On the Acceleration Processes in Solar Flares

The paper summarizes what we know about the acceleration processes on the Sun. Four different instabilities are distinguished: (1) One with purely thermal consequences giving rise to the origin of any flare. (2) A non-thermal process at the flash phase of flares giving rise to ∼ 100 keV electrons and protons, manifested through hard X-ray and impulsive microwave bursts (current interruption?). (3) An instability giving rise to streams of electrons, without accelerating protons, manifested by type III bursts (tearing-mode instability?). When (2) and (3) are linked, flare associated electron events in space are often recorded. (4) Finally an explosive instability produces a shock wave which manifests itself as a type II burst. This instability leads to a second-step acceleration of particles preaccelerated in (2) and gives origin to >10 MeV protons and relativistic electrons (probably stochastic acceleration).

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The Flash Phase of Solar Flares: Satellite Observations of Electrons

Symposium - International Astronomical Union ◽

10.1017/s0074180900234244 ◽

1974 ◽

Vol 57 ◽

pp. 201-223

Author(s):

R. P. Lin

Keyword(s):

Solar Flares ◽

Plasma Waves ◽

Quantitative Information ◽

Satellite Observations ◽

Acceleration Process ◽

Relativistic Electrons ◽

Lower Corona ◽

X Ray ◽

Type Iii ◽

Second Stage

Satellite observations of solar electrons bearing on flare particle acceleration and the generation of radio and X-ray emission are reviewed. The observations support a two stage acceleration process for electrons, one stage commonly occurring at the flare flash phase and accelerating electrons up to ~ 100 keV, and a second stage occurring only in large proton flares and accelerating electrons up to relativistic energies. The location of the acceleration region appears to be no lower than the lower corona.The accelerated non-relativistic electrons generate type III radio burst emission as they escape from the Sun. Direct spacecraft observations of the type III emission generated near 1 AU and the energetic electrons, provide quantitative information on the characteristics of the electrons exciting type III emission, the production of plasma waves, and the conversion from plasma waves to electromagnetic radiation.

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First-order Fermi acceleration in solar flares as a mechanism for the second-step acceleration of prompt protons and relativistic electrons

The Astrophysical Journal ◽

10.1086/160881 ◽

1983 ◽

Vol 267 ◽

pp. 433 ◽

Cited By ~ 73

Author(s):

T. Bai ◽

H. S. Hudson ◽

R. M. Pelling ◽

R. P. Lin ◽

R. A. Schwartz ◽

...

Keyword(s):

Solar Flares ◽

Second Step ◽

Relativistic Electrons ◽

Fermi Acceleration ◽

First Order

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Major solar flares without coronal mass ejections

Proceedings of the International Astronomical Union ◽

10.1017/s174392130902941x ◽

2008 ◽

Vol 4 (S257) ◽

pp. 283-286 ◽

Cited By ~ 7

Author(s):

N. Gopalswamy ◽

S. Akiyama ◽

S. Yashiro

Keyword(s):

Open Field ◽

Coronal Mass Ejections ◽

Solar Flares ◽

Active Regions ◽

High Energy ◽

X Ray ◽

Type Iii ◽

High Energy Electrons ◽

Field Lines ◽

Microwave Bursts

AbstractWe examine the source properties of X-class soft X-ray flares that were not associated with coronal mass ejections (CMEs). All the flares were associated with intense microwave bursts implying the production of high energy electrons. However, most (85%) of the flares were not associated with metric type III bursts, even though open field lines existed in all but two of the active regions. The X-class flares seem to be truly confined because there was no material ejection (thermal or nonthermal) away from the flaring region into space.

