The Flash Phase of Solar Flares: Satellite Observations of Electrons

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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Nonthermal Processes in Large Solar Flares

Symposium - International Astronomical Union ◽

10.1017/s0074180900071862 ◽

1975 ◽

Vol 68 ◽

pp. 425-426

Author(s):

H. S. Hudson ◽

T. W. Jones ◽

R. P. Lin

Keyword(s):

Shock Wave ◽

Solar Flares ◽

Interplanetary Medium ◽

Interplanetary Shock ◽

Energy Output ◽

Shock Acceleration ◽

List Type ◽

Lower Corona ◽

X Ray ◽

Interplanetary Shock Wave

SummaryIn many small solar flares the ∼10–100 keV electrons accelerated during the flash phase contain the bulk of the total flare energy output. In large flares, such as those in the period 1972, August 2–7, the flash phase electrons are present in substantially greater numbers. These electrons can explosively heat the chromosphere-lower corona and eject flare material. The ejected matter can produce a shock wave which will then accelerate nucleons and electrons to relativistic energies. We analyze energetic particle, radio, X-ray, gamma ray and interplanetary shock observations of the 1972 August flares to obtain quantitative estimates of the energy contained in each facet of these large flares. In general these observations are consistent with the above hypothesis. In particular: (1)From the X-ray emission (van Beek et al., 1973) the energy contained in >25 keV electrons is calculated to be 2 × 1032 erg for the 1972, August 4 event. Since the lower energy cutoff to the electron spectrum is known to be below 25 keV and possibly below 10 keV, the electrons contain enough energy to produce the following interplanetary shock wave, which has by far the bulk of the energy dissipated in the flare. Similar numbers are obtained for the large August 7 flare event.(2)From the γ-ray emission (Chupp et al., 1973) the energy in protons dumped at the same level of the atmosphere, assuming a thick target situation, is at least a factor of three smaller than the electrons. Moreover the γ-ray emission indicates that the bulk of the protons are accelerated at least several minutes after the electrons. Thus it is more likely that the electrons are responsible for the flare optical (Hα and white light) emissions which occur in the chromosphere.(3)Approximately 5% of the electrons and 99% of the protons escape into the interplanetary medium to be observed by spacecraft. This situation is consistent with the hypothesis of shock acceleration of the protons high in the solar corona.(4)The four most intense X-ray bursts observed during the period July 31–August 11 are the only bursts followed by an interplanetary shock wave and a new injection of energetic protons into the interplanetary medium.

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Production of Different Non-Thermal Electron Groups in Small Solar Flares

International Astronomical Union Colloquium ◽

10.1017/s0252921100089983 ◽

1972 ◽

Vol 14 ◽

pp. 822-823

Author(s):

S. R. Kane

Keyword(s):

Electron Emission ◽

Solar Flares ◽

Electron Acceleration ◽

X Rays ◽

Thermal Electron ◽

X Ray ◽

Type Iii

Using the measurements of impulsive solar X-rays made with the OGO-5 satellite to identify the flash phase electron acceleration in solar flares of Hα-importance ≲ 1, the satellite and ground based observations are analyzed to study the origin of the different groups of non-thermal electrons responsible for the impulsive X-ray, impulsive microwave, type III radio and interplanetary electron emission.

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Quiescent Microwave Emission from Late-Type Stars

International Astronomical Union Colloquium ◽

10.1017/s0252921100078052 ◽

1994 ◽

Vol 142 ◽

pp. 743-751

Author(s):

Manuel Güdel

Keyword(s):

Microwave Emission ◽

Radio Continuum ◽

Acceleration Process ◽

Relativistic Electrons ◽

Late Type ◽

Model Calculations ◽

X Ray ◽

Gyrosynchrotron Emission ◽

Crucial Information ◽

High Level

AbstractA diversity of stellar classes has been detected to be prolific sources of low-level, “quiescent” microwave radiation. This emission is, in most cases, attributed to the persistent presence of mildly relativistic electrons in the coronae. Frequent or continuous particle acceleration is required to maintain a high level of gyrosynchrotron emission. In this paper, observations relevant to our understanding of quiescent microwave emission from stars are reviewed, with emphasis on nondegenerate, late-type stars. Though the nature of the acceleration process remains unknown, a large amount of crucial information is presently available, in particular observations of timescales of slow variations, estimates of particle energies, model calculations based on spectral observations, and correlative studies with soft X-ray emission.Subject headings: acceleration of particles — radio continuum: stars — stars: activity — stars: coronae — stars: late-type — stars: magnetic fields

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A study on the relationship of type III radio bursts CME and solar flares during the active period October-November 2003

Proceedings of the International Astronomical Union ◽

10.1017/s1743921309029573 ◽

2008 ◽

Vol 4 (S257) ◽

pp. 361-363

Author(s):

Michaella Thanassa ◽

Eleftheria Mitsakou ◽

Panagiota Preka-Papadema ◽

Xenophon Moussas ◽

Panagiotis Tsitsipis ◽

...

