Acceleration In Solar Flares Research Articles

The debate on protons and electrons in solar flares

The series of articles in Eos (September 10, 1996) on particles in solar flares caught my eye. As I am not directly working in, but certainly am interested in, the field of particle acceleration in solar flares, I expected comprehensive, easy to understand summaries of two conflicting points of view I had read about before. The introduction by Peter Cargill set the stage perfectly for such a discussion.Too bad the debate did not fulfill my expectations. It became very clear as I read the articles that [the authors of the debate] were not dealing with the subject on an equal basis. Clearly, the article by George M. Simnett about protons had been written without prior insight into the opposing view. Unfortunately, the case for the electrons by A. Gordon Emslie was basically a reply to the proton case, sometimes specifically citing parts of the article. This would have been appropriate if Simnett had had the chance to reply, but unfortunately no such reply was added to the series.

The Scaling Law between Electron Time-of-Flight Distances and Loop Lengths in Solar Flares

view Abstract Citations (90) References (47) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Scaling Law between Electron Time-of-Flight Distances and Loop Lengths in Solar Flares Aschwanden, Markus J. ; Kosugi, Takeo ; Hudson, Hugh S. ; Wills, Meredith J. ; Schwartz, Richard A. Abstract From the complete data set of solar flares simultaneously observed with the Burst and Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory (CGRO) in the high-time resolution mode (64 ms) and the Hard X-ray Telescope (HXT) on board Yohkoh, we were able to determine the electron time-of-flight (TOF) distance l' and the flare loop geometry in 42 events. The electron TOF distances were determined from time delays (of ≍ 10-100 ms) of hard X-ray (HXR) pulses (measured in 16 channel spectra over ≍ 20-200 keV), produced by the velocity difference of the HXR-producing electrons. The flare loops were mostly identified from double footpoint sources in ≥30 keV HXT images, with radii in the range r = 3000-25,000 km. We find a scaling law between the electron TOF distance l' and the flare loop half-length 5 = r(π/2), having a mean ratio (and standard deviation) of l'/s = 1.4±0.3. In five flares, we observe coronal ≥ 30 keV HXR sources of the Masuda type in the cusp region above the flare loop and find that their heights are consistent with the electron TOF distance to the footpoints. These results provide strong evidence that particle acceleration in solar flares occurs in the cusp region above the flare loop and that the coronal HXR sources discovered by Masuda et al. are a signature of the acceleration site, probably controlled by a magnetic reconnection process. Publication: The Astrophysical Journal Pub Date: October 1996 DOI: 10.1086/177943 Bibcode: 1996ApJ...470.1198A Keywords: ACCELERATION OF PARTICLES; RADIATION MECHANISMS: NONTHERMAL; SUN: CORONA; SUN: FLARES; SUN: X-RAYS; GAMMA RAYS full text sources ADS | data products HEASARC (1)

Sequences of correlated hard X-ray and type III bursts during solar flares

Acceleration and injection of electron beams in solar flares can be traced from radio type III (or type U) bursts and correlated hard X-ray pulses with similar timescales and nonthermal spectra. We perform a systematic survey of such correlated radio and hard X-ray (HXR) pulses with timescales of less than or approximately 2 s in flares simultaneously observed with the radio spectrometer Ikarus and the Hard X-Ray Burst Spectrometer (HXRBS) on solar maximum mission (SMM). We applied an epoch-folding technique to enhance correlated time patterns in burst sequences at the two wavelengths. We present the results from the strongest (10) flares with a HXRBS count rate greater than or = 3000 counts/s, which have a satisfactory signal-to-noise ratio for subsecond pulses. The major findings of this study are presented. These observations strongly suggest that particle acceleration in solar flares occurs in a pulsed mode where electron beams are simultaneously injected in upward and downward directions. Since the sequences of correlated HXR and radio bursts show identical durations and intervals at the two wavelengths, they are believed to reflect most directly the temporal dynamics of the underlying common accelerator. As a consequence, thick-target models should be reconsidered under the aspect of electron injection with pulse durations of 0.2-2.0 s and duty cycles of approximately = 50%.

