Solar Sources of Heliospheric Energetic Electron Events—Shocks or Flares?

Electrons with near-relativistic (E≳30 keV, NrR) and relativistic (E≳0.3 MeV) energies are often observed as discrete events in the inner heliosphere following solar transient activity. Several acceleration mechanisms have been proposed for the production of those electrons. One candidate is acceleration at MHD shocks driven by coronal mass ejections (CMEs) with speeds ≳1000 km s⁻¹. Many NrR electron events are temporally associated only with flares while others are associated with flares as well as with CMEs or with radio type II shock waves. Since CME onsets and associated flares are roughly simultaneous, distinguishing the sources of electron events is a serious challenge. On a phenomenological basis two classes of solar electron events were known several decades ago, but recent observations have presented a more complex picture. We review early and recent observational results to deduce different electron event classes and their viable acceleration mechanisms, defined broadly as shocks versus flares. The NrR and relativistic electrons are treated separately. Topics covered are: solar electron injection delays from flare impulsive phases; comparisons of electron intensities and spectra with flares, CMEs and accompanying solar energetic proton (SEP) events; multiple spacecraft observations; two-phase electron events; coronal flares; shock-associated (SA) events; electron spectral invariance; and solar electron intensity size distributions. This evidence suggests that CME-driven shocks are statistically the dominant acceleration mechanism of relativistic events, but most NrR electron events result from flares. Determining the solar origin of a given NrR or relativistic electron event remains a difficult proposition, and suggestions for future work are given.     

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.     

Energy Supply Processes for Solar Flares and Magnetospheric Substorms

It is shown that solar flares and magnetospheric substorms must primarily be caused by a dynamo process, rather than magnetic reconnection – a spontaneous, explosive annihilation of magnetic energy stored prior to the onset. Magnetic energy in the vicinity of solar flares and in the magnetotail shows often an increase at their onset, not a decrease. It is unfortunate that many observed features of solar flares and substorms have tacitly been ascribed to unproven (3-D) characteristics of the neutral line for a long time. In the future, it is necessary to study carefully their driving process and examine how the driven magnetic field system evolves, leading to solar flares and substorms.     

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.     

A sequence of five proton producing flares in McMath Plage 15266 28 April–7 May 1978 and their interplanetary consequences

McMath Plage 15266, which transited the solar disk during Carrington Rotation 1667, gave rise during its passage to a spectacular sequence of five proton producing flares. Solar circumstances leading up to the formation of the active plage are described. An account is given of the magnetic affiliations and optical characteristics of the flares themselves, and it is suggested that four of these events might be interpreted as two twin phase flares displaying secondary maxima and minima such that the second phase in each case could in some sense be deemed a consequence of phenomena initiated during the first phase. Those particle phenomena associated with the observed activity are reviewed, and it is suggested that the azimuthal propagation of solar cosmic rays in the corona may occur more efficiently for flares at eastern longitudes in which the magnetic axis is aligned in a roughly north to south rather than an east-to-west direction.An invited paper presented at STIP Workshop on Shock Waves in the Solar Corona and Interplanetary Space, 15–19 June, 1980, Smolenice, Czechoslovakia.     

Solar Gamma-Ray Spectroscopy

Interactions of ions accelerated in solar flares produce gamma-ray lines and continuum and neutrons. These emissions contain a rich set of observables that provides information about both the accelerated ions and the environment where the ions are transported and interact. Ion interactions with the various nuclei present in the ambient medium produce gamma-ray lines at unique energies. How abundance information is extracted from the measurements is discussed and results from analyses of a number of solar flares are presented. The analyses indicate that the composition of the ambient gas where the ions interact (typically at chromospheric densities) is different from that of the photosphere and more like the composition of the corona, exhibiting low-FIP elemental enhancements that may vary from flare to flare. Evidence for increased Ne/O and the photospheric ³He abundance is also discussed.     

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.     

