Relativistic electron events in interplanetary space

This review is concerned with relativistic electron events observed in interplanetary space. The different types of event are identified and illustrated. The relationships between solar X-ray and radio emissions and relativistic electrons are examined, and the relevance of the observations to solar flare acceleration models is discussed. A statistical analysis of electron spectra, the electron/proton ratio and propagation from the flare site to the Earth is presented. A model is outlined which can account for the release of electrons from the Sun in a manner consistent with observations of energetic solar particles and electromagnetic solar radiation.The literature survey for this review was concluded in May 1973.     

Structure and development of quiet loops in the solar corona

X-ray emission from solar coronal loops changes on two different timescales: a) flare loops and transient active region brightenings show a rapid variability, b) quiet region loops are quasi-steady and change only slowly with time. This different time behavior has been analyzed on the basis of Yohkoh SXT observations and we report here on the results from our analysis, mainly focussing on quiet loop variability.     

Soft X-ray observations of helmet structures from SXT/Yohkoh

We present soft X-ray observations of helmet structures in solar active regions obtained from SXT/Yohkoh. These helmet structures are observed to form in the flare decay phase and to be associated with active region loop interactions. Their morphology is similar to the much larger scale helmet streamers that appear in the outer corona as shown in optical images of solar eclipse. The observed X-ray helmet structures appear to be in quasi-equilibrium with lifetimes greater than the MHD time scale. Using the filter ratio method for the X-ray observations, we find that the cusp region has lower temperature and higher density than that in the stalk region above it. The plasma pressure in the cusp region is about the same or slightly higher than that in the stalk region.     

Heating Diagnostics with MHD Waves

The heating of the solar atmosphere is a fundamental problem of modern solar and astrophysics. A review of the seismological aspects of magnetohydrodynamic (MHD) waves with an emphasis on standing longitudinal waves in the context of coronal heating is presented. Efforts made recently may be split into two categories: forward modelling and data inversion. Forward modelling can be applied to predict the observational footprints of various heating scenarios. A new diagnostic method based on the analysis of Doppler shift time series is outlined with specific application to solar coronal conditions. The power of the method is demonstrated and tested using synthetic data and comparing them with actual high-resolution (e.g. SoHO/SUMER) observations. Further, related recent examples of standing longitudinal oscillations in coronal loop structures observed with the new Hinode/EIS instrument are also presented. These latter observations provide an advanced ground for MHD seismology as a tool for plasma heating diagnostics in the atmosphere of the 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.     

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.     

Observational studies of gamma-rays and neutrons from solar flares

Gamma-ray observations from HINOTORI satellite and possible neutron observations from the Tokyo neutron monitor are reviewed. Time histories of gamma-ray and X-ray emissions for both typical impulsive and gradual flares are discussed in connection with the particle acceleration time. The gamma-ray spectral hardening observed around 400 keV is explained from superimposition of two different electron bremsstrahlung spectra. Proton-energy spectra derived from the gamma-ray observations are compared with the solar energetic particle spectra in interplanetary space. The weak correlation between the gamma-ray fluence and the proton flux is discussed in connection with the particle trapping and escaping in the flare region. The limb darkening of the 2.22 MeV line resulting from neutron-proton capture is interpreted in terms of the attenuation by the Compton scattering in the photosphere. Possible solar neutron events recorded by the Tokyo neutron monitor are presented and the correlation between the gamma-ray fluence and the neutron fluence are described.     

Microwave spectrum analysis as solar energy release diagnostics

The possibilities of using spectrographic observations of microwave radio emission as a solar flare plasma diagnostic are discussed. The spectral fine structure of the emission is interpreted in the context of plasma emission mechanisms. The balance equations for particles and plasma turbulence together with the transfer equations for electromagnetic waves in a plasma are solved for a model containing a diverging magnetic loop. As a result of the analysis of the blip-type spectral feature, the structure of energy release region and the unperturbed plasma concentration in the preflare loop are evaluated. The number of accelerated electrons and the intensity of the plasma turbulence in the source region are estimated using the properties of the weak continuum emission following the blip. Based on the degree of circular polarization of both the narrow band and the continuum emission, estimates for the external magnetic field strength and the angular width of the radiating plasma turbulence have been obtained.     

Optical observations of solar flares

This paper gives a review of the results of optical observations of solar flares. Observations carried out in the H_α line, flare spectra, and the methods of analysing the flare spectra are briefly discussed. Great attention is paid to the continuous emission of flares in the optical spectral region. In the last section, optical aspects of proton flares are summarized.     

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.     

