Plasma and radiation processes (invited review)

The physical processes leading to metre-wave and microwave emissions from the solar corona are reviewed and the possibility of observing analogous phenomena on other stars is discussed. Particular emphasis is placed on three alternative processes for the emission from flare stars: cyclotron maser emission, second harmonic plasma emission, and gyro-synchrotron emission.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.     

Particle acceleration and nonthermal radiation in space plasmas

Acceleration processes for fast particles in astrophysical and space plasmas are reviewed with emphasis on stochastic acceleration by MHD turbulence and on acceleration by shock waves. Radiation processes in astrophysical and space plasmas are reviewed with emphasis on plasma emission from the solar corona and electron cyclotron maser emission from planets and stars.     

Certain problems of solar and heliospheric physics in the light of novel satellite data

Recent satellite data have given a better insight into the possible nature of extremely strong disturbances on the Sun and in the heliosphere by relating them to processes in the solar interior. The energy, momentum, and mass transfer on various spatiotemporal scales are organized in the Sun into a hierarchy of coupled nonlinear processes. Confirmation has been given to the fact that coronal mass ejections and solar flares are not linked causally but merely reflect the existence of two channels of free-energy dissipation in the solar atmosphere in the form of plasma motion and plasma emission; their relative role can be described by a corresponding nondimensional parameter. Information on the global asymmetry of the solar emission and active processes has been gained. A great diversity in the geometry of eruptive events (not necessarily associated with magnetic reconnection) has been revealed. In our opinion, the basic unresolved problems in the investigation of solar activity dictate the necessity of carrying out more accurate, absolutely calibrated measurements of the whitelight solar emission at appropriately high spatiotemporal resolutions. The development of direct and indirect techniques of measuring the electric fields and currents with the aim of reconstructing the solar and heliospheric current system remains a challenging task.     

A new mechanism for pulsar gamma-ray emission

Gamma-ray emission in pulsar magnetospheres is attributed to synchrotron radiation, which tends to decrease the pitch angle of the particle, being balanced by plasma processes tending to increase the pitch angle. The plasma processes are non-resonant instabilities that drive non-resonant quasilinear diffusion (NQD), thereby pumping energy from waves and the parallel motion of the particle into the perpendicular motion of the particle. It is shown that NQD can maintain the pitch angles for particles near the light-cylinder such that they radiate synchrotron radiation at MeV energies. Compared to conventional emission mechanisms (such as polar cap or outer gap models), the resulting spectrum has a relatively low upper cut-off from about a few to 100 MeV. Possible observational consequences of this mechanism are discussed.     

The models for radio emission from pulsars–The outstanding issues

The theory of pulsar radio emission is reviewed critically, emphasizing reasons why there is no single, widely-accepted emission mechanism. The uncertainties in our understanding of how the magnetosphere is populated with plasma preclude predicting the properties of the emission from first principles. Some important observational features are incorporated into virtually all the proposed emission mechanisms, and other observational features are either controversial or fail to provide criteria that clearly favor one mechanism over others. It is suggested that the criterion that the emission mechanism apply to millisecond, fast young, and slow pulsars implies that it is insensitive to the magnetic field strength. It is argued that coherent emission processes in all astrophysical and space plasmas consist of emission from many localized, transient subsources, that any theory requires both an emission mechanism and a statistical theory for the subsource, and, that this aspect of coherent emission has been largely ignored in treatments of pulsar radio emission. Several specific proposed emission mechanisms are discussed critically: coherent curvature emission by bunches, relativistic plasma emission, maser curvature emission, cyclotron instability and free electron maser emission. It is suggested that some form of relativistic plasma emission is the most plausible candidate although one form of maser curvature emission and free electron maser emission are not ruled out. Propagation effects are discussed, emphasizing the interpretation of jumps between orthogonal polarizations.     

Relativistic anisotropic pair plasmas

The properties of waves able to propagate in a relativistic pair plasma are at the basis of the interpretation of several astrophysical observations. For instance, they are invoked in relation to radio emission processes in pulsar magnetospheres and to radiation mechanisms for relativistic radio jets. In such physical environments, pair plasma particles probably have relativistic, or even ultrarelativistic, temperatures. Besides, the presence of an extremely strong magnetic field in the emission region constrains the particles to one-dimensional motion: all the charged particles strictly move along magnetic field lines.
We take anisotropic effects and relativistic effects into account by choosing one-dimensional relativistic Jűttner–Synge distribution functions to characterize the distribution of electrons and/or positrons in a relativistic, anisotropic pair plasma. The dielectric tensor, from which the dispersion relation associated with plane wave perturbations of such a pair plasma is derived, involves specific coefficients that depend on the distribution function of particles. A precise determination of these coefficients, using the relativistic one-dimensional Jűttner–Synge distribution function, allows us to obtain the appropriate dispersion relation. The properties of waves able to propagate in anisotropic relativistic pair plasmas are deduced from this dispersion relation. The conditions in which a beam and a plasma, both ultrarelativistic, may interact and trigger off a two-stream instability are obtained from this same dispersion relation. Two astrophysical applications are discussed.     