Solar flares and particle acceleration

Energy release in solar flares occurs during the impulsive phase, which is a period of a few to about ten minutes, during which energy is injected into the flare region in bursts with durations of various time scales, from a few tens of seconds down to 0.1 s or even shorter. Non-thermal heating is observed during a short period, not longer than a few minutes, in the very first part of the impulsive phase; in average flares, with ambient particle densities not larger than a few times 10¹⁰ cm^–3 it is due to thick-target electron beam injection, causing chromospheric ablation followed by convection. In flares with larger densities the heating is due to thermal fronts (Section 1). The average energy released in chromospheric regions is a few times 10³⁰ erg, and an average number of 10³⁸ electrons with E 15 keV is accelerated. In subsecond pulses these values are about 10³⁵ electrons and about 10²⁷ erg per subsecond pulse. The total energy released in flares is larger than these values (Section 2). Energization occurs gradually, in a series of fast non-explosive flux-thread interactions, on the average at levels about 10⁴ km above the solar photosphere, a region permeated by a large number ( 10) of fluxthreads, each carrying electric currents of 10¹⁰–10¹¹ A. The energy is fed into the flare by differential motions of magnetic fields driven by photospheric-chromospheric movements (Section 3). In contrast to these are the high-energy flares, characterized by the emission of gamma-radiation and/or very high-frequency (millimeter) radiobursts. Observations of such flares, of the flare neutron emission, as well as the observation of ³He-rich interplanetary plasma clouds from flares all point to a common source, identified with shortlived ( 0.1 s) superhot ( 10⁸ K) flare knots, situated in chromospheric levels (Section 4). Pre-flare phenomena and the existence of homologous flares prove that flare energization can occur repeatedly in the same part of an active region: the consequent conclusions are that only seldom the full energy of an active region is exhausted in one flare, or that the flare energy is generated anew between homologous flares; this latter case looks more probable (Section 5). Flare energization requires the formation of direct electric fields, in value comparable with, or somewhat smaller than the Dreicer field (Section 6). Such fields originate by current-thread reconnection in a regime in which the current sheet is thin enough to let resistive instability originate (Section 7). Particle acceleration occurs by fast reconnection in magnetic fields 100 G and electric fields exceeding about 0.3 times the Dreicer field at fairly low particle densities ( 10¹⁰ cm^–3); for larger densities plasma heating is expected to occur (Section 8). Transport of accelerated particles towards interplanetary space demands a field-line configuration open to space. Such a configuration originates mainly after the gradual gamma-ray/proton flares, and particularly after two-ribbon flares; these flares belong to the dynamic flares in Sturrock and vestka's flare classification. Acceleration to GeV energies occurs subsequently in shock waves, probably by first-order Fermi acceleration (Section 9).     

Hydrodynamic shock wave formation in the solar chromosphere and corona during flares

Basic mechanisms of the hydrodynamic shock wave formation in the solar atmosphere during flares are considered. Hydrodynamic plasma flows during flares arise due to fast energy release which is accumulated in the magnetic field of currents in the solar atmosphere. Shock waves arise as a result of rapid heating of the chromospheric upper layers from accelerated particles or heat fluxes. Powerful hydrodynamic phenomena can also arise due to explosive current sheet disruption in the region of strong magnetic field reconnection. Fundamental questions of shock wave formation and propagation in a non-homogeneous emitting solar atmosphere are discussed.An invited paper presented at STIP Workshop on Shock Waves in the Solar Corona and Interplanetary Space, 15–19 June, 1980, Smolenice, Czechoslovakia.     

Nuclear processes and accelerated particles in solar flares

Nuclear processes and particle acceleration in solar flares are reviewed. The theory of gamma-ray and neutron production is discussed and results of calculations are compared to gamma-ray, neutron, and charged-particle observations from solar flares. The implications of these comparisons on particle energy spectra, total numbers, anisotropies, electron-to-proton ratios, as well as on acceleration mechanisms and the interaction site, are presented. The information on elemental and isotopic abundances derived from gamma-ray observations is compared to abundances obtained from escaping accelerated particles and other sources.NAS/NRC Resident Research Associate.     