Properties of solar flare acceleration from multiwavelength observations

We present a brief review of some high-energy solar flares for which multiwavelength observations have revealed new properties of solar flare acceleration mechanisms. The observations encompass measurements of radio, optical, X-ray, γ-ray, and high-energy neutron emissions.

Gamma-ray emission and electron acceleration in solar flares

view Abstract Citations (31) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Gamma-Ray Emission and Electron Accleration in Solar Flares Petrosian, Vahe ; McTiernan, James M. ; Marschhauser, Holger Abstract Recent observations have extended the spectra of the impulsive phase of flares to the GeV range. Such high-energy photons can be produced either by electron bremsstrahlung or by decay of pions produced by accelerated protons. In this paper we investigate the effects of processes which become important at high energies. We examine the effects of synchrotron losses during the transport of electrons as they travel from the acceleration region in the corona to the gamma-ray emission sites deep in the chromosphere and photosphere, and the effects of scattering and absorption of gamma rays on their way from the photosphere to space instruments. These results are compared with the spectra from so-called electron-dominated flares, observed by GRS on the Solar Maximum Mission, which show negligible or no detectable contribution from accelerated protons. The spectra of these flares show a distinct steepening at energies below 100 keV and a rapid falloff at energies above 50 MeV. Following our earlier results based on lower energy gamma-ray flare emission we have modeled these spectra. We show that neither the radiative transfer effects, which are expected to become important at higher energies, nor the transport effects (Coulomb collisions, synchrotron losses, or magnetic field convergence) can explain such sharp spectral deviations from a simple power law. These spectral deviations from a power law are therefore attributed to the acceleration process. In a stochastic acceleration model the low-energy steepening can be attributed to Coulomb collision and the rapid high-energy steepening can result from synchrotron losses during the acceleration process. Publication: The Astrophysical Journal Pub Date: October 1994 DOI: 10.1086/174777 Bibcode: 1994ApJ...434..747P Keywords: Bremsstrahlung; Coulomb Collisions; Electron Acceleration; Gamma Ray Astronomy; Gamma Ray Spectra; Pions; Radiative Transfer; Solar Flares; Solar Protons; Stellar Models; Synchrotron Radiation; Transport Properties; Chromosphere; Gamma Ray Observatory; Photosphere; Solar Corona; Solar Maximum Mission; Solar Physics; ACCELERATION OF PARTICLES; RADIATION MECHANISMS: MISCELLANEOUS; SUN: FLARES; SUN: X-RAYS; GAMMA RAYS full text sources ADS |

Pulsed acceleration in solar flares

We study the nonlinear dynamics of particle acceleration in solar flares by analyzing the time series of various quasi-periodic radio signatures during flares. In particular we present the radio and hard X-ray data of three flares which suppport the following tentative conclusions: (1) Particle acceleration and injection into magnetic structures occurs intrinsically in a pulsed mode (with a typical period of 1-2 s), produced by a single, spatially coherent, nonlinear system, rather than by a stochastic system with many spatially independent components ('statistical flare' produced by a fragmented primary energy release). (2) The nonlinear (quasi-periodic) mode of pulsed particle acceleration and injection into a coronal loop can be stabilized by phase locking with an MHD wave (oscillation) mode, if both periods are close to each other. (3) Pulsed injection of electron beams into a coronal loop may trigger nonlinear relaxational oscillations of wave-particle interactions. This is particularly likely when the limit cycles of both systems are similar.

