Positions and sizes of X-ray solar flare sources

Abstract

We investigate the positions and source sizes of X-ray solar flare sources taking into account Compton backscattering (albedo). Using a Monte Carlo simulation of X-ray photon transport including photo-electric absorption and Compton scattering, we calculate the apparent source sizes and positions of X-ray sources at the solar disk for various source sizes, spectral indices and directivities of the primary source. We show that the albedo effect will alter the true source positions and substantially increase the measured source sizes. The source positions are shifted up to $\sim 0.5"$ radially towards the disk centre and 5 arcsecond source sizes can be two times larger even for an isotropic source (minimum albedo effect) at 1 Mm above the photosphere. X-ray sources therefore should have minimum observed sizes, thus FWHM source size (2.35 times second-moment) will be as large as $\sim 7"$ in the 20-50 keV range for a disk-centered point source at a height of 1 Mm ($\sim 1.4"$) above the photosphere. The source size and position change is the largest for flatter primary X-ray spectra, stronger downward anisotropy, for sources closer to the solar disk centre, and between the energies of 30 and 50 keV. Albedo should be taken into account when X-ray footpoint positions, footpoint motions or source sizes from e.g. RHESSI or Yohkoh data are interpreted, and suggest that footpoint sources should be larger in X-rays than in optical or EUV ranges.

4 pages, 2 figures, submitted to Astronomy and Astrophysics Letters

References in corpus (7)

• The chemical composition of the Sun
• RHESSI Observation of Chromospheric Evaporation
• Chromospheric magnetic field and density structure measurements using hard X-rays in a flaring coronal loop
• A Statistical Survey of Hard X-ray Spectral Characteristics of Solar Flares with Two Footpoints
• Hard X-ray Spectra and Positions of Solar Flares observed by RHESSI: photospheric albedo, directivity and electron spectra
• Low-energy cutoffs in electron spectra of solar flares: statistical survey
• Exploring the connection between coronal and footpoint sources in a thin-thick target solar flare model

Cited by in corpus (19)

• A Review of Solar Type III Radio Bursts
• Deducing Electron Properties From Hard X-Ray Observations
• Unprecedented Fine Structure of a Solar Flare Revealed by the 1.6~m New Solar Telescope
• Anisotropic Radio-Wave Scattering and the Interpretation of Solar Radio Emission Observations
• The height of a white-light flare and its hard X-ray sources
• Characteristics of the flare acceleration region derived from simultaneous hard X-ray and radio observations
• Observing the formation of flare-driven coronal rain
• On the variation of solar flare coronal x-ray source sizes with energy
• The Low-High-Low Trend of Type III Radio Burst Starting Frequencies and Solar Flare Hard X-rays
• Height structure of X-ray, EUV and white-light emission in a solar flare
• Spatially resolved hard X-ray polarization in solar flares: effects of Compton scattering and bremsstrahlung
• Hard X-ray footpoint sizes and positions as diagnostics of flare accelerated energetic electrons in the low solar atmosphere
• Statistically-derived flaring chromospheric-coronal density structure from non-thermal X-ray observations of the Sun
• The influence of albedo on the size of hard X-ray flare sources
• Measuring X-ray anisotropy in solar flares. Prospective stereoscopic capabilities of STIX and MiSolFA
• Statistical study of spatio-temporal distribution of precursor solar flares associated with major flares
• A Very Narrow RHESSI X-ray Flare on 25 September 2011
• Time variations of observed H$\alpha$ line profiles and precipitation depths of non-thermal electrons in a solar flare
• Spectral component imaging of solar X-ray flares