The directivity of high-energy emission from solar flares - Solar Maximum Mission observations

Vestrand, W. T.; Forrest, D. J.; Chupp, E. L.; Rieger, E.; Share, G. H.

doi:10.1086/165796

Cited by 94 publications

(65 citation statements)

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“…It can be attributed most easily to albedo. Given the albedo interpretation of the source, we find that the true source size is likely to be of the order of 7 arcsec for realistic values of the directivity of the order of α = 2, which would be consistent with earlier observational studies of anistropy of albedo (Kane 1974;Vestrand et al 1987;Kašparová et al 2007;. A model involving increasing directivity with energy can be ruled out by the data.…”

Section: Discussionsupporting

confidence: 87%

The influence of albedo on the size of hard X-ray flare sources

Battaglia

Kontar

Hannah

2010

A&A

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Context. Hard X-rays from solar flares are an important diagnostic of particle acceleration and transport in the solar atmosphere. However, any observed X-ray flux from on-disc sources is composed of direct emission plus Compton backscattered photons (albedo). This affects both the observed spectra and images and the physical quantities derived from them, such as the spatial and spectral distributions of accelerated electrons or characteristics of the solar atmosphere (e.g. density). Aims. We propose a new indirect method to measure albedo and to infer the directivity of X-rays in imaging using RHESSI data. We describe this method and demonstrate its application to a compact disc event observed with RHESSI. Methods. Visibility forward fitting is used to determine the size (second moment) of a disc event observed by RHESSI as a function of energy. Using a Monte Carlo simulation code of photon transport in the chromosphere, maps for different degrees of downward directivity and true source sizes are computed. The resulting sizes from the simulated maps are compared with the sizes from the observations to find limits on the true source size and the directivity. Results. The observed full width half maximum of the source varies in size between 7.4 arcsec and 9.1 arcsec with the maximum between 30 and 40 keV. Such behaviour is expected in the presence of albedo and is found in the simulations. The uncertainties in the data are not small enough to make unambiguous statements about the true source size and the directivity simultaneously. However, a source size smaller than 6 arcsec is improbable for modest directivities, and the true source size is likely to be around 7 arcsec for small directivities. Conclusions. While it is difficult to image the albedo patch directly, the effect of backscattered photons on the observed source size can be estimated. This is demonstrated here on observations for the first time. The increase in source size caused by albedo has to be accounted for when computing physical quantities that include the size as a parameter, such as flare energetics. At the same time, studying the albedo signature provides vital information about the directivity of X-rays and related electrons.

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Section: Discussionsupporting

confidence: 87%

The influence of albedo on the size of hard X-ray flare sources

Battaglia

Kontar

Hannah

2010

A&A

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“…The delays in that flare were successfully explained by efficient trapping of the energetic electrons in a magnetic arch. The constant energetic electron spectral index of 3 used in the Vilmer, Trotter, and Kane (1982) model is similar to what we deduce here; however, because the delays we observe are smaller, this would imply a higher ion ambient density region in the case of the 2 November 2003 flare. Another possibility to account for the delays of the higher energy emission is a second-stage acceleration (Frost and Dennis, 1971) of the electrons, where the >1 MeV electrons would be accelerated later from an original seed population, thus giving rise to the ∼20 seconds delay of the 800-keV and 212 -405 GHz emissions.…”

Section: Discussionsupporting

confidence: 81%

“…In contrast we have found that the time profiles of the bremsstrahlung radiation >800 keV and the >200 GHz component are similar and delayed from the microwaves, suggesting that higher energy electrons accelerated with a delay are responsible for these emissions. Similar delays between hard X rays of different energies were observed during the 14 August 1979 flare (Vilmer, Trotter, and Kane, 1982) for example, the 398 -873 keV were delayed by 36 s from the 24 -43 keV emission. The delays in that flare were successfully explained by efficient trapping of the energetic electrons in a magnetic arch.…”

Section: Discussionsupporting

confidence: 65%

Evidence that Synchrotron Emission from Nonthermal Electrons Produces the Increasing Submillimeter Spectral Component in Solar Flares