Microwave burst timescales and solar flare acceleration processes

view Abstract Citations (14) References (30) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Microwave Burst Timescales and Solar Flare Acceleration Processes Krueger, A. ; Kliem, B. ; Hildebrandt, J. ; Zaitsev, V. V. Abstract The occurrence of quite different timescales in solar microwave bursts is considered and possible ways of their physical interpretation are discussed. An interesting feature is the existence of hierarchic time structures, an example of which is provided by the solar event of 1991 March 23. Plasma parameter sets to be invoked for the interpretation of timescales are estimated on the base of different models such as the collisionless conduction front model. The twisted magnetic loop model, the coalescence model, and the electric circuit model. With emphasis on the interpretation of burst fine structures with timescales of a few seconds the coalescence model has been favored. On the other hand, the simultaneous occurrence of a large range of different timescales appears well suited to be described by the circuit model. Publication: The Astrophysical Journal Supplement Series Pub Date: February 1994 DOI: 10.1086/191889 Bibcode: 1994ApJS...90..683K Keywords: Centimeter Waves; Mathematical Models; Millimeter Waves; Particle Acceleration; Solar Flares; Collisionless Plasmas; Field Aligned Currents; Fine Structure; Hierarchies; Magnetic Fields; Time; Solar Physics; ACCELERATION OF PARTICLES; SUN: FLARES; SUN: RADIO RADIATION full text sources ADS |

Pulsed Acceleration in Solar Flares

AbstractWe study the nonlinear dynamics of particle acceleration in solar flares by analyzing the time series of various quasi-periodic radio signatures during flares. In particular we present the radio and hard X-ray data of three flares which support the following tentative conclusions:1.Particle acceleration and injection into magnetic structures occurs intrinsically in a pulsed mode (with a typical period of 1-2 s), produced by a single, spatially coherent, nonlinear system, rather than by a stochastic system with many spatially independent components (“statistical flare” produced by a fragmented primary energy release).2.The nonlinear (quasi-periodic) mode of pulsed particle acceleration and injection into a coronal loop can be stabilized by phase locking with an MHD wave (oscillation) mode, if both periods are close to each other.3.Pulsed injection of electron beams into a coronal loop may trigger nonlinear relaxational oscillations of wave-particle interactions. This is particularly likely when the limit cycles of both systems are similar.Subject headings: acceleration of particles— Sun: corona — Sun: flares — Sun: radio radiation — Sun: X-rays, gamma rays

Collective acceleration in solar flares

Solar flare data are examined with an eye to seeing if they suggest collective acceleration of ions. That, in fact, seems to be the case. The collective acceleration mechanism of Gershtein is reviewed and the possibilities of the mechanism are discussed.

Regular versus chaotic motion of charged particles in non-neutral current sheets

Charged particle motion in reconnecting current sheets (CS) can be both regular and chaotic, depending on the values of transverse (perpendicular to the CS plane) and longitudinal (parallel to the electric field inside the CS) components of the magnetic field. The non-zero transverse field gives rise to chaos, whereas a sufficiently large longitudinal field tends to stabilize the motion. The longitudinal field change in time may be the cause of different regimes of electron acceleration in solar flares and magnetospheric substorms.

Acceleration in solar flares: Interacting particles versus interplanetary particles

We study the properties of flare accelerated particles using gamma ray observations. We derive ion and electron energy spectra, electron to proton ratios, and numbers of interacting particles. We investigate the effects of the abundance variations implied by the gamma ray data on these parameters. We compare the results with interplanetary observations of solar flare particles.

Magnetohydrodynamic turbulence dissipation and stochastic proton acceleration in solar flares

The Alfven and fast magnetosonic wave MHD modes can stochastically accelerate protons from super-Alfvenic to ultrarelativistic energies in solar flares. It is the Landau resonance, however, which generates most of the magnetosonic wave energy being dissipated on electron heating rather than on stochastic proton acceleration. Alfven waves are also subject to a nonlinear wave-particle interaction, and nonlinear Landau damping can selectively and efficiently heat the ambient protons to preaccelerate many to super-Alfvenic speeds. A spectrum of Alfven waves can therefore energize protons from low-temperature thermal to ultrarelativistic energies through a combination of linear and nonlinear particle-wave interactions.

