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.     

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.     

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.     

Energy Deposition in Planetary Atmospheres by Charged Particles and Solar Photons

We discuss here the energy deposition of solar FUV, EUV and X-ray photons, energetic auroral particles, and pickup ions. Photons and the photoelectrons that they produce may interact with thermospheric neutral species producing dissociation, ionization, excitation, and heating. The interaction of X-rays or keV electrons with atmospheric neutrals may produce core-ionized species, which may decay by the production of characteristic X-rays or Auger electrons. Energetic particles may precipitate into the atmosphere, and their collisions with atmospheric particles also produce ionization, excitation, and heating, and auroral emissions. Auroral energetic particles, like photoelectrons, interact with the atmospheric species through discrete collisions that produce ionization, excitation, and heating of the ambient electron population. Auroral particles are, however, not restricted to the sunlit regions. They originate outside the atmosphere and are more energetic than photoelectrons, especially at magnetized planets. The spectroscopic analysis of auroral emissions is discussed here, along with its relevance to precipitating particle diagnostics. Atmospheres can also be modified by the energy deposited by the incident pickup ions with energies of eV’s to MeV’s; these particles may be of solar wind origin, or from a magnetospheric plasma. When the modeling of the energy deposition of the plasma is calculated, the subsequent modeling of the atmospheric processes, such as chemistry, emission, and the fate of hot recoil particles produced is roughly independent of the exciting radiation. However, calculating the spatial distribution of the energy deposition versus depth into the atmosphere produced by an incident plasma is much more complex than is the calculation of the solar excitation profile. Here, the nature of the energy deposition processes by the incident plasma are described as is the fate of the hot recoil particles produced by exothermic chemistry and by knock-on collisions by the incident ions.     

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.     

Protons in flares

This work addresses the role of non-thermal protons as a means of transporting energy in stellar atmospheres. The most dramatic transient visible phenomena are flares, the best studied of which are from the Sun. It is believed that energetic particles take a fundamental part in flare development, but it is controversial as to whether protons or electrons play the dominant role. This review is aimed at helping resolve the controversy. We start by outlining acceleration mechanisms for energetic particles, on the premise that the acceleration site is in the corona. The propagation of a proton beam through the atmosphere is discussed, together with the radiation signatures it would produce. Chromospheric evaporation is expected as the beam reaches the dense part of the atmosphere. Direct observational evidence for energetic protons is reviewed, from gamma-ray production involving energies >30 MeV to H polarization, which is significant at energies 100 keV. Proton beams can be detected in the corona via slowly-drifting type III bursts, while they can be directly sampled by spacecraft and, at energies >1 GeV, by detectors on the Earth. A number of key flare observations and energy arguments are debated from the viewpoint of protons versus electrons. The conclusion is that primary non-thermal protons are much more important, in terms of total energy, than non-thermal electrons in flares, and that the bulk of the energetic electrons are secondary.     

10Be Production in the Atmosphere by Galactic Cosmic Rays

Galactic cosmic ray nuclei and energetic protons produced in solar flares and accelerated by coronal mass ejections are the main sources of high-energy particles of extraterrestrial origin in near-Earth space and inside the Earth’s atmosphere. The intensity of galactic cosmic rays inside the heliosphere is strongly influenced by the modulation of the interstellar source particles on their way through interplanetary space. Among others, this modulation depends on the activity of the Sun, and the resulting intensity of the energetic particles in the atmosphere is an indicator of the solar activity. Therefore, rare isotopes found in historical archives and produced by spallation reactions of primary and secondary hadrons of cosmic origin in the atmosphere, so-called cosmogenic nuclides, can be used to reconstruct the solar activity in the past. The production rate of ¹⁰Be, one of the cosmogenic nuclides most adequate to study the solar activity, is presented showing its variations with geographic latitude and altitude and the dependence on different production cross-sections present in literature. In addition, estimates for altitude integrated production rates of ¹⁰Be at different locations since the early nineteen sixties are shown.     

Imaging diagnostics of solar non-thermal particles

Non-thermal hard X-ray, gamma-ray and radio emission are the most direct signatures of the presence of energetic particles in the solar atmosphere. This paper lays emphasis on hard X-ray and radio imaging data, obtained during and outside flares, which reveal the sites where particles interact with the ambient medium. These observations, which provide more or less direct information on the topology and dynamics of the magnetic structures in which particles are accelerated and propagate, are discussed in the framework of the statistical flare scenario.     

