Analysis of RHESSI Flares Using aRadio Astronomical Technique

Schmahl, E. J.; Pernak, Rick; Hurford, G. J.; Lee, J.; Bong, Su-Chan

doi:10.1007/s11207-007-0263-1

Cited by 61 publications

(91 citation statements)

References 9 publications

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“…They instead prefer a magnetic mirroring model (Melrose & Brown 1976) in which the magnetic field converges more quickly at one of the footpoints. This convergence of the magnetic field lines has been found to be consistent with area measurements of HXR footpoints (Schmahl et al 2006) and has been observed in one M-class limb event with RHESSI (Kontar et al 2008).…”

Section: Introductionsupporting

confidence: 86%

The spectral difference between solar flare HXR coronal and footpoint sources due to wave-particle interactions

Hannah

Kontar

2011

A&A

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Aims. We investigate the spatial and spectral evolution of hard X-ray (HXR) emission from flare accelerated electron beams subject to collisional transport and wave-particle interactions in the solar atmosphere. Methods. We numerically follow the propagation of a power-law of accelerated electrons in 1D space and time with the response of the background plasma in the form of Langmuir waves using the quasilinear approximation. Results. We find that the addition of wave-particle interactions to collisional transport for a transient initially injected electron beam flattens the spectrum of the footpoint source. The coronal source is unchanged and so the difference in the spectral indices between the coronal and footpoint sources is Δγ > 2, which is larger than expected from purely collisional transport. A steady-state beam shows little difference between the two cases, as has been previously found, as a transiently injected electron beam is required to produce significant wave growth, especially at higher velocities. With this transiently injected beam the wave-particle interactions dominate in the corona whereas the collisional losses dominate in the chromosphere. The shape of the spectrum is different with increasing electron beam density in the wave-particle interaction case whereas with purely collisional transport only the normalisation is changed. We also find that the starting height of the source electron beam above the photosphere affects the spectral index of the footpoint when Langmuir wave growth is included. This may account for the differing spectral indices found between double footpoints if asymmetrical injection has occurred in the flaring loop.

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Section: Introductionsupporting

confidence: 86%

The spectral difference between solar flare HXR coronal and footpoint sources due to wave-particle interactions

Hannah

Kontar

2011

A&A

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“…4 (left). To better quantify the size of the X-ray source, the visibility forward fitting technique was also used (see Schmahl et al 2007 for the definition of visibilities; and Xu et al 2008 for examples of application). This algorithm allows the size of the source to be computed and also the error on the size once a model has been chosen.…”

Section: Temporal and Spatial Evolution Of The X-ray Emissionsmentioning

confidence: 99%

Hard X-ray emitting energetic electrons and photospheric electric currents

Musset

Vilmer

Bommier

2015

A&A

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Context. The energy released during solar flares is believed to be stored in non-potential magnetic fields associated with electric currents flowing in the corona. While no measurements of coronal electric currents are presently available, maps of photospheric electric currents can now be derived from SDO/HMI observations. Photospheric electric currents have been shown to be the tracers of the coronal electric currents. Particle acceleration can result from electric fields associated with coronal electric currents. We revisit here some aspects of the relationship between particle acceleration in solar flares and electric currents in the active region. Aims. We study the relation between the energetic electron interaction sites in the solar atmosphere, and the magnitudes and changes of vertical electric current densities measured at the photospheric level, during the X2.2 flare on February 15, 2011, in AR NOAA 11158. Methods. X-ray images from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) are overlaid on magnetic field and electric current density maps calculated from the spectropolarimetric measurements of the Helioseismic and Magnetic Imager (HMI) on the Solar Dynamics Observatory (SDO) using the UNNOFIT inversion and Metcalf disambiguation codes. X-ray images are also compared with extreme ultraviolet (EUV) images from the SDO Atmospheric Imaging Assembly (AIA) to complement the flare analysis. Results. Part of the elongated X-ray emissions from both thermal and non-thermal electrons overlay the elongated narrow current ribbons observed at the photospheric level. A new X-ray source at 50−100 keV (produced by non-thermal electrons) is observed in the course of the flare and is cospatial with a region in which new vertical photospheric currents appeared during the same period (an increase of 15%). These observational results are discussed in the context of the scenarios in which magnetic reconnection (and subsequent plasma heating and particle acceleration) occurs at current-carrying layers in the corona.

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“…Dennis & Pernak (2009) and Kontar et al (2010) have pointed out that CLEAN images usually have systematically larger sizes than other algorithms when using the default beam width factor of 1.0. To ensure the best possible determination of the source sizes using CLEAN images, we applied the visibility forward-fitting procedure (Schmahl et al 2007) on the footpoints of each flare, adjusted the CLEAN beam size, and re-calculated the images until the FWHM of the footpoint from both algorithms had the same size. The integration time interval …”

Section: Imaging Spectroscopymentioning

confidence: 99%

“…We emphasize that the inference of imaging spectroscopy parameters relies strongly on the precise knowledge of the X-ray source area, which was only developed fairly recently (e.g. Schmahl et al 2007;Kontar et al 2008;Dennis & Pernak 2009;Kontar et al 2010). In addition, the full Spectral Response Matrix (SRM) of RHESSI detectors (including non-diagonal terms) for imaging spectroscopy became available relatively recently, since 2006 February.…”

Section: Introductionmentioning

confidence: 99%

Implications for electron acceleration and transport from non-thermal electron rates at looptop and footpoint sources in solar flares

Simões¹,

Kontar²

2013

A&A

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The interrelation of hard X-ray (HXR) emitting sources and the underlying physics of electron acceleration and transport presents one of the major questions in high-energy solar flare physics. Spatially resolved observations of solar flares often demonstrate the presence of well-separated sources of bremsstrahlung emission, so-called coronal and footpoint sources. Using spatially resolved X-ray observations by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and recently improved imaging techniques, we investigate in detail the spatially resolved electron distributions in a few well-observed solar flares. The selected flares can be interpreted as having a standard geometry with chromospheric HXR footpoint sources related to thick-target X-ray emission and the coronal sources characterised by a combination of thermal and thin-target bremsstrahlung. Using imaging spectroscopy techniques, we deduce the characteristic electron rates and spectral indices required to explain the coronal and footpoint X-ray sources. We found that, during the impulsive phase, the electron rate at the looptop is several times (a factor of 1.7−8) higher than at the footpoints. The results suggest that a sufficient number of electrons accelerated in the looptop explain the precipitation into the footpoints and imply that electrons accumulate in the looptop. We discuss these results in terms of magnetic trapping, pitch-angle scattering, and injection properties. Our conclusion is that the accelerated electrons must be subject to magnetic trapping and/or pitch-angle scattering, keeping a fraction of the population trapped inside the coronal loops. These findings put strong constraints on the particle transport in the coronal source and provide quantitative limits on deka-keV electron trapping/scattering in the coronal source.

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Analysis of RHESSI Flares Using aRadio Astronomical Technique

Cited by 61 publications

References 9 publications

The spectral difference between solar flare HXR coronal and footpoint sources due to wave-particle interactions

The spectral difference between solar flare HXR coronal and footpoint sources due to wave-particle interactions

Hard X-ray emitting energetic electrons and photospheric electric currents

Implications for electron acceleration and transport from non-thermal electron rates at looptop and footpoint sources in solar flares

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