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NOAO Newsletter - National Solar Observatory - March 2000 - Number 61


Coronal Holes Coming to You

Harry Jones

Harry Jones and NSO summer REU student Chad Bender (Illinois/Urbana-Champaign) analyzed imaging spectroscopy in the He I 1083 nm line of a solar coronal hole observed near disk center, and reported first results at the Atlanta AAS meeting in January. Observations were made on 12 January and 26 February 1999 with the NASA/NSO Spectromagnetograph at the NSO/Kitt Peak Vacuum Telescope. Jones and Bender computed images of the equivalent width of the He I absorption feature, along with its wing asymmetry as determined by the difference in wavelength positions between the line bisector at 30% of central line depth and line center. Dupree, Penn, and Jones (ApJ 467, L121, 1996) previously used a similar measure to show that excess blue absorption, indicative of line-of-sight motion towards the observer, is found preferentially in some polar coronal holes where the line is weakest. They suggested, on the basis of the variation of this asymmetry with heliocentric angle over the limited span of their observations, that these motions are vertical and are candidates for source regions of high-speed solar wind. Jones and Bender confirmed that this phenomenon can be observed in coronal holes at disk center with a magnitude (7-9 km/s) and center-to-limb behavior consistent with the results of Dupree et al. Motions parallel to the solar surface cannot explain the observations so that the areas of large blue asymmetry point to solar outflows low in the transition region. Whether these flows continue higher in the atmosphere can, in principal, be clarified by spacecraft EUV spectroscopy, although results to date do not present a clear picture. Preliminary analysis of observations obtained in November, 1999 show similar outflows and were obtained in collaboration with the SUMER EUV spectrometer on the SOHO spacecraft. Detailed comparison with the spacecraft data by Jones and Don Hassler, of the Southwest Research Institute in Boulder, will begin shortly.


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