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.     

Solar cosmic radiation during August 1972

Low energy cosmic rays produced by the spectacular series of solar flares in August 1972 are reviewed. Satellite observations of electrons, protons and alpha particles are compared. The proton differential energy spectrum is discussed at certain key times during the events. Three energetic storm particle events were produced over the time period covered by the detailed analysis, 2–11 August. The origin of the cosmic ray square wave on 5 August is discussed. Measurements of heavy ions are available both from Pioneer 10 and from a high latitude rocket flight on 4 August.The literature survey for this review was concluded in August 1975.     

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.     

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.     

The STEREO Mission: An Introduction

The twin STEREO spacecraft were launched on October 26, 2006, at 00:52 UT from Kennedy Space Center aboard a Delta 7925 launch vehicle. After a series of highly eccentric Earth orbits with apogees beyond the moon, each spacecraft used close flybys of the moon to escape into orbits about the Sun near 1 AU. Once in heliospheric orbit, one spacecraft trails Earth while the other leads. As viewed from the Sun, the two spacecraft separate at approximately 44 to 45 degrees per year. The purposes of the STEREO Mission are to understand the causes and mechanisms of coronal mass ejection (CME) initiation and to follow the propagation of CMEs through the inner heliosphere to Earth. Researchers will use STEREO measurements to study the mechanisms and sites of energetic particle acceleration and to develop three-dimensional (3-D) time-dependent models of the magnetic topology, temperature, density and velocity of the solar wind between the Sun and Earth. To accomplish these goals, each STEREO spacecraft is equipped with an almost identical set of optical, radio and in situ particles and fields instruments provided by U.S. and European investigators. The SECCHI suite of instruments includes two white light coronagraphs, an extreme ultraviolet imager and two heliospheric white light imagers which track CMEs out to 1 AU. The IMPACT suite of instruments measures in situ solar wind electrons, energetic electrons, protons and heavier ions. IMPACT also includes a magnetometer to measure the in situ magnetic field strength and direction. The PLASTIC instrument measures the composition of heavy ions in the ambient plasma as well as protons and alpha particles. The S/WAVES instrument uses radio waves to track the location of CME-driven shocks and the 3-D topology of open field lines along which flow particles produced by solar flares. Each of the four instrument packages produce a small real-time stream of selected data for purposes of predicting space weather events at Earth. NOAA forecasters at the Space Environment Center and others will use these data in their space weather forecasting and their resultant products will be widely used throughout the world. In addition to the four instrument teams, there is substantial participation by modeling and theory oriented teams. All STEREO data are freely available through individual Web sites at the four Principal Investigator institutions as well as at the STEREO Science Center located at NASA Goddard Space Flight Center.     

The streaming of solar flare particles and plasma in interplanetary space

A review is given of the features of solar particle emissions which cause various terrestrial disturbances. Three types of corpuscular emissions, namely, solar cosmic rays, energetic storm protons and plasma clouds, are associated with intense solar flares. Outward streaming of the solar wind and of beams of enhanced activity originate from the quiescent solar corona. It is shown that these solar particles propagate through interplanetary space, being modulated in a systematic way by existing magnetic fields. Time variations of solar flare particle flux, and their energy spectrum, are discussed.     

