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Craig DeForest   Submitted: 2008-08-28 17:18

Using the 2004 Venus transit of the Sun to constrain a semi-empirical point-spread function for the TRACE EUV solar telescope, we have measured the effect of stray light in that telescope. We find that 43% of 171A EUV light that enters TRACE is scattered, either through diffraction off the entrance filter grid or through other nonspecular effects. We carry this result forward, via known-PSF deconvolution of TRACE images, to identify its effect on analysis of TRACE data. Known-PSF deconvolution by this derived PSF greatly reduces the effect of visible haze in the TRACE 171A images, enhances bright features, and reveals that the smooth background component of the corona is considerably less bright (and hence much more rarefied) than commonly supposed. Deconvolution reveals that some prior conlclusions about the Sun appear to have been based on stray light in the images. In particular, the diffuse background ''quiet corona'' becomes consistent with hydrostatic support of the coronal plasma; feature contrast is greatly increased, possibly affecting derived parameters such as the form of the coronal heating function; and essentially all existing differential emission measure studies of small features appear to be affected by contamination from nearby features. We speculate on further implications of stray light for interpretation of EUV images from TRACE and similar instruments, and advocate deconvolution as a standard tool for image analysis with future instruments such as SDO/AIA.

Authors: C.E. DeForest, P.C.H. Martens, M.J. Wills-Davey
Projects: None

Publication Status: Accepted by APJ
Last Modified: 2008-08-28 21:18
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Solar Coronal Structures and Stray Light in TRACE  

Craig DeForest   Submitted: 2008-08-28 17:18

Using the 2004 Venus transit of the Sun to constrain a semi-empirical point-spread function for the TRACE EUV solar telescope, we have measured the effect of stray light in that telescope. We find that 43% of 171A EUV light that enters TRACE is scattered, either through diffraction off the entrance filter grid or through other nonspecular effects. We carry this result forward, via known-PSF deconvolution of TRACE images, to identify its effect on analysis of TRACE data. Known-PSF deconvolution by this derived PSF greatly reduces the effect of visible haze in the TRACE 171A images, enhances bright features, and reveals that the smooth background component of the corona is considerably less bright (and hence much more rarefied) than commonly supposed. Deconvolution reveals that some prior conlclusions about the Sun appear to have been based on stray light in the images. In particular, the diffuse background ''quiet corona'' becomes consistent with hydrostatic support of the coronal plasma; feature contrast is greatly increased, possibly affecting derived parameters such as the form of the coronal heating function; and essentially all existing differential emission measure studies of small features appear to be affected by contamination from nearby features. We speculate on further implications of stray light for interpretation of EUV images from TRACE and similar instruments, and advocate deconvolution as a standard tool for image analysis with future instruments such as SDO/AIA.

Authors: C.E. DeForest, P.C.H. Martens, M.J. Wills-Davey
Projects:

Publication Status: Accepted by APJ
Last Modified: 2008-08-29 12:35
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The Sensitivity of Hybrid Differential Stereoscopy for Spectral Imaging  

Craig DeForest   Submitted: 2007-04-19 09:06

Stereoscopic spectral imaging is an observing technique that affords rapid acquisition of limited spectral information over an entire image plane simultaneously. Light from a telescope is dispersed into multiple spectral orders, which are imaged separately, and two or more of the dispersed images are combined using an analogy between the (x,y,lambda) spectral data space and conventional (x,y,z) three-space. Because no photons are deliberately destroyed during image acquisition, the technique is much more photon-efficient in some observing regimes than existing techniques such as scanned-filtergraph or scanned-slit spectral imaging. Hybrid differential stereoscopy, which uses a combination of conventional cross-correlation stereoscopy and linear approximation theory to extract the central wavelength of a spectral line, has been used to produce solar Stokes-V (line-of-sight) magnetograms in the 617.34 nm Fe I line, and more sophisticated inversion techniques are currently being used to derive Doppler and line separation data from EUV images of the solar corona collected in the neighboring lines of He-II and Si-XI at 30.4 nm. In this paper we develop an analytic a priori treatment of noise in the line shift signal derived from hybrid differential stereoscopy. We use the analysis to estimate the noise level and measurement precision in a high resolution solar magnetograph based on stereoscopic spectral imaging, compare those estimates to a test observation made in 2003, and discuss implications for future instruments.

Authors: DeForest, C.E. and Kankelborg, C.C.
Projects: None

Publication Status: APJ Suppl. Ser. (submitted)
Last Modified: 2007-04-20 01:48
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On the Size of Structures in the Solar Corona  

Craig DeForest   Submitted: 2007-02-22 14:28

Fine-scale structure in the corona appears not to be well resolved by current imaging instruments. Assuming this to be true offers a simple geometric explanation for several current puzzles in coronal physics, including: the apparent uniform cross-section of bright threadlike structures in the corona; the low EUV contrast (long apparent scale height) between the top and bottom of active region loops; and the inconsistency between loop densities derived by spectral and photometric means. Treating coronal loops as a mixture of diffuse background and very dense, unresolved filamentary structures address these problems with a combination of high plasma density within the structures, which greatly increases the emissivity of the structures, and geometric effects that attenuate the apparent brightness of the feature at low altitudes. It also suggests a possible explanation for both the surprisingly high contrast of EUV coronal loops against the coronal background, and the uniform ``typical'' height of the bright portion of the corona (about 0.3 solar radii) in full-disk EUV images. Some ramifications of this picture are discussed, including an estimate (10-100 km) of the fundamental scale of strong heating events in the corona.

Authors: C.E. DeForest
Projects: None

Publication Status: In press for APJ, June 2007
Last Modified: 2007-02-23 10:08
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Fluxon Modeling of Low-Beta Plasmas  

Craig DeForest   Submitted: 2006-10-25 16:57

We have developed a new, quasi-Lagrangian approach for numerical modeling of magnetohydrodynamics in low to moderate eta plasmas such as the solar corona. We introduce the concept of a ``fluxon'', a discretized field line. Fluxon models represent the magnetic field as a skeleton of such discrete field lines, and interpolate field values from the geometry of the skeleton where needed, reversing the usual direction of the field line transform. The fluxon skeleton forms the grid for a collection of 1-D Eulerian models of plasma along individual flux tubes. Fluxon models have no numerical resistivity, because they preserve topology explicitly. Our prototype code, ''FLUX'', is currently able to find 3-D nonlinear force-free field solutions with a specified field topology, and work is ongoing to validate and extend the code to full magnetohydrodynamics. FLUX has significant scaling advantages over conventional models: for ``magnetic carpet'' models, with photospheric line-tied boundary conditions, FLUX simulations scale in complexity like a conventional 2-D grid although the full 3-D field is represented. The code is free software and is available online. In this current paper we introduce fluxons and our prototype code, and describe the course of future work with the code.

Authors: C.E. DeForest, C.C. Kankelborg
Projects: None

Publication Status: Has been in press at JASTP for over 1 year
Last Modified: 2006-10-26 11:13
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Abstracts by Author
Subject will be restored when possible
Solar Coronal Structures and Stray Light in TRACE
The Sensitivity of Hybrid Differential Stereoscopy for Spectral Imaging
On the Size of Structures in the Solar Corona
Fluxon Modeling of Low-Beta Plasmas

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