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Impulsiveness and energetics in solar flares with and without type II radio bursts - A comparison of hard X-ray characteristics for over 2500 solar flares

The Astrophysical Journal ◽

10.1086/167074 ◽

1989 ◽

Vol 336 ◽

pp. 1050 ◽

Cited By ~ 23

Author(s):

Douglas H. Pearson ◽

Robert Nelson ◽

Gabriel Kojoian ◽

James Seal

Keyword(s):

Solar Flares ◽

Radio Bursts ◽

Type Ii ◽

X Ray ◽

Type Ii Radio Bursts

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An Interpretation of the Millisecond Time Variation in Hard X-Ray Solar Flares

Symposium - International Astronomical Union ◽

10.1017/s0074180900075756 ◽

1985 ◽

Vol 107 ◽

pp. 299-301

Author(s):

V. Krishan ◽

M. R. Kundu

Keyword(s):

Growth Rate ◽

Solar Flares ◽

Characteristic Time ◽

Heating Time ◽

Time Variability ◽

Fast Time ◽

Tearing Mode ◽

X Ray ◽

Impulsive Heating ◽

Good Agreement

Recent observations of the fast time variability in the hard X-ray emission from solar flares are interpreted. The fast spikes are assumed to be superimposed on the thermal X-ray emission. The rise and fall of a spike are caused by disruptions in the plasma. The rise time represents the impulsive heating time and the decay or fall time represents a quick cooling of the plasma due to the accelerating growth rate of the m=1 tearing mode. The estimated characteristic time durations of the spike are found to be in good agreement with the observed ones.

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Probing Current Sheet Instabilities from Flare Ribbon Dynamics

The Astrophysical Journal ◽

10.3847/1538-4357/ac256f ◽

2021 ◽

Vol 922 (2) ◽

pp. 117

Author(s):

Ryan J. French ◽

Sarah A. Matthews ◽

I. Jonathan Rae ◽

Andrew W. Smith

Keyword(s):

Solar Flare ◽

Current Sheet ◽

Solar Flares ◽

Spatial Scales ◽

Tearing Mode ◽

Scale Growth ◽

Mode Instability ◽

Field Lines ◽

Flare Ribbon ◽

High Cadence

Abstract The presence of current sheet instabilities, such as the tearing mode instability, are needed to account for the observed rate of energy release in solar flares. Insights into these current sheet dynamics can be revealed by the behavior of flare ribbon substructure, as magnetic reconnection accelerates particles down newly reconnected field lines into the chromosphere to mark the flare footpoints. Behavior in the ribbons can therefore be used to probe processes occurring in the current sheet. In this study, we use high-cadence (1.7 s) IRIS Slit Jaw Imager observations to probe for the growth and evolution of key spatial scales along the flare ribbons—resulting from dynamics across the current sheet of a small solar flare on 2016 December 6. Combining analyses of spatial scale growth with Si iv nonthermal velocities, we piece together a timeline of flare onset for this confined event, and provide evidence of the tearing mode instability triggering a cascade and inverse cascade toward a power spectrum consistent with plasma turbulence.

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Current Sheets in Solar Flares

Symposium - International Astronomical Union ◽

10.1017/s0074180900075677 ◽

1985 ◽

Vol 107 ◽

pp. 233-244

Author(s):

E. R. Priest

Keyword(s):

Numerical Simulations ◽

Magnetic Reconnection ◽

Solar Flares ◽

Tearing Mode ◽

Mode Instability ◽

Current Sheets ◽

Magnetic Arcade

Until recently magnetic reconnection in solar flares was discussed simplistically in terms of either a spontaneous tearing mode instability or a driven Petschek mode. Now the subtle relationship between these two extremes is much better understood. Current sheets may form and reconnection may be initiated in many different ways. There are also a variety of nonlinear pathways from a reconnection instability and several types of driven reconnection.In solar flares current sheets may be important as new flux emerges from below the photosphere and also as a magnetic arcade closes down after being blown open by an eruptive instability. Numerical simulations of these sheets will be described, including new features such as the presence of a fast shock in Petschek's mechanism and impulsive bursty reconnection due to secondary tearing and coalescence.

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