X-Ray Polarization of Solar Flares Measured with Rhessi

Suarez-Garcia, E.; Hajdas, W.; Wigger, C.; Arzner, K.; Güdel, M.; Zehnder, A.; Grigis, P. C.

doi:10.1007/s11207-006-0268-1

Cited by 48 publications

(38 citation statements)

References 27 publications

(45 reference statements)

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“…9, respectively , one observes an increase in the initial energy flux of beam electrons in the two consecutive times by a factor 2 -from 2 × 10 10 to 4 × 10 10 erg cm −2 s −1 , which is confirmed by the light curves Tindo et al (1970Tindo et al ( , 1972a at 15 keV, triangles -to the observations of Tramiel et al (1984) at 16-21 keV, squares -to the observations of Suarez-Garcia et al (2006) at 100−350 keV, and asterisks -to the observations of Boggs et al (2006) at 200−400 keV.…”

Section: Comparison With Observationssupporting

confidence: 63%

“…The bulk of observations also detect a noticeable increase of the HXR polarization and directivity in flares located closer to the limb, although some deviations from this trend have been recently observed by RHESSI (Suarez-Garcia et al 2006). The similar increase in the HXR directivity at 20−150 keV in flares located towards the limb is reported by Kašparová et al (2007).…”

Section: Introductionsupporting

confidence: 54%

“…The polarimetric observations are indicative of the high variability of the HXR polarization degrees in different events: from a few percent (Tramiel et al 1984) up to 40% (Tindo et al 1976;Suarez-Garcia et al 2006), or even above 50% as observed for the flare of 23 July 2002(McConnell et al 2003 or 75% as observed by the CORONAS payload (Zhitnik et al 2006). The bulk of observations also detect a noticeable increase of the HXR polarization and directivity in flares located closer to the limb, although some deviations from this trend have been recently observed by RHESSI (Suarez-Garcia et al 2006).…”

Section: Introductionmentioning

confidence: 93%

“…This pitch angle anisotropy of precipitating particles can be detected by polarimetric observations in flares located between close to the solar center and the limb. These observations are rather scattered since they were carried out over many years in different energy bands: e.g., 15 keV in the early rocket observations by Tindo et al (1970Tindo et al ( , 1972a, 16−21 keV later by Tramiel et al (1984), 100−350 keV in the RHESSI observations Suarez-Garcia et al (2006), and at 200−400 keV in the ACT observations by Boggs et al (2006).…”

Section: Introductionmentioning

confidence: 99%

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Diagnostics of energetic electrons with anisotropic distributions in solar flares

Zharkova

Kuznetsov²,

Siversky

2010

A&A

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Aims. The paper aims are to simulate steady-state distributions of electrons beams precipitating in collisional and Ohmic losses with pitch angle anisotropy into a flaring atmosphere with converging magnetic field and to apply these to the interpretation of HXR photon spectra, directivity and polarization observed for different photon energies and flare positions on the solar disk. Methods. Summary approximation method is applied to a time-dependent Fokker-Planck equation by splitting the temporal derivative equally between the derivatives in depth, energy and pitch angles and finding the solutions in forward and backward directions for each variable. Results. For softer beams, there is a noticeable flattening of the photon spectra at lower energies caused by the self-induced electric field that increases for larger viewing angles. For the models with an electric field, the HXR emission with lower energies (30 keV) becomes directed mainly upwards at upper atmospheric levels owing to the increased number of particles moving upwards, while in deeper layers it again becomes directed downwards. The polarization maximum shifts to higher energies with every precipitation depth approaching 25 keV for the models with pure collisions and 100 keV for the models with return currents. At deeper layers, the polarization decreases because of the isotropization of electrons by collisions. The maximum polarization is observed at the viewing angle of 90 • , becoming shifted to lower angles for softer beams. The integrated polarization and directivity shows a dependence on a magnetic field convergence for harder beams, while for softer beams the directivity is strongly affected by the self-induced electric field changing from a downward motion to an upward one at upper atmospheric depths. Conclusions. The proposed precipitation model for an electron beam with wider pitch angle dispersion of 0.2 taking into account collisional and Ohmic losses allowed us to fit the double power law HXR photon spectra with a spectrum flattening at lower energies observed in the flares of 20 and 23 July 2002. The observed directivity of HXR photons of 20 keV derived for a large number of flares located from the disk center to limb is also reproduced well by the theoretical directivity calculated for an electron beam with a very narrow pitch angle dispersion of 0.02. The simulated polarization of this narrowly-directed electron beam fits up to 90% of all the available polarimetric observations carried out at various locations across the solar disk.

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Section: Comparison With Observationssupporting

confidence: 63%

Section: Introductionsupporting

confidence: 54%

Section: Introductionmentioning

confidence: 93%

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Diagnostics of energetic electrons with anisotropic distributions in solar flares

Zharkova

Kuznetsov²,

Siversky

2010

A&A

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“…Currently, this is the only available diagnostic of downward electron beaming, except for contradictory measurements of polarisation (see e.g. Suarez-Garcia et al 2006;Boggs et al 2006). Numerical modelling (e.g.…”

Section: Introductionmentioning

confidence: 99%

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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References

2012

Electron and Proton Kinetics and Dynamics in Flaring Atmospheres

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X-Ray Polarization of Solar Flares Measured with Rhessi

Cited by 48 publications

References 27 publications

Diagnostics of energetic electrons with anisotropic distributions in solar flares

Diagnostics of energetic electrons with anisotropic distributions in solar flares

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

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