Silva

Murphy

et al. 2007

Sol Phys

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We investigate the origin of the increasing spectra observed at submillimeter wavelengths detected in the flare on 2 November 2003 starting at 17:17 UT. This flare, classified as an X8.3 and 2B event, was simultaneously detected by RHESSI and the Solar Submillimeter Telescope (SST) at 212 and 405 GHz. Comparison of the time profiles at various wavelengths shows that the submillimeter emission resembles that of the high-energy X rays observed by RHESSI whereas the microwaves observed by the Owens Valley Solar Array (OVSA) resemble that of ∼50 keV X rays. Moreover, the centroid position of the submillimeter radiation is seen to originate within the same flaring loops of the ultraviolet and X-ray sources. Nevertheless, the submillimeter spectra are distinct from the usual microwave spectra, appearing to be a distinct spectral component with peak frequency in the THz range. Three possibilities to explain this increasing radio spectra are discussed: (1) gyrosynchrotron radiation from accelerated electrons, (2) bremsstrahlung from thermal electrons, and (3) gyrosynchrotron emission from the positrons produced by pion or radioactive decay after nuclear interactions. The latter possibility is ruled out on the grounds that to explain the submillimeter observations requires 3000 to 2 × 10 5 more positrons than what is inferred from X-ray and γ -ray observations. It is possible to model the emission as thermal; however, such sources would produce too much flux in the ultraviolet and soft X-ray wavelengths. Nevertheless we are able to explain both spectral components at microwave and submillimeter wavelengths by gyrosynchrotron emission from the same population of accelerated electrons that emit hard X rays and γ rays. We find that the same 5 × 10 35 electrons inferred from RHESSI observations are responsible for the compact submillimeter source (0.5 arcsec in radius) in a region of 4500 G low in the atmosphere, and for the traditional microwave spectral component by a more extended source (50 arcsec) in a 480 G magnetic field located higher up in the loops. The extreme values in magnetic field and source size required to account for the submillimeter emission can be relaxed if anisotropy and transport of the electrons are taken into account.

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“…It can be seen that the observed spectral index at 200-300 keV cannot be less than 5 ( Fig. 1; much softer than typical spectral indices 2-3 (Vestrand et al 1987;Li et al 1994). …”

Section: Photospheric Albedo As a Natural Constraint On Downward Beammentioning

confidence: 79%

“…The first is direct simultaneous measurement of flux spectra from at least two spacecraft at well-separated locations (Li et al 1994;Kane et al 1988). The second method is based on the statistical study of the distribution of HXR fluxes and spectral indices over heliocentric angle (Datlowe et al 1977;Bogovalov et al 1985;Vestrand et al 1987). However, neither of these methods provides direct information about downwardemitted photons, which are crucial in model testing.…”

Section: Introductionmentioning

confidence: 99%

Stereoscopic Electron Spectroscopy of Solar Hard X-Ray Flares with a Single Spacecraft

Kontar

Brown

2006

ApJ

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Hard X-ray (HXR) spectroscopy is the most direct method of diagnosing energetic electrons in solar flares. Here we present a technique that allows us to use a single HXR spectrum to determine an effectively stereoscopic electron energy distribution. Considering the Sun's surface to act as a "Compton mirror" allows us to look at emitting electrons also from behind the source, providing vital information on downward-propagating particles. Using this technique we determine simultaneously the electron spectra of downward-and upward-directed electrons for two solar flares observed by the Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The results reveal surprisingly near-isotropic electron distributions, which contrast strongly with the expectations from the standard model that invokes strong downward beaming, including a collisional thick-target model.

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The directivity of high-energy emission from solar flares - Solar Maximum Mission observations

Cited by 94 publications

References 0 publications

The influence of albedo on the size of hard X-ray flare sources

The influence of albedo on the size of hard X-ray flare sources

Evidence that Synchrotron Emission from Nonthermal Electrons Produces the Increasing Submillimeter Spectral Component in Solar Flares

Stereoscopic Electron Spectroscopy of Solar Hard X-Ray Flares with a Single Spacecraft

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