Solar flux and its variations

We present data on the solar irradiance as derived from a number of sources. An attempt was made to bring these data onto a uniform scale. The results are presented in Table IV and Figure 6. Summation of fluxes at all wavelengths yields a figure of 1357.826 W m^-2 for the solar constant. Estimates are made of the solar flux variations due to flares, active regions (slowly varying component), solar rotation and the 11-year cycle.Solar activity does not produce a significant variation in the value of the solar constant. Nevertheless, variations in the X-ray and extreme ultraviolet portions of the solar flux may be several orders of magnitude during solar activity, especially at times of major flares. It is well established that these short wavelength flux enhancements cause significant changes in the terrestrial ionosphere.     

Seismic Transients from Flares in Solar Cycle 23

Some solar flares are known to drive seismic waves into the sub-photospheres of the magnetic regions that host them. Sunquakes, which are identified as a wave-packet of ripples are observed on the solar surface emanating from a focal region, known as seismic source or sometimes as a transient. Not all seismic transients from flares generate sunquakes. How these are produced is still a puzzle. In this paper, I will give an overview of the observed properties of sunquakes and efforts to understanding physics underlying them, including numerical modelling of flare-driven oscillations.     

Observational support of reconnection in solar flares

We present a detailed analysis of the magnetic topology of flaring active region. TheH kernels are found to be located at the intersection of the separatrices with the chromosphere when the shear, deduced from the fibrils or/and transverse magnetic field direction, is taken into account. We show that the kernels are magnetically connected by field lines passing close to the separator. We confirm, for other flares, previous studies which show that photospheric current concentrations are located at the borders of flare ribbons. Moreover we found two photospheric current concentrations of opposite sign, linked in the corona by field lines which follow separatrices. These give evidence that magnetic energy is released by reconnection processes in solar flares.     

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 rays

The first observations of solar cosmic rays were made simultaneously by many investigators at worldwide cosmic-ray stations in the periods of powerful chromospheric flares on February 28 and March 7, 1942. The discovery of these and the investigation of cosmic-ray solar-daily variations with maximum time near noon led some authors (Richtmyer and Teller, 1948; Alfvén, 1949, 1950) to a model of apparent cosmic-ray solar origin. We present here the results of the properties of solar cosmic rays from ground events (experimental and theoretical investigations). We also discuss important information from solar experimental data relating to these ground events observed in September and October 1989 and May 1990. Some experimental evidence of acceleration processes in associated phenomena with flares and long-term (solar cycle) variation of the average flux of solar cosmic rays is discussed as also cornal and interplanetary propagation, and that in the terrestrial magnetosphere. Note that the energy spectrum of solar cosmic rays varied very strongly from one flare to another. What are the causes of these phenomena? What is the nature of chemical and isotopic contents of solar cosmic rays? How can its changes occur in the energy spectrum and chemical contents of solar cosmic rays in the process of propagation? Is it possible to recalculate these parameters to the source? What makes solar cosmic rays rich in heavy nucleus and³He? The important data about electrons, positrons, gamma-quanta and neutrons from flares will be discussed in a subsequent paper (Dorman and Venkatesan, 1992). The question is: What main acceleration mechanism of solar flare and associated phenomena are reliable? These problems are connected with the more general problem on solar flare origin and its energetics. In Dorman and Venkatesan (1993) we will consider these problems as well as the problem of prediction of radiation hazard from solar cosmic rays (not only in space, but also in the Earth's atmosphere too).     

Global Properties of Solar Flares

This article broadly reviews our knowledge of solar flares. There is a particular focus on their global properties, as opposed to the microphysics such as that needed for magnetic reconnection or particle acceleration as such. Indeed solar flares will always remain in the domain of remote sensing, so we cannot observe the microscales directly and must understand the basic physics entirely via the global properties plus theoretical inference. The global observables include the general energetics—radiation in flares and mass loss in coronal mass ejections (CMEs)—and the formation of different kinds of ejection and global wave disturbance: the type II radio-burst exciter, the Moreton wave, the EIT “wave”, and the “sunquake” acoustic waves in the solar interior. Flare radiation and CME kinetic energy can have comparable magnitudes, of order 10³² erg each for an X-class event, with the bulk of the radiant energy in the visible-UV continuum. We argue that the impulsive phase of the flare dominates the energetics of all of these manifestations, and also point out that energy and momentum in this phase largely reside in the electromagnetic field, not in the observable plasma.     

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.     

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.     

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.