Keyword(s):

Solar Flares ◽

Relativistic Electrons ◽

Active Period ◽

Radio Bursts ◽

Characteristic Type ◽

Type Iii ◽

Intense Activity ◽

Relationship Of ◽

The Relationship

AbstractWithin a period of intense activity (20 October to 5 November 2003), the injection and propagation of near relativistic electrons, resulted in hundreds of type III bursts recorded by the ARTEMISIV radio spectrograph (20–650 MHz). For a number of these type III events association with GOES SXR/Hα flare and/or SOHO/LASCO CME was established. We study the variation of characteristic type III parameters and their relationship with features of the associated flares and/or CMEs.

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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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Impulsive (Flash) Phase of Solar Flares: Hard X-Ray, Microwave, EUV and Optical Observations

Symposium - International Astronomical Union ◽

10.1017/s0074180900234165 ◽

1974 ◽

Vol 57 ◽

pp. 105-141 ◽

Cited By ~ 3

Author(s):

S. R. Kane

Keyword(s):

Solar Flares ◽

Electromagnetic Emission ◽

Impulsive Phase ◽

Point Of View ◽

Optical Observations ◽

Acceleration Process ◽

Short Pulses ◽

X Ray ◽

High Acceleration ◽

Electromagnetic Emissions

Recent observations of impulsive hard X-ray, microwave, EUV and optical emissions during solar flares are briefly reviewed in order to deduce the characteristics of the impulsive (flash) phase phenomenon in small solar flares particularly from the point of view of the acceleration of electrons and their role in producing the various impulsive phase emissions. Observed and deduced characteristics of the various electromagnetic emission sources are summarized (Table II). The deduced characteristics of the electron acceleration process (Table III) indicate a process with high acceleration efficiency. The observations are found to be consistent with a model in which electrons are accelerated in a series of short pulses each lasting for ≲ 1 s and the accelerated electrons provide the energy necessary for all the observed electromagnetic emissions produced during the flash phase of small solar flares. Models of the impulsive phase emissions in which energetic electrons play a prominant role are examined and crucial tests to check the accuracy of these models are indicated (Table IV).

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On the Acceleration Processes in Solar Flares

Symposium - International Astronomical Union ◽

10.1017/s0074180900071874 ◽

1975 ◽

Vol 68 ◽

pp. 427-439

Author(s):

Z. Švestka

Keyword(s):

Solar Flares ◽

Thermal Process ◽

Second Step ◽

Relativistic Electrons ◽

Type Ii ◽

Tearing Mode ◽

Mode Instability ◽

X Ray ◽

Mev Protons ◽

Microwave Bursts

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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X-Ray Emission in Absence of Flares Related to Hα Activity and Type III Burst Production

Symposium - International Astronomical Union ◽

10.1017/s0074180900234189 ◽

1974 ◽

Vol 57 ◽

pp. 147-148 ◽

Cited By ~ 1

Author(s):

S. R. Kane ◽

R. W. Kreplin ◽

M. J. Martres ◽

M. Pick ◽

I. Soru-Escaut

Keyword(s):

Line Center ◽

Acceleration Process ◽

X Ray ◽

Type Iii ◽

Lyot Filter ◽

Rectangular Area ◽

Band Pass ◽

S Period ◽

The University ◽

The Relationship

(Solar Phys.). The relationship between Hα absorption features, type III radio bursts and soft X-ray emission has been examined in order to determine the characteristics of the particle acceleration process operating when a Hα-flare may or may not be detectable. The Hα observations were made by Meudon Observatory with a Hα telescope fitted with a 0.75 Å band pass Lyot filter. During a 10 s period, three pictures were obtained – one at the Hα line center, one at Hα + 0.75 Å and one at Hα −0.75 Å. This sequence of three pictures was repeated every one minute. Each picture covered a rectangular area 18 × 24 mm2, the diameter of the complete solar image being 38 mm on this scale. In addition, Meudon Hα films of the whole solar disc were also used. The X-ray observations were made with the University of California (Berkeley) experiment aboard the OGO-5 satellite and the NRL experiment aboard Solrad-9. The wavelength range covered was 0.5–20 Å. The type III radio data was obtained from two sources: The 169 MHz radio-heliograph at Nancay which provided east–west position of the radio burst on the Sun with an accuracy of ~ 1′ and the radio spectra measured by various ground based observatories. The findings are as follows:Transient Hα activity observed in the absence of reported flares is associated with production of type III radio and soft X-ray emission. Since such optical phenomena are much more frequent than flares themselves, we conclude that instabilities generating fast particles may be produced in the corona in a quasi-continuous way with coincident perturbations in the lower solar atmosphere.The soft X-ray component is not necessarily the direct product of fast particles, but is probably associated with some type of heating since both the soft X-ray emission and the Hα features exhibit a comparable evolution. The type III bursts, when they are produced, occur near the maximum of this perturbation.We identify the transient Hα activity (emission or absorption) with the existence of a metastable situation which may or may not lead to the triggering of a flare.

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