Physics
Scientific paper
May 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agusmsp13a..07w&link_type=abstract
American Geophysical Union, Spring Meeting 2005, abstract #SP13A-07
Physics
7522 Helioseismology, 7524 Magnetic Fields
Scientific paper
Dopplergrams of magnetic regions are used in time-distance helioseismology and for the investigation of oscillations in sunspots. MDI Dopplergrams are produced onboard SOHO by measuring four filtergrams which are sensitive to intensity fluctuations in the wings of the Ni i absorption line at 6768 Å. The line of sight velocity is inferred from a lookup table which is based on the shape of the line in the quiet sun. In magnetic regions, however, the line shape changes drastically and molecular absorption lines appear close to the line's wavelength. This problem is equally relevant for the upcoming Helioseismic and Magnetic Imager (HMI) which measures velocities based on the same principles as MDI, using however a different absorption line (Fe i at 6173 Å). Based on high spectral resolution images of magnetic regions obtained by the ASP (Advanced Stokes Polarimeter) instrument we show that the MDI Doppler velocities are systematically underestimated in magnetic regions. We discuss possibilities to correct the velocity measurements in magnetic regions using intensity and magnetic field data.
Kosovichev Aleksandr G.
Rajaguru Paul
Schou Jepser
Wachter R.
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