HR: 1330h
AN: SP13A-07    [Abstracts]
TI: Systematic errors in Dopplergrams of active regions
AU: * Wachter, R
EM: richard@quake.stanford.edu
AF: Stanford University, Hansen Experimental Physics Laboratory Annex A 455 Via Palou, Stanford, CA 94305 United States
AU: Schou, J
EM: schou@quake.stanford.edu
AF: Stanford University, Hansen Experimental Physics Laboratory Annex A 455 Via Palou, Stanford, CA 94305 United States
AU: Kosovichev, A G
EM: sasha@quake.stanford.edu
AF: Stanford University, Hansen Experimental Physics Laboratory Annex A 455 Via Palou, Stanford, CA 94305 United States
AU: Rajaguru, P
EM: rajaguru@quake.stanford.edu
AF: Stanford University, Hansen Experimental Physics Laboratory Annex A 455 Via Palou, Stanford, CA 94305 United States
AB: 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.
DE: 7522 Helioseismology
DE: 7524 Magnetic fields
SC: Solar Physics Division - AAS [SP]
MN: 2005 Joint Assembