A Comparison of Ground Level Event e/p and Fe/O Ratios with Associated Solar Flare and CME Characteristics

Solar energetic particle (SEP) events reaching rigidities >1 GV are observed at 1?AU as ground-level events (GLEs). They are considered to be extreme cases of gradual SEP events, produced by shocks driven by wide and fast CMEs that are usually associated with long-duration (>1 hour) soft X-ray (SXR) flares. However, some large gradual SEP events, including GLEs, are associated with flares of short-duration (<1 hour) timescales comparable to those of flares seen with impulsive, low-energy SEP events with enhanced charge states, heavy-element abundances, and e/p ratios. The association of some GLEs with short-duration SXR events challenges us to understand the GLE event-to-event variation with SXR durations and whether it truly reflects the nature of the particle acceleration processes or simply the characteristics of the solar regions from which large, fast CMEs arise. We examine statistically the associated flare, active region (AR), and CME characteristics of ～40?GLEs observed since 1976 to determine how the GLE e/p and Fe/O ratios, each measured in two energy ranges, depend on those characteristics. The abundance ratios trend weakly to lower, more coronal, and less scattered values with increasing flare timescales, thermal and nonthermal peak fluxes, and measures of source AR sizes. These results and the wide range of solar longitude connections for GLEs with high abundance ratios argue against a significant role for flare effects in the GLEs. We suggest that GLE SEPs are accelerated predominately in CME-driven shocks and that a coupling of flare size and timescales with CME properties could explain the SEP abundance correlations with flare properties.     

Observations of fragmented energy release

Energy release into coronal plasmas is observable in the forms of heating and acceleration. In flares and active stars, heating and acceleration have been found to be related as indicated by an approximately constant ratio of microwave (synchrotron) and soft X-ray (thermal) emission. The discovery suggests a flare-like heating process for the quiescent coronae of active stars.The energy release in solar flares involves several time scales: (i) The largest is the rate of homologous flares in an active region of the order of one per five hours. (ii) Hard X-ray andH emissions suggest a total flare duration of ten minutes, (iii) with individual episodes of contiguous acceleration of one minute. (iv) Elementary hard X-ray peaks have 5–10 s duration, corresponding to groups of beams observable as type III radio bursts. (v) The effective injection time of these beams is of the order 0.1 s. (vi) The smaller time scale is observed in narrowband radio spikes in the 0.2–8 GHz range with durations of a few times 0.01 s.     

High-Frequency Radio Signatures of Solar Eruptive Flares

Several examples of the radio emission of eruptive solar flares with high-frequency slowly drifting structures and type II bursts are presented. Relationships of these radio bursts with eruptive phenomena such as soft X-ray plasmoid ejection and shock formation are shown. Possible underlying physical processes are discussed in the framework of the plasmoid ejection model of eruptive solar flares. On the other hand, it is shown that these radio bursts can be considered as radio signatures of eruptive solar flares and thus used for the prediction of heliospheric effects. This revised version was published online in August 2006 with corrections to the Cover Date.     

The transient highly excited solar flare plasma

Recent observations of the energetic particles produced in solar flares indicate that the production of electrons, with energies up to about 100 keV, is a fairly common feature of small flares. In those flares the acceleration of protons and other nuclei does not extend beyond about 1 MeV.The X-ray emission often exhibits two distinct components of which the first one is produced by non-thermal, the second by thermal electrons through bremsstrahlung collisions with the ambient ions. Along with these X rays, radio emission, in the microwave region, is observed. This radio emission is usually interpreted as due to gyrosynchrotron radiation from the same electrons.In this review a discussion is presented of the processes occurring in solar flares with special reference to the acceleration and radiation processes.     

Dissipative Solitary Kinetic Alfvén Waves and Electron Acceleration in the Solar Corona

Nonthermal electrons play a major role during solar flares since not only they contain a large amount of the released energy but also they provide important information of the flaring physics through their nonthermal radiation in radio and hard X-ray bands. In a recent work Wu (Phys. Plasmas 10 (2003) 1364) proposed that dissipative solitary kinetic Alfvén wave (DSKAW) with a local shock-like structure could provide an efficient acceleration mechanism for energetic electrons in a low-β plasma. In the present paper dynamical characteristics of the DSKAW acceleration mechanism in solar coronal plasmas are studied and its application to the acceleration of flaring electrons is discussed.     

Japanese satellite program on the solar research for the coming solar maximum

The Institute of Space and Astronautical Science (ISAS) is developing a satellite dedicated to high-energy observations of solar flares. The Solar-A will be launched in August–September, 1991, from the Kagoshima Space Center on board a M3S-II vehicle. The instrument complement emphasizes hard X-ray and soft X-ray imaging, with both high resolution (2.4 arc sec pixel size) and full-Sun field of view. Solar-A contains instruments supplied in part by U.S. and U.K. experimenters. This paper describes the instrumentation and the tentative observing program.     