Solar gamma rays

The theory of gamma-ray production in solar flares is treated in detail. Both lines and continuum are produced. The strongest line predicted at 2.225 MeV with a width of less than 100 eV and detected at 2.24±0.02 MeV, is due to neutron capture by protons in the photosphere. Its intensity is dependent on the photospheric ³He abundance. The neutrons are produced in nuclear reactions of flare accelerated particles which also produce positrons and prompt nuclear deexcitation lines. The strongest prompt lines are at 4.43 MeV from ¹²C and at 6.2 from ¹⁶O and ¹⁵N. These lines result from both direct excitation and spallation. The widths of individual prompt lines are determined by nuclear kinematics. The width of the 4.43 MeV line is 100 keV and that of the 6.2 MeV feature is 300 keV. Both these lines have been observed from a solar flare. Other potentially observable lines are predicted at 0.845 and 1.24 MeV from ⁵⁶Fe, at 1.63 MeV principally from ¹⁴N and ²⁰Ne, at 1.78 MeV from ²⁸Si, at 5.3 MeV from ¹⁵O and ¹⁵N, and at 7.12 MeV from ¹⁶O. The widths of the iron lines are only a few keV, while those of the other lines are about 100 keV. The only other observed line is at 0.511 MeV from positron annihilation. The width of this line is determined by the temperature, and its temporal variation depends on the density of the ambient medium in the annihilation region. Positrons can also annihilate from the ³ S state of positronium to produce a 3-photon continuum below 0.511 MeV. In addition, the lines of ⁷Li and ⁷Be at 0.478 keV and 0.431 keV, which have kinematical widths of 30 keV, blend into a strong feature just below the 0.511 MeV line.From the comparison of the observed and calculated intensities of the line at 4.4 MeV to that of the 2.2 MeV line it is possible to obtain information on the spectrum of accelerated nuclei in flares. Moreover, from the absolute intensities of these lines the total number of accelerated nuclei at the Sun and their heating of the flare region can be estimated. We find that about 10³³ protons of energies greater than 30 MeV were produced in the 1972, August 4 flare.The gamma-ray continuum, produced by electron bremsstrahlung, allows the determination of the spectrum and number of accelerated electrons in the MeV region. From the comparison of the line and continuum intensities we find a proton-to-electron ratio of about 10 to 10² at the same energy for the 1972, August 4 flare. For the same flare the protons above 2.5 MeV which are responsible for the gamma-ray emission produce a few percent of the heat generated by the electrons which make the hard X-rays above 20 keV.NAS-NRC Resident Research Associate.Research supported by the National Science Foundation under Grant GP 31620.     

GeV Particle Acceleration in Solar Flares and Ground Level Enhancement (GLE) Events

Ground Level Enhancement (GLE) events represent the most energetic class of solar energetic particle (SEP) events, requiring acceleration processes to boost ?1?GeV ions in order to produce showers of secondary particles in the Earth’s atmosphere with sufficient intensity to be detected by ground-level neutron monitors, above the background of cosmic rays. Although the association of GLE events with both solar flares and coronal mass ejections (CMEs) is undisputed, the question arises about the location of the responsible acceleration site: coronal flare reconnection sites, coronal CME shocks, or interplanetary shocks? To investigate the first possibility we explore the timing of GLE events with respect to hard X-ray production in solar flares, considering the height and magnetic topology of flares, the role of extended acceleration, and particle trapping. We find that 50% (6 out of 12) of recent (non-occulted) GLE events are accelerated during the impulsive flare phase, while the remaining half are accelerated significantly later. It appears that the prompt GLE component, which is observed in virtually all GLE events according to a recent study by Vashenyuk et al. (Astrophys. Space Sci. Trans. 7(4):459–463, 2011), is consistent with a flare origin in the lower corona, while the delayed gradual GLE component can be produced by both, either by extended acceleration and/or trapping in flare sites, or by particles accelerated in coronal and interplanetary shocks.     

Energy Release and Particle Acceleration in Flares: Summary and Future Prospects

RHESSI measurements relevant to the fundamental processes of energy release and particle acceleration in flares are summarized. RHESSI??s precise measurements of hard X-ray continuum spectra enable model-independent deconvolution to obtain the parent electron spectrum. Taking into account the effects of albedo, these show that the low energy cut-off to the electron power-law spectrum is typically ?tens of keV, confirming that the accelerated electrons contain a large fraction of the energy released in flares. RHESSI has detected a high coronal hard X-ray source that is filled with accelerated electrons whose energy density is comparable to the magnetic-field energy density. This suggests an efficient conversion of energy, previously stored in the magnetic field, into the bulk acceleration of electrons. A new, collisionless (Hall) magnetic reconnection process has been identified through theory and simulations, and directly observed in space and in the laboratory; it should occur in the solar corona as well, with a reconnection rate fast enough for the energy release in flares. The reconnection process could result in the formation of multiple elongated magnetic islands, that then collapse to bulk-accelerate the electrons, rapidly enough to produce the observed hard X-ray emissions. RHESSI??s pioneering ??-ray line imaging of energetic ions, revealing footpoints straddling a flare loop arcade, has provided strong evidence that ion acceleration is also related to magnetic reconnection. Flare particle acceleration is shown to have a close relationship to impulsive Solar Energetic Particle (SEP) events observed in the interplanetary medium, and also to both fast coronal mass ejections and gradual SEP events. New instrumentation to provide the high sensitivity and wide dynamic range hard X-ray and ??-ray measurements, plus energetic neutral atom (ENA) imaging of SEPs above ??2 R_??, will enable the next great leap forward in understanding particle acceleration and energy release is large solar eruptions??solar flares and associated fast coronal mass ejections (CMEs).     

