Particle Acceleration In Solar Flares Research Articles

Transient Particle Acceleration Associated with Solar Flares

Understanding how individual charged particles can be accelerated to extreme energies (10(20) electron volts), remains a foremost problem in astrophysics. Within our solar system, the active sun is capable of producing, on a short time scale, ions with energies higher than 25 gigaelectron volts. Satellite and ground-based observation over the past 30 years have greatly increased our knowledge of the properties of transient bursts of energetic particles emitted from the sun in association with solar flares, but a real understanding of the solar flare particle acceleration process requires greatly refined experimental data. On the practical side, it is also imperative that this problem be solved if man is to venture, for long periods of time, beyond the protective umbrella of Earth's magnetic field, which excludes much of the biologically damaging solar energetic particles. It is only through an understanding of the basic acceleration problem that we can expect to be able to predict the occurrence of a solar flare with lethal solar radiations. For our knowledge of these effects to advance, a new space mission dedicated to studying the high-energy aspects of solar flares at high spatial and energy resolution will be required.

Particle acceleration in solar flares

Particle acceleration in solar flares was the topic of a workshop held at the University of Tasmania in Hobart, Australia, January 24–26, 1990. The workshop was held in conjunction with the 21st International Cosmic Ray Conference in Adelaide, South Australia, January 6–19, 1990. Forty‐six scientists participated in the workshop, which was organized by Hilary Cane and John Humble of the University of Tasmania; Marc Duldig, Australian Antarctic Division, University of Tasmania; Ed Cliver, Geophysics Laboratory, U.S. Air Force Systems Command; and Steve Kahler, Boston College, Mass.

Signature of the solar cosmic-ray event on 1982 June 3

view Abstract Citations (13) References (13) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Signature of the Solar Cosmic-Ray Event on 1982 June 3 Debrunner, H. ; Flueckiger, E. O. ; Lockwood, J. A. Abstract Observations of the solar H-alpha, radio, and hard X-ray radiation and measurements made with the gamma-ray spectrometer on the Solar Maximum Mission satellite, the Goddard cosmic-ray detector on Helios 1, and the Jungfraujoch neutron monitor during the solar cosmic-ray event on June 3, 1982 were used to determine the onset times of the various electromagnetic emissions, the injection of the flare protons into the interplanetary magnetic field, and the production of solar neutrons. The results of this analysis show that the time to accelerate the first protons up to energies of a few GeV was about 3.5 minutes longer than the time needed to accelerate the first 20 MeV protons. The accelerations process of the high-energy protons in this event is consistent with the conventional picture of solar flare particle acceleration where energy is dissipated from the flare region, heating the coronal plasma and producing a strong shock wave. The shock, as it then moves through the corona, accelerates the higher energy protons. Publication: The Astrophysical Journal Supplement Series Pub Date: June 1990 DOI: 10.1086/191458 Bibcode: 1990ApJS...73..259D Keywords: Particle Acceleration; Solar Cosmic Rays; Solar Radiation; H Alpha Line; Shock Waves; Solar Flares; Solar Magnetic Field; Solar Maximum Mission; Solar Radio Emission; Solar X-Rays; Solar Physics; COSMIC RAYS: GENERAL; PARTICLE ACCELERATION; SUN: FLARES; SUN: PARTICLE EMISSION; SUN: RADIO RADIATION; SUN: X-RAYS full text sources ADS |

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.

Soft X-ray emissions, meter-wavelength radio bursts, and particle acceleration in solar flares

A detailed study of the relationship between metric radio bursts and soft X-ray flares has been made using an extensive data set covering 15 yr. It is found that type IV emission is mainly associated with long-duration 1-8 A events that are known to be well associated with coronal mass ejections. In contrast, type II and type III bursts originate primarily in impulsive soft X-ray events that are not necessarily accompanied by mass ejection. Strong type III bursts, in particular, appear to occur only in association with relatively impulsive flares. It is suggested that coronal shocks responsible for type II bursts are blast waves generated in impulsive energy releases.

Nuclear processes and accelerated particles in solar flares

A detailed review of nuclear processes and particle acceleration in solar flares has been completed recently (Ramaty and Murphy, 1987). Included in this review were a comprehensive discussion of the theory of gamma-ray and neutron production, as well as the results of comparisons of calculations with gamma-ray, neutron and charged-particle observations of solar flares. The implications of these comparisons on particle energy spectra, total numbers, anisotropies and electron-to-proton ratios, as well as on acceleration mechanisms and the interaction site were also discussed. In addition, elemental and isotopic abundances of the ambient gas, derived from gamma-ray observations, were compared to abundances obtained from observations of escaping accelerated particles and other sources. The present paper is a synopsis of this review

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.