High Energy Particles at and from the Sun

A large amount of the energy released during solar flares is converted into high energy particle acceleration: electrons and ions. These particles are detected either in the interplanetary space or through the radiations that they produce in the solar atmosphere (radio, X-ray and γ-ray emissions). After a brief discussion of the hard X-ray and γ-ray emission processes, we focus on the observational constraints on the accelerated particles: occurrence of ion acceleration in solar flares, time scale for particle acceleration, transport from the acceleration site to the emitting regions. The relative abundances of the accelerated nuclei and in the interaction site deduced from γ-ray spectroscopy are discussed. Finally the relationship between gamma-ray line and solar energetic events will be examined.

Stochastic Fermi acceleration in solar flares

Proton spectra, valid from non- to ultra-relativistic energies, resulting from stochastic Fermi acceleration in solar flares are calculated. These spectra were obtained by numerically solving the Fokker-Planck equation, in which the escape of the particles from the acceleration region is characterized by an energy-independent escape time. In addition to equilibrium spectra, time-dependent energy spectra showing the approach to equilibrium are also presented. These numerical equilibrium spectra are compared with previous results which were obtained either by Monte Carlo simulations or approximate analytical treatments. There are no analytic solutions valid in the transrelativistic regime, which is very important for the production of pions and neutrons in solar flares. The acceleration efficiency is related to physical parameters, in particular the energy density in either magnetosonic or Alfven waves, and a lower limit is placed on either of these energy densities from acceleration times implied by gamma-ray observations. Also discussed is the physical interpretation of the escape time.

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.

Impact line linear polarization as a diagnostic of 100 keV proton acceleration in solar flares

view Abstract Citations (84) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Impact Line Linear Polarization as a Diagnostic of 100 keV Proton Acceleration in Solar Flares Henoux, J. C. ; Chambe, G. ; Smith, D. ; Tamres, D. ; Feautrier, N. ; Rovira, M. ; Sahal-Brechot, S. Abstract The use of impact linear polarization of chromospheric lines as a diagnostic of 100 keV protons is proposed. Observations of hard X-ray emission are examined, noting the possibility that 100 keV proton beams bombarding the atmosphere could create a hot thermal plasma at the origin of X-ray emission. The application of impact linear line polarization as a diagnostic of the existence of energetic particles with anisotropic velocity distribution functions is described. The mechanisms leading to an anisotropic proton distribution in the solar chromosphere are discussed and the required initial energy and energy flux for accelerated protons in the corona are derived. It is suggested that it is possible to infer the original proton anisotropy in the corona and the effect of this anisotropy on the proton energy and proton energy flux required to reach the H-alpha-forming layers. Publication: The Astrophysical Journal Supplement Series Pub Date: June 1990 DOI: 10.1086/191466 Bibcode: 1990ApJS...73..303H Keywords: Linear Polarization; Particle Acceleration; Solar Flares; Solar Protons; Chromosphere; Emission Spectra; Proton Beams; Solar Atmosphere; Solar Spectra; Solar X-Rays; Solar Physics; PARTICLE ACCELERATION; POLARIZATION; SUN: FLARES; SUN: PARTICLE EMISSION; 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.

Isotope abundances of solar coronal material derived from solar energetic particle measurements

Coronal isotopic abundances for the elements He, C, N, O, Ne, and Mg are derived from previously published measurements of the isotopic composition of solar energetic particles by first measuring, and then correcting for, the charge-to-mass-dependent fractionation due to solar flare acceleration and propagation processes. The resulting coronal composition generally agrees with that of other samples of solar system material, but the previously noted difference between the solar flare and solar wind Ne-22/Ne-20 ratios remains unresolved. 18 references.

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.

Solar neutron observations and their relation to solar flare acceleration problems

We review the current observational knowledge on the production of neutrons in association with solar flares. From a study of the observations it is shown that unique information can be obtained on the spectral properties of accelerated ions produced during the flare. Also, the abundance of 3He/H in the photosphere can be directly determined. We also review the current interpretations of all available neutron observations and in particular highlight the uncertainties, and provide guide posts for future experiments.