The Cusp as a Source of Magnetospheric Energetic Particles, Currents, and Electric Fields: A New Paradigm

Energetic particle instrumentation on the Polar satellite has discovered that significant fluxes of energetic particles are continuously present in the region of the dayside magnetosphere where they cannot be stably trapped. This region is associated with either open magnetic field lines or a magnetic topology associated with pseudo-trapping. Two distinct features [Time-Energy Dispersion (TED) signatures and Cusp Energetic Particle (CEP) events] are observed in these energetic particle fluxes that strongly suggest a local acceleration of mostly shocked solar wind particles. As the solar wind particles ram themselves into the cusp geometry, they form diamagnetic cavities with strong turbulence that are capable of accelerating particles to energies of 100s and 1000s of kiloelectronvolts. This process forms a layer of energetic particles on the magnetopause as well as permits such particles to enter via drift the equatorial nightside magnetosphere to distances as close as six Earth radii under the influence of gradient and curvature effects in the local magnetic field. The fluxes of these particles have all of the properties associated with the ring current and can supply the magnitude of the cross tail current required. ISEE-1 energetic particle data and their pitch angle distributions [PAD] are examined at the magnetic equatorial plane on the night side to investigate and possibly validate the insights gains from the Polar data and energetic particle trajectory tracing in a realistic magnetic field. The existence and properties of butterfly-type PADs strongly supports the concept of a dayside high latitude source of energetic particle fluxes. Because the CEP process is impulsive and time variable the charge separation produced by the drifting electrons (eastward) and ions (westward) on the magnetospheric nightside may be responsible for the cross tail electric field that has been ascribed to the reconnection/convection process.     

Particle acceleration in solar flares and some related phenomena

Observational features concerning solar energetic particles are compactly reviewed with some emphasis on the spectra and time histories. Velocity dependent characteristics in the energy spectra are pointed out, and compared to the results of the interplanetary shocks. A shock drift acceleration is introduced in order to interpret the observational features, especially a very fast acceleration to MeV energies within an order of second. There is a strong evidence of the shock drift acceleration in the interplanetary shocks. When some conditions are satisfied in the corona, only one or several encounters of particles with a near perpendicular shock accelerates protons to gamma-ray emitting energies (> 10 MeV). Pre-acceleration is inevitable for any kind of acceleration mechanisms in solar flares. To fulfill the requirements from the abundance ratios between various species of accelerated ions, pre-acceleration to the same velocities before the injection into a main acceleration process turns out to be absolutely necessary.     

High-Energy Atmospheric Physics: Terrestrial Gamma-Ray Flashes and Related Phenomena

It is now well established that both thunderclouds and lightning routinely emit x-rays and gamma-rays. These emissions appear over wide timescales, ranging from sub-microsecond bursts of x-rays associated with lightning leaders, to sub-millisecond bursts of gamma-rays seen in space called terrestrial gamma-ray flashes, to minute long glows from thunderclouds seen on the ground and in or near the cloud by aircraft and balloons. In particular, terrestrial gamma-ray flashes (TGFs), which are thought to be emitted by thunderclouds, are so bright that they sometimes saturate detectors on spacecraft hundreds of kilometers away. These TGFs also generate energetic secondary electrons and positrons that are detected by spacecraft in the inner magnetosphere. It is generally believed that these x-ray and gamma-ray emissions are generated, via bremsstrahlung, by energetic runaway electrons that are accelerated by electric fields in the atmosphere. In this paper, we review this newly emerging field of High-Energy Atmospheric Physics, including the production of runaway electrons, the production and propagation of energetic radiation, and the effects of both on atmospheric electrodynamics.