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C. E. Thornton (Virginia Polytechnic and State University), H. P. Jones (NASA's GSFC)
The Sun's magnetic field controls the structure and activity of the outer solar atmosphere. Although the surface magnetic flux is now frequently observed over the entire visible hemisphere by several instruments, accurate quantitative measurment is still difficult as reflected by comparisons of different instruments which can show relative differences of 50% or more. We extend a previous study (Jones and Ceja, 2001, ASP Conference Series 236, 87)comparing data via histogram equating from three widely used sources: the SOHO Michelson Doppler Imager (MDI), the refurbished Global Oscillations Network Group (GONG+) and the NASA/NSO Spectromagnetograph (SPM). Data from GONG+ and SPM are quite comparable but underestimate flux values in comparison with MDI by 20-40%. Our analysis shows that MDI and SPM magnetograms agree better in later time periods when solar activity is higher, suggesting that strong-field regions are better measured than weak-field ones. This is probably due to the broader extents and higher filling factors for strong-field areas which cause them to be less sensitive to blurring and cancellation by atmospheric seeing in ground-based instruments. That seeing affects instrumental comparisons is reinforced by our examination of contrasts in cotemporal and cospatial intensity images accompanying SPM magnetograms; overall agreement between MDI and SPM data is generally better for higher rms contrasts in the SPM intensities.
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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.