The Transparency of Solar Coronal Active Regions

Abstract

Resonance scattering has often been invoked to explain the disagreement between the observed and predicted line ratios of Fe XVII 15.01 A to Fe XVII 15.26 A (the ``3C/3D'' ratio). In this process photons of 15.01, with its much higher oscillator strength, are preferentially scattered out of the line of sight, thus reducing the observed line ratio. Recent laboratory measurements, however, have found significant inner-shell Fe XVI lines at 15.21 and 15.26 Angstroms, suggesting that the observed 3C/3D ratio results from blending. Given our new understanding of the fundamental spectroscopy, we have re-examined the original solar spectra, identifying the Fe XVI 15.21 line and measuring its flux to account for the contribution of Fe XVI to the 15.26 flux. Deblending brings the 3C/3D ratio into good agreement with the experimental ratio; hence, we find no need to invoke resonance scattering. Low opacity in Fe XVII 15.01 also implies low opacity for Fe XV 284.2, ruling out resonance scattering as the cause of the fuzziness of TRACE and SOHO EIT 284-Angstrom images. The images must, instead, be unresolved due to the large number of structures at this temperature. Insignificant resonance scattering implies that future instruments with higher spatial resolution could resolve the active region plasma into its component loop structures.

accepted to Ap J Letters

References in corpus (11)

• Collisional Plasma Models with APEC/APED: Emission Line Diagnostics of Hydrogen-like and Helium-like Ions
• Ionization balance for optically thin plasmas: rate coefficients for all atoms and ions of the elements H to Ni
• Flares from small to large: X-ray spectroscopy of Proxima Centauri with XMM-Newton
• The X-ray spectrum of Fe XVII revisited with a multi-ion model
• Are stellar coronae optically thin in X-rays? - A systematic investigation of opacity effects
• Influence of Resonances on Spectral Formation of X-Ray Lines in Fe XVII
• An Assessment of the Fe XVIII and Fe XIX Line Ratios from the Chandra Grating Observations of Capella
• Detection of X-ray Resonance Scattering in Active Stellar Coronae
• Resonant Scattering of Emission Lines in Coronal Loops: Effects on Image Morphology and Line Ratios
• Some Like it Hot: The X-Ray Emission of The Giant Star YY Mensae
• Flare X-ray observations of AB Dor: evidence for Stellar Coronal Opacity

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