Microflares and the Statistics of X-ray Flares

This review surveys the statistics of solar X-ray flares, emphasising the new views that RHESSI has given us of the weaker events (the microflares). The new data reveal that these microflares strongly resemble more energetic events in most respects; they occur solely within active regions and exhibit high-temperature/nonthermal emissions in approximately the same proportion as major events. We discuss the distributions of flare parameters (e.g., peak flux) and how these parameters correlate, for instance via the Neupert effect. We also highlight the systematic biases involved in intercomparing data representing many decades of event magnitude. The intermittency of the flare/microflare occurrence, both in space and in time, argues that these discrete events do not explain general coronal heating, either in active regions or in the quiet Sun.     

Solar flares as sources of energetic particles

In this paper a review is presented of the present status of our knowledge of solar flare phenomena with special emphasis on the production of suprathermal particles and their solar effects. Of these energetic particles electrons play an important role since they produce the X-ray and radiobursts observed during many flares. Also, during their slowing down to thermal energies they contribute to the heating of localized regions in the solar atmosphere, through energy exchange with the ambient electrons. Observable radiations of energetic protons, and other nuclei, are produced through nuclear interactions leading to the emissions of gamma-ray lines. Detectable fluxes of these gamma-ray lines are produced only in the most powerful flares. Also the nuclei that enter into deeper layers of the solar atmosphere transfer most of their kinetic energy to the ambient plasma.     

High-energy phenomena on the Sun: An introductory review

Large solar flares are often accompanied by both emissions of high-energy quanta and particles. The emissions such as gamma-ray and hard X-ray photons are generated due to the interaction of high-energy nuclei and electrons with gases ambient in the flare regions and the solar atmosphere. Nonthermal radio emissions of wide frequency band are produced from energetic electrons while being decelerated by the action of plasmas and magnetic fields ambient in the flare site and its neighboring region. To understand the emission mechanism of these high-energy quanta on the Sun, it is, therefore, necessary to find the acceleration mechanism for both nuclei and electrons, which begins almost simultaneously with the onset of solar flares.A part of the accelerated nuclei and electrons are later released from the solar atmosphere into the outer space and eventually lost from the space of the solar system. Their behavior in the interplanetary space is considered to study the large-scale structure of plasmas and magnetic fields in this space.The observations and studies of high-energy phenomena on the Sun are thus thought of as giving some crucial hint important to understand the nature of various high-energy phenomena being currently observed in the Universe.     

An Observational Overview of Solar Flares

We present an overview of solar flares and associated phenomena, drawing upon a wide range of observational data primarily from the RHESSI era. Following an introductory discussion and overview of the status of observational capabilities, the article is split into topical sections which deal with different areas of flare phenomena (footpoints and ribbons, coronal sources, relationship to coronal mass ejections) and their interconnections. We also discuss flare soft X-ray spectroscopy and the energetics of the process. The emphasis is to describe the observations from multiple points of view, while bearing in mind the models that link them to each other and to theory. The present theoretical and observational understanding of solar flares is far from complete, so we conclude with a brief discussion of models, and a list of missing but important observations.     

Soft X-Ray and Extreme Ultraviolet Excess Emission from Clusters of Galaxies

An excess over the extrapolation to the extreme ultraviolet and soft X-ray ranges of the thermal emission from the hot intracluster medium has been detected in a number of clusters of galaxies. We briefly present each of the satellites (EUVE, ROSAT PSPC and BeppoSAX, and presently XMM-Newton, Chandra and Suzaku) and their corresponding instrumental issues, which are responsible for the fact that this soft excess remains controversial in a number of cases. We then review the evidence for this soft X-ray excess and discuss the possible mechanisms (thermal and non-thermal) which could be responsible for this emission.     

EXOSAT medium energy observations of Cyg X-2

On July 5.–6. 1983, during the EXOSAT performance verification (PV) and calibration phase, a raster scan of Cygnus X-2 was performed. In contrast to the previously observed smooth intensity variations on timescales of hours, the source revealed a behaviour unknown until now: active periods with high energy flares recurring on time scales of 300–500 s were interrupted by quiet periods of several hours. At all intensity levels the source spectra clearly require a two component continuum (blackbody + thermal bremsstrahlung). In addition, a weak iron emission line with equivalent widths between 39 an 70 eV was detected. The source has a much harder spectrum during the flares than during quiet periods, indicating drastic temperature changes within the emission region, while the absolute iron line flux does not vary. From the spectral characteristics it becomes clear that self-comptonization of the thermal bremsstrahlung spectrum plays an important role. The time variability and spectral behaviour in this peculiar state allow Cyg X-2 to be classified as a Low Mass X-ray Binary System (LMXB) very similar to the prototype of this class, Sco X-1.     