High-Energy Particle Production in Solar Flares (SEP, Gamma-Ray and Neutron Emissions)

Electrons and ions, over a wide range of energies, are produced in association with solar flares. Solar energetic particles (SEPs), observed in space and near Earth, consist of electrons and ions that range in energy from 10 keV to ∼ 100 MeV and from 1 MeV to 20 GeV, respectively. SEPs are directly recorded by charged particle detectors, while x-ray, γ-ray, and neutron detectors indicate the properties of the accelerated particles (electrons and ions) which have interacted in the solar atmosphere. A major problem of solar physics is to understand the relationship between these two groups of charged particles; in particular whether they are accelerated by the same mechanism.We review the physics of γ-ray and neutron production in the solar atmosphere and the method by which properties of the primary charged particles produced in the solar flare can be deduced. Recent observations of energetic photons and neutrons in space and at the Earth are used to present a current picture of the properties of impulsively flare accelerated electrons and ions. Some important properties discussed are time scale of production, composition, energy spectra, accelerator geometry. The correlation of the SEP flux with the yield of γ-rays at the Sun is briefly discussed. We review, in some detail, energetic particle production in the large flare on 1982 June 3. Discussed also are gamma ray and neutron observational constraints on particle acceleration mechanisms and a summary of some recent work on solar flare particle acceleration mechanisms.

Particle propagation effects on solar flare X- and ?-ray time profiles

Much of the evidence for second stage particle acceleration in solar flares lies in the temporal variation of solar X- and γ-ray emissions. However, the solar flare X- and γ-ray burst time-intensity profiles are governed not only by the production or acceleration of electrons and protons but by the propagation of these particles in the solar atmosphere. The effects of particle propagation on X-ray and γ-ray time profiles are illustrated and compared through the use of three models with the result that a variety of particle propagation schemes reproduce effects commonly associated with second stage acceleration. The first model is that of a closed uniform density trap. The other two models employ particle diffusion from a trap to denser regions of the solar atmosphere to produce the high energy radiation. These calculations show that delayed peaking of the photon flux with respect to particle production and reduction in the impulsiveness of the high energy emission is to be expected, effects commonly associated with second stage acceleration. Thus, well understood physical processes are capable of producing so-called time delays in the high energy emission independent of any delays produced by additional particle acceleration processes. Diagnostic differences between these models are also discussed.

Investigations of turbulent motions and particle acceleration in solar flares

Investigations of X-ray spectra of solar flares show that intense random (turbulent) motions are present in hot flare plasma. Here we argue that the turbulent motions are of great importance for flare development. They can efficiently enhance flare energy release and accelerate particles to high energies.

Simultaneous acceleration of electrons and ions in solar flares

Two very intense and impulsive nuclear γ-ray and electron bremsstrahlung X-ray emitting solar flares, on 7 and 21 June 1980, were observed with 40 keV and γ-ray emission show that electrons and ions were accelerated within seconds of each other. The near simultaneous peaking, together with the observation of >400-MeV neutrons, show that the solar flare acceleration process can produce the full intensity and spectrum of electrons (>1 MeV) and ions (>102 MeV) in a time scale of <5 s. These results are in direct contradiction to the widely accepted concept of solar flare particle acceleration.

High energy particle acceleration in solar flares ? observational evidence

High energy particle acceleration in solar flares ? observational evidence

Observations with the SMM gamma-ray spectrometer

The Gamma Ray Spectrometer on the SMM satellite has observed solar cosmic energetic photon transients since 17 February 1980. Using the data available through 1981, new results have been obtained on ion acceleration phenomena in solar flares. It now is evident that both ion and electron acceleration can take place impulsively, simultaneously or within seconds of one another. That the impulsive acceleration process can produce ions with energies as high as GeV/nucleon is directly shown by observations of neutrons at the Earth with energies of several hundred MeV. These two facts and the relative timing of hard X-ray emissions provide new constraints on solar flare particle acceleration theory. New flare spectra have also been observed showing new nuclear γ-ray lines not previously observed from 24Mg, 20Ne and 56Fe as well as from other elements. These spectral observations provide new information on the relative abundances of the accelerated and target nuclei. Following a review of the solar data and implications for flare theories we will also give a brief review of the results obtained on nonsolar γ-ray bursts. Most such bursts have photon spectra extending to MeV energies but with little, if any, evidence for spectral features.