X-ray emission from solar flares

Solar X-ray Spectrometer (SOXS), the first space-borne solar astronomy experiment of India was designed to improve our current understanding of X-ray emission from the Sun in general and solar flares in particular. SOXS mission is composed of two solid state detectors, viz., Si and CZT semiconductors capable of observing the full disk Sun in X-ray energy range of 4–56 keV. The X-ray spectra of solar flares obtained by the Si detector in the 4–25 keV range show evidence of Fe and Fe/Ni line emission and multi-thermal plasma. The evolution of the break energy point that separates the thermal and non-thermal processes reveals increase with increasing flare plasma temperature. Small scale flare activities observed by both the detectors are found to be suitable to heat the active region corona; however their location appears to be in the transition region.     

X-ray spectroscopy of stars

Non-degenerate stars of essentially all spectral classes are soft X-ray sources. Their X-ray spectra have been important in constraining physical processes that heat plasma in stellar environments to temperatures exceeding one million degrees. Low-mass stars on the cooler part of the main sequence and their pre-main sequence predecessors define the dominant stellar population in the galaxy by number. Their X-ray spectra are reminiscent, in the broadest sense, of X-ray spectra from the solar corona. The Sun itself as a typical example of a main-sequence cool star has been a pivotal testbed for physical models to be applied to cool stars. X-ray emission from cool stars is indeed ascribed to magnetically trapped hot gas analogous to the solar coronal plasma, although plasma parameters such as temperature, density, and element abundances vary widely. Coronal structure, its thermal stratification and geometric extent can also be interpreted based on various spectral diagnostics. New features have been identified in pre-main sequence stars; some of these may be related to accretion shocks on the stellar surface, fluorescence on circumstellar disks due to X-ray irradiation, or shock heating in stellar outflows. Massive, hot stars clearly dominate the interaction with the galactic interstellar medium: they are the main sources of ionizing radiation, mechanical energy and chemical enrichment in galaxies. High-energy emission permits to probe some of the most important processes at work in these stars, and put constraints on their most peculiar feature: the stellar wind. Medium and high- resolution spectroscopy have shed new light on these objects as well. Here, we review recent advances in our understanding of cool and hot stars through the study of X-ray spectra, in particular high-resolution spectra now available from XMM-Newton and Chandra. We address issues related to coronal structure, flares, the composition of coronal plasma, X-ray production in accretion streams and outflows, X-rays from single OB-type stars, massive binaries, magnetic hot objects and evolved WR stars.     

Lower bound on the magnetic field strength of a magnetar from analysis of SGR giant flares

Based on the magnetar model, we have studied in detail the processes of neutrino cooling of an electron-positron plasma generating an SGR giant flare and the influence of the magnetar magnetic field on these processes. Electron-positron pair annihilation and synchrotron neutrino emission are shown to make a dominant contribution to the neutrino emissivity of such a plasma. We have calculated the neutrino energy losses from a plasma-filled region at the long tail stage of the SGR 0526-66, SGR 1806–20, and SGR 1900+14 giant flares. This plasma can emit the energy observed in an SGR giant flare only in the presence of a strongmagnetic field suppressing its neutrino energy losses. We have obtained a lower bound on the magnetic field strength and showed this value to be higher than the upper limit following from an estimate of the magnetic dipole losses for the magnetars being analyzed in a wide range of magnetar model parameters. Thus, it is problematic to explain the observed energy release at the long tail stage of an SGR giant flare in terms of the magnetarmodel.     

Cosmic radio sources with the peaks of spectral flux density at decameter wavelengths (According to the Grakovo catalog)

Spectral characteristics of extragalactic radio sources of synchrotron emission with spectral flux density maxima located at decameter wavelengths are studied on the basis of the Grakovo catalog data. Physical processes in space that can give rise to such spectra are considered. Angular sizes and possible characteristics of relevant cosmic plasma are determined for a number of sources.     

