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A. Sterling (NASA/MSFC/UAT), Louise Harra (Mullard Space Science Lab.)
``Coronal dimmings,'' localized regions showing a precipitous drop in EUV or X-ray emission, are a key coronal signature of the sources of Coronal Mass Ejections (CMEs). Researchers often assume that dimmings result from a depletion of coronal material (i.e., a decrease in density along the line-of-sight), even when no obvious moving structure can be detected in images. Yet, this explanation has not been unambiguously established; in principle the dimmings could, for example, be due to a dramatic change in the temperature of the emitting material instead. Here we present the most direct evidence to-date that the dimmings result from mass loss, by observing Doppler motions of material leaving the regions as they dim. Using spectral data from the Coronal Diagnostic Spectrometer (CDS) on SOHO, we observe Doppler shifts in two different events. One of these, from 1998 March~31 near 9~UT, was near the solar limb and was associated with a CME traveling in the plane of the sky, while the other event, from 1999 July~19 near 1:50~UT, was on the solar disk and was associated with an Earth-directed ``halo'' CME\@. The limb event shows Doppler signatures of \approx 30 km~s-1 in coronal (Fe~{\sc xvi} and Mg~{\sc ix}) emission lines, where the enhanced velocities coincide with the locations of coronal dimming. An ``EIT wave'' accompanies the disk event, and a dimming region behind the wave shows strong blue-shifted Doppler signatures of \approx 100 km~s-1 in the O~{\sc v} transition region line. These results provide strong evidence that material from the dimming regions feeds into the CMEs.
\noindent This work was supported by NASA's SR&T and GI Programs, and by PPARC.
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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.