3He-rich solar flares

The formation of the observed solar cosmic ray (SCR) composition remains an open question. It has become particularly acute after the discovery of ³He-rich events. The ³He/⁴He abundance ratio revealed in such events exceeds substantially (up to four orders of magnitude) that in the solar atmosphere.The data available on the ³He-rich events are discussed and a list of all such events known up to date is presented. Most of the ³He-rich SCR events can be associated with the corresponding optical flares on the Sun, with X-ray and radiobursts.An analysis of the models of ³He enrichement proposed up to now shows that only preferential ³He heating by plasma mechanisms can provide the observed high enrichment levels (³He/⁴He 1).A model involving preferential heating of ³He by induced scattering on ions of the ion acoustic waves generated by flare associated electrons in the solar atmosphere is considered in detail. This model can account for the major properties of the ³He-rich flares.Observational implications of the ³He-rich solar flare model are analyzed; the predictions of the theory are compared with the experimental data available, and promising avenues of further relevant experimental and theoretical research are considered.However it is shown that all main conclusions made based on the expressions ((3.2), (3.11), (3.12)) remain the same.     

Laboratory-produced radiation related to the solar flare emission

A review is given of continuing efforts to generate laboratory spectra at photon energies above 1 keV from highly ionized atoms. Emphasis is placed on the most easily-interpreted highest ion stages such as the hydrogenic, helium-like and lithium-like species. The close similarities to solar flare observations of certain signatures, such as the line and continuum X-ray spectra and microwave emission as well as their sequence of occurrence, are pointed out and scaling factors are given.     

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.     

Non-relativistic solar electrons

This review summarizes both the direct spacecraft observations of non-relativistic solar electrons, and observations of the X-ray and radio emission generated by these particles at the Sun and in the interplanetary medium. These observations bear on three physical processes basic to energetic particle phenomena: (1) the acceleration of particles in tenuous plasmas; (2) the propagation of energetic charged particles in a disordered magnetic field, and (3) the interaction of energetic charged particles with tenuous plasmas to produce electromagnetic radiation. Because these electrons are frequently accelerated and emitted by the Sun, mostly in small and relatively simple flares, it is possible to define a detailed physical picture of these processes.In many small solar flares non-relativistic electrons accelerated during flash phase constitute the bulk of the total flare energy. Thus the basic flare mechanism in these flares essentially converts the available flare energy into fast electrons. Non-relativistic electrons exhibit a wide variety of propagation modes in the interplanetary medium, ranging from diffusive to essentially scatter-free. This variability in the propagation may be explained in terms of the distribution of interplanetary magnetic field fluctuations. Type III solar radio burst emission is generated by these electrons as they travel out to 1 AU and beyond. Recent in situ observations of these electrons at 1 AU, accompanied by simultaneous observations of the low frequency radio emission generated by them at 1 AU provide quantitative information on the plasma processes involved in the generation of type III bursts.     

Solar cosmic ray phenomena

This review attempts to present an integrated view of the several types of solar cosmic ray phenomena. The relevant large and small scale properties of the interplanetary medium are first surveyed, and their use in the development of a quantitative understanding of the cosmic ray propagation processes summarised. Solar cosmic ray events, in general, are classified into two phenomenological categories: (a) prompt events, and (b) delayed events. The properties of both classes of events are summarised. The properties considered are the frequency of occurrence, dependence on parent flare position, the time profile, energy spectra, anisotropies, particle species, velocity dispersions, etc. A single model is presented to explain the various species of delayed event. Thus the halo and core events, energetic storm particle events, EDP events and proton recurrent regions are suggested to be essentially of common origin. The association of flare particle events with electromagnetic phenomena, including optical, X-ray and microwave emissions is summarised. The conditions in a sunspot group, and solar flare that are considered to be conducive to cosmic ray acceleration processes are discussed. Considerable discussion is devoted to physical processes occurring near the Sun. Near Sun particle storage, and diffusion, and secondary injection processes that are triggered by a far distant solar flare are reviewed. In order to explain the considerable differences between aspects of the prompt and delayed events, we propose selective diffusion processes that only occur at early times in a solar flare. The type IV radio emissions at metric wave-lengths are suggested to yield direct evidence for the storage processes that are necessary to explain the properties of the delayed events, and also as yielding direct evidence of secondary injection processes. We conclude by briefly summarising the ionospheric effects of the solar cosmic radiation.     

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.     

Working Group 2 Report: Energy Input, Heating, and Solar Wind Acceleration in Coronal Holes

Theories and observations of energy input, heating and acceleration mechanisms in the low corona were presented and discussed. The main topics of discussion were large-scale solar wind simulations, theoretical heating mechanisms, observational constraints, confronting theory with observations and observational issues. This revised version was published online in June 2006 with corrections to the Cover Date.