Instability of Electrons Trapped by the Coronal Magnetic Field and Its Evidence in the Fine Structure (Zebra Pattern) of Solar Radio Spectra

Solar radio emission is a significant source of information regarding coronal plasma parameters and the processes occurring in the solar atmosphere. High resolution frequency, space, and time observations together with the developed theory make it possible to retrieve physical conditions in the radiation source and recognize the radiation mechanisms responsible for various kinds of solar radio emission. In particular, the high brightness temperature of many bursts testifies to coherent radiation mechanisms, that is, to plasma instabilities in the corona. As an example, the fine structure of solar radio spectra looking like a set of quasi-harmonic stripes of enhanced and lowered radiation, which is observed against the type IV continuum at the post-flare phase of activity, is considered. It is shown that such emission arises from a trap-like source filled with a weakly anisotropic equilibrium plasma and a small addition of electrons which have a shortage of small velocities perpendicular to the magnetic field. For many recorded events with the mentioned fine spectral structure the instability processes responsible for the observed features are recognized. Namely, the background type IV continuum is due to the loss-cone instability of hot non-equilibrium electrons, and the enhanced striped radiation results from the double-plasma-resonance effect in the regions where the plasma frequency f _p coincides with the harmonics of electron gyrofrequency f _B ; f _p=sf _B . Estimations of the electron number density and magnetic field in the coronal magnetic traps, as well as the electron number density and velocities of hot electrons necessary to excite the radiation with the observed fine structure, are given. It is also shown that in some cases several ensembles of non-equilibrium electrons can coexist in magnetic traps during solar flares and that its radio signature sensitively depends on the parameters of the distribution functions of the various ensembles.     

Vlasov – Maxwell,Self-consistent Electromagnetic Wave Emission Simulations in the Solar Corona

1.5D Vlasov – Maxwell simulations are employed to model electromagnetic emission generation in a fully self-consistent plasma kinetic model for the first time in the context of solar physics. The simulations mimic the plasma emission mechanism and Larmor-drift instability in a plasma thread that connects the Sun to Earth with the spatial scales compressed appropriately. The effects of spatial density gradients on the generation of electromagnetic radiation are investigated. It is shown that a 1.5D inhomogeneous plasma with a uniform background magnetic field directed transverse to the density gradient is aperiodically unstable to the Larmor-drift instability. The latter results in a novel effect of generation of electromagnetic emission at plasma frequency. The generated perturbations consist of two parts: i) non-escaping (trapped) Langmuir type oscillations, which are localised in the regions of density inhomogeneity, and are highly filamentary, with the period of appearance of the filaments close to electron plasma frequency in the dense regions; and ii) escaping electromagnetic radiation with phase speeds close to the speed of light. When the density gradient is removed (i.e. when plasma becomes stable to the Larmor-drift instability) and a low density super-thermal, hot beam is injected along the domain, in the direction perpendicular to the magnetic field, the plasma emission mechanism generates non-escaping Langmuir type oscillations, which in turn generate escaping electromagnetic radiation. It is found that in the spatial location where the beam is injected, standing waves, oscillating at the plasma frequency, are excited. These can be used to interpret the horizontal strips (the narrow-band line emission) observed in some dynamical spectra. Predictions of quasilinear theory are: i) the electron free streaming and ii) the long relaxation time of the beam, in accord with the analytic expressions. These are corroborated via direct, fully-kinetic simulation. Finally, the interplay of the Larmor-drift instability and plasma emission mechanism is studied by considering a dense electron beam in the Larmor-drift unstable (inhomogeneous) plasma. The latter case enables one to study the deviations from the quasilinear theory.     

Evidence of harmonic microwave radiation during solar flares

First observational evidence of harmonic radiation at microwave frequencies during solar bursts is presented for the event of April 28, 1983. The recordings between 3.1 and 19.6 GHz show a typical continuum with a spectral maximum near 5.2 GHz. Superimposed fine structures with durations in the order of some seconds exhibit a very unusual spectral behaviour. Narrow-banded intensity peaks appeared at 5.2 and 11.8 GHz which were barely visible at 3.1, 8.4 and 19.6 GHz. These structures can be interpreted as harmonic emission. Harmonic radio emission can be generated either by plasma radiation, gyroradiation, electron-cyclotron maser or by nonlinear conversion processes. However, all of those mechanisms require extreme assumptions on the source and the ambient plasma in order to account for the observations.Proceedings of the Second CESRA Workshop on Particle Acceleration and Trapping in Solar Flares, held at Aubigny-sur-Nère (France), 23–26 June, 1986.     

The emission and absorption of waves by charged particles in magnetized plasmas

A semiclassical theory describing the emission and absorption of waves is applied to the interaction of charged particles with waves in magnetized plasmas. Spontaneous emission of all cold plasma wave modes is calculated in detail. The method gives the absorption coefficient for the waves and a diffusion equation in momentum space for the particles describing the effects of the induced processes.Coefficients describing the systematic change of particle parameters follow from the diffusion equation. Applications of astrophysical interest are outlined.     

