena in astrophysical plasma.

There exist two different sorts of stationary MHD flows depending on whether or not a plasma can be considered as ideal or non-ideal medium. Both cases have interesting applications in modern astrophysics.

Magnetic fields are easily generated in astrophysical plasma owing to its high conductivity. Magnetic fields, having strengths of order few 10-6 G, correlated on several kiloparsec scales are seen in spiral galaxies. Their origin could be due to amplification of a small seed field by a turbulent galactic dynamo. In several galaxies, like the famous M51, magnetic fields are well correlated (or anti-correlated) with the optical spiral arms. These are the weakest large-scale fields observed in cosmic space. The strongest magnets in space are presumably the so-called magnetars, the highly magnetized (with the strength of the field of about 1015 G) young neutron stars formed in the supernova explosions.

Magnetic reconnection is a fundamental feature of astrophysical and laboratory plasmas, which takes place under definite but quit general conditions and creates a sudden release of magnetic energy, an original electrodynamical explosion or flare. Surprisingly, the simplest approximation — a single particle in given force fields — gives us clear approach to several facets of reconnection and particle acceleration.

When two oppositely directed magnetic fields are pressed together, the conductive plasma is squeezed out from between them, causing the field gradient to steepen until a reconnecting current layer (RCL) appears and becomes so thin that the resistive dissipation determines the magnetic reconnection rate. In this Chapter, the basic magnetohydrodynamic properties of such a process are considered in the approximation of a strong magnetic field.

The physics of flares on the Sun now becomes ‘an étalon’ for contemporary astrophysics, in particular for gamma and X-ray astronomy. In contrast to flares on other stars and to many analogous phenomena in the Universe, solar flares are accessible to a broad variety of observational methods to see and investigate the magnetic reconnection process in high-temperature strongly-magnetized plasma of the corona as well as in low-temperature weakly-ionized plasma in the photosphere.

The famous ‘Bastille day 2000’ flare was well observed by several space- and ground-based observatories and stidied extensively by many researchers. The modern observations in multiple wavelengths demonstrate, in fact, that the Bastille day flare has the same behavior as many large solar flares. In this Chapter, the flare is studied from observational and topological points of view in terms of three-dimentional magnetic reconnection.

The topological model of a flare, with a reasonable accuracy, predicts the location of a flare energy source in the corona. In order to clarify an origin of this energy, we have to consider the non-potential part of magnetic field in an active region. In this Chapter, we discuss the main electric currents related to magnetic reconnection in a large solar flare. More specificaly, we continue a study of the Bastille day 2000 flare which topological model was considered in a previous Chapter

Reconnection in cosmic plasma serves as a highly efficient engine to convert magnetic energy into thermal and kinetic energies of plasma flows and accelerated particles. Stationary models of the reconnection in current layers are considered in this Chapter. Properties of a stationary current layer strongly depends on a state of plasma turbulence inside it.

The super-hot turbulent-current layer (SHTCL) model fits well for solar flares with different properties: impulsive and gradual, compact and large-scale, thermal and non-thermal. Reconnection in SHTCLs creates collapsing magnetic traps. In this Chapter, we discuss the possibility that coronal HXR emission is generated as bremsstrahlung of the fast electrons accelerated in the collapsing traps due to joint action of the Fermi-type first-order mechanism and betatron acceleration.

The super-hot turbulent-current layer (SHTCL) theory offers an attractive opportunity for laboratory and astrophysical applications of the magnetic reconnection.

The inductive electric field is directed along the current inside a collisionless reconnecting current layer (RCL). This strong field does positive work on charged particles, thus increasing their energy impulsively, for example, in solar flares of flares in the accretion disk coronae of compact astrophysical objects.

The interrelation between the stability and the structure of current layers governs their nonlinear evolution and determines a reconnection regime. In this Chapter we study the structural instability of the reconnecting current layer, i.e. its evolutionarity.

The tearing instability can play a significant role in reconnecting current layers, but it is well stabilized in many cases of interest. For this reason, quasi-stationary current layers can exist for a long time in astrophysical plasma, for example in the solar corona, in the Earth magnetospheric tail.

The open issues focused on in this Chapter presumably will determine the nearest future as well as the most interesting perspectives of plasma astrophysics.

Magnetic reconnection, while being well established in the solar corona, is successfully invoked for explanation of many phenomena in the low-temperature weakly-ionized plasma in the solar atmosphere.

Magnetic reconnection reconnects field lines together with field-aligned electric currents. This process may play a significant role in the dynamics of astrophysical plasma because of a topological interruption of the electric currents.