X-rays from quasars and active galaxies

We review some of the most important theoretical ideas and observations for quasars and the nuclei of active galaxies, and suggest areas of future research. Emphasis is on the nature of the power source, the radiation processes, and the mechanism for formation and collimation of jets. Phenomena that produce X-rays are of particular concern. Particular topics discussed are the observed and expected time variability, the gas supply mechanisms and luminosity evolution, thermal and nonthermal radiation processes, observed and theoretical spectra, criteria for thermalization of electrons and ions, effects of electron-positron pairs on relativistic plasmas, hydrodynamic, electrodynamic and inertial methods for producing and confining jets. We conclude with a list of needed observations.Based on a lecture given at the Goddard Workshop on X-ray Astronomy (October 1981).     

The diffuse soft X-ray sky

The current status of the investigation of the soft X-ray diffuse background in the energy range 0.1–2.0 keV is reviewed. A consistent model, based on the soft X-ray brightness distribution and the energy spectrum over the sky, is derived. The observed diffuse background is predominantly of galactic origin and considered as thermal emission for the most part from a local hot region of temperature ≈10⁶ K which includes the solar system. Several pronounced features of enhanced emission are interpreted in terms of hot regions with temperatures up to 3×10⁶K, some of which are probably old supernova remnants. The properties of the soft X-ray emitting regions are discussed in relation to the observational results on O vi absorption.     

The nonthermal energy content and gamma ray emission of starburst galaxies and clusters of galaxies

The nonthermal particle production in contemporary starburst galaxies and in galaxy clusters is estimated from the Supernova rate, the iron content, and an evaluation of the dynamical processes which characterize these objects. The primary energy derives from SN explosions of massive stars. The nonthermal energy is transformed by various secondary processes, like acceleration of particles by Supernova Remnants as well as diffusion and/or convection in galactic winds. If convection dominates, the energy spectrum of nonthermal particles will remain hard. At greater distances from the galaxy almost the entire enthalpy of thermal gas and Cosmic Rays will be converted into wind kinetic energy, implying a fatal adiabatic energy loss for the nonthermal component. If this wind is strong enough then it will end in a strong termination shock, producing a new generation of nonthermal particles which are subsequently released without significant adiabatic losses into the external medium. In clusters of galaxies this should only be the case for early type galaxies, in agreement with observations. Clusters should also accumulate their nonthermal component over their entire history and energize it by gravitational contraction. The pion decay -ray fluxes of nearby contemporary starburst galaxies is quite small. However rich clusters should be extended sources of very high energy -rays, detectable by the next generation of systems of air Cherenkov telescopes. Such observations will provide an independent empirical method to investigate these objects and their cosmological history.     

Spectral observation of the soft x-ray background and of the north polar spur with solid state spectrometers

Conclusions The present results confirm the thermal nature of the interstellar soft X-ray emission and give rather clear evidence of the Oxygen and Carbon contribution.     

Particle Acceleration by the Sun: Electrons, Hard X-rays/Gamma-rays

Observations of hard X-ray (HXR)/γ-ray continuum and γ-ray lines produced by energetic electrons and ions, respectively, colliding with the solar atmosphere, have shown that large solar flares can accelerate ions up to many GeV and electrons up to hundreds of MeV. Solar energetic particles (SEPs) are observed by spacecraft near 1 AU and by ground-based instrumentation to extend up to similar energies, but it appears that a different acceleration process, one associated with fast Coronal Mass Ejections (CMEs) is responsible. Much weaker SEP events are observed that are generally rich in electrons, ³He, and heavy elements. The energetic particles in these events appear to be similar to those accelerated in flares. The Ramaty High Energy Solar Spectroscopic Imager (RHESSI) mission provides high-resolution spectroscopy and imaging of flare HXRs and γ-rays. The observations of the location, energy spectra, and composition of the flare accelerated energetic particles at the Sun strongly imply that the acceleration is closely related to the magnetic reconnection that releases the energy in solar flares. Here preliminary comparisons of the RHESSI observations with observations of both energetic electrons and ions near 1 AU are reviewed, and the implications for the particle acceleration and escape processes are discussed.     

Microphysics of Cosmic Ray Driven Plasma Instabilities

Energetic nonthermal particles (cosmic rays, CRs) are accelerated in supernova remnants, relativistic jets and other astrophysical objects. The CR energy density is typically comparable with that of the thermal components and magnetic fields. In this review we discuss mechanisms of magnetic field amplification due to instabilities induced by CRs. We derive CR kinetic and magnetohydrodynamic equations that govern cosmic plasma systems comprising the thermal background plasma, comic rays and fluctuating magnetic fields to study CR-driven instabilities. Both resonant and non-resonant instabilities are reviewed, including the Bell short-wavelength instability, and the firehose instability. Special attention is paid to the longwavelength instabilities driven by the CR current and pressure gradient. The helicity production by the CR current-driven instabilities is discussed in connection with the dynamo mechanisms of cosmic magnetic field amplification.