Some constraints on particles in Saturn's spokes

The consequences of taking into account some physical processes previously ignored in the context of Saturn's spokes were studies. Secondary emission, which can ensure large changes of potential following small environment variations, may provide a plausible mechanism for triggering spoke formation. Taking into account the grains' centrifugal disruption due to their spin, should modify the constraints on size previously derived on the basis of electrostatic disruption alone. Taking into account the dielectric polarization when calculating the electrostatic forces on grains resting on parent bodies gives ejection conditions more stringent than previously expected. Finally, a simplified evaluations shows that if secondary emission properties of ring particle are of the order of those of many other oxides, and if the plasma temperature is near one of their threshold temperatures, then a very small environmental variation can trigger the ejection of about 0.1- to 1-μm grains from about 1-mm to 10-cm parent bodies in a very short time scale.     

Spectroscopic evidence of charge exchange X‐ray emission from galaxies

What are the origins of the soft X‐ray line emission from non‐AGN galaxies? XMM‐Newton RGS spectra of nearby non‐AGN galaxies (including starforming ones: M82, NGC 253, M51, M83, M61, NGC 4631, M94, NGC 2903, and the Antennae galaxies, as well as the inner bulge of M31) have been analyzed. In particular, the Kα triplet of O VII shows that the resonance line is typically weaker than the forbidden and/or inter‐combination lines. This suggests that a substantial fraction of the emission may not arise directly from optically thin thermal plasma, as commonly assumed, and may instead originate at its interface with neutral gas via charge exchange. This latter origin naturally explains the observed spatial correlation of the emission with various tracers of cool gas in some of the galaxies. However, alternative scenarios, such as the resonance scattering by the plasma and the relic photo‐ionization by AGNs in the recent past, cannot be ruled out, at least in some cases, and are being examined. Such X‐ray spectroscopic studies are important to the understanding of the relationship of the emission to various high‐energy feedback processes in galaxies (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Collective neutrino-plasma interactions

Nonlinear processes describing the interaction of neutrinos with collective plasma oscillations and the excitation of plasma turbulence by a large neutrino flux is discussed. The excitation considered is the inverse processes of neutrino emission by plasma waves first considered by Tsytovich (V.N. Tsytovich, Soviet Fiz. Dokl. 9 (1965) 1114). The process is similar to a beam plasma instability considered as inverse Landau damping in which the usual electromagnetic interactions are important. In the neutrino beam relaxation the weak interaction can play a similar role. We emphasize here the possibility of another process namely the interaction of an intense neutrino flux with a strongly turbulent plasma. The turbulence can also be assumed to be due to the shock produced at the early stages of a type II supernova (SN) explosion. The scattering of the neutrinos in the turbulent plasma is shown to be sufficient for transferring momentum and energy from the neutrino flux to the plasma causing the shock to continue moving outward and eventually creating the blow-off of the mantle of the star producing type II SN.     

A deep X-ray observation of M82 with XMM–Newton

We report on the analysis of a deep (100-ks) observation of the starburst galaxy M82 with the EPIC and RGS instruments onboard the X-ray telescope XMM–Newton . The broad-band (0.5–10 keV) emission is due to at least three spectral components: (i) continuum emission from point sources; (ii) thermal plasma emission from hot gas; and (iii) charge-exchange emission from neutral metals (Mg and Si). The plasma emission has a double-peaked differential emission measure, with the peaks at ∼0.5 and ∼7 keV. Spatially resolved spectroscopy has shown that the chemical absolute abundances are not uniformly distributed in the outflow, but are larger in the outskirts and smaller close to the galaxy centre. The abundance ratios also show spatial variations. The X-ray-derived oxygen abundance is lower than that measured in the atmospheres of red supergiant stars, leading to the hypothesis that a significant fraction of oxygen ions have already cooled off and no longer emit at energies ≳0.5 keV.     

The effect of photon capture and field ionisation on high magnetic field pulsars

We have investigated the formation of polar gaps in pulsars as envisaged by Ruderman and Sutherland (1975) in the presence of photon conversion into positronia (Shabad and Usov, 1985; Heroldet al., 1985) and dissociation of positronia by strong electric fields. For the emission of curvature radiation near the pulsar's surface, we have used formulae which are valid in the extreme relativistic regime. We find that the gap formation is not affected, only the gap height is generally slightly increased. In the sample of pulsars that we have studied, in most cases there is likely to be enough plasma in the gap to initiate processes producing radio emission in accordance with the theory. In some cases we found the possibility of emission of high-energy photons outside the pulsar magnetosphere, as was suggested earlier by us (Bhatiaet al., 1987) in conformity with some of the observations.