Gamma astronomy of the Sun and study of solar cosmic rays

A detailed discussion is given of the various nuclear reactions proceeding in the Sun's atmosphere under the influence of flare-accelerated particles. The role of such reactions in formation of the line spectrum and continuum of gamma-rays from the disturbed and quiet Sun is discussed. The gamma-ray fluxes in individual lines and in the continuum are estimated. The possibility of applying data on gamma-ray emission from the Sun to analysis of particle acceleration in solar flares and the conditions of their ejection into interplanetary space is analyzed.

Nonlinear processes involved in particle acceleration in solar flares

Requirements for nonrelativistic electron accelaration in solar flares are reviewed. Candidate mechanisms for the primary energy release in flares, steady-state reconnection and the tearing mode instability, are examined to see to what extent they can satisfy these requirements. By examining direct electric field acceleration and electron plasma wave acceleration into their nonlinear phases it is concluded that at most 0.1% of the flare energy can be deposited into nonthermal streaming electrons with the primary energy release mechanisms as they have been proposed and analyzed. Implications for hard X-ray emission models and ion acceleration are given.

Particle acceleration in solar flares by cyclotron damping of cascading turbulence

It is pointed out that cyclotron damping of cascading turbulence can be a very efficient acceleration process because the turbulence is generally damped by the suprathermal tail of the particle distribution. Using a simplified approach to the transfer of wave energy down the spectrum, analytic time-dependent solutions are derived which describe the growth of the tail and its effect back on the turbulence. They suggest that with this mechanism, a sizable fraction of all the available energy ends up in a very small minority of the particles at the flare site, in accord with observations. Preferential acceleration of heavy elements is generally expected. The enhancement mechanism is very sensitive; and if He-3 is preaccelerated by even a modest amount, it could be enhanced by many orders of magnitude, as observed in some flares.

Particle Acceleration in Solar Flares

A review of observational data supports the proposal that there are two distinct phases of particle acceleration in solar flares. ‘Phase 1’ is associated with the flash phase and is here interpreted as acceleration during field-line reconnection. ‘Phase 2’ is associated with type II and type IV radio bursts, and is ascribed to stochastic acceleration in the turbulent plasma behind a magnetohydrodynamic shock formed ahead of an ejected plasmoid.

The Differential Energy Spectra of Solar-Flare 'h , 3 He, and 'he

Solar-flare-accelerated H-1, H-2, He-3, and He-4 isotopes are considered, and the importance of nuclear interactions during solar-flare particle acceleration is discussed. The relative abundances ratios of He-3/He-4 are determined and found to be so large as to impose limits on the production of He-3 by nuclear interactions during acceleration processes.

A New Model of Solar Flares.

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A New Model of Solar Flares. Sturrock, P. A. ; Coppi, B. Abstract A review of observational data concerning solar flares, including in particular data concerning homologous flares, leads to a statement of the principal requirements to be met by a flare model. It is submitted that earlier theories do not meet these requirements. A new model of solar flares is proposed in which the pre-flare state consists of a heavy mass of gas partially supported by a compressed magnetic field. Such a configuration is likely to form in those parts of the low chromosphere of a center of activity for which the magnetic field is nearly horizontal and for which the radius of curvature is directed upwards. The pre-flare state may have its origin in gas circulation due either to photo spheric temperature differences at a center of activity or to the nonthermal (acoustic) energy flux, or to a small vertical (upward) displacement of the magnetic field configuration. The instability appropriate to this configuration, to be identified with the flash phase of solar flares, is a gravitational resistive instability. This instability is sufficiently rapid, and it develops electric fields of sufficient magnitude to explain observed particle acceleration in solar flares. The instability also causes the plasma-field configuration to break up into fine filaments, in agreement with observations. This finescale structure effects a decoupling of the plasma and magnetic fields, permitting the overlying mass of gas to sink to the photosphere and the compressed magnetic field to relax to a force-free state; energy released in this phase corresponds to the main part of the light curve of a flare. Publication: The Astronomical Journal Pub Date: 1965 DOI: 10.1086/109651 Bibcode: 1965AJ.....70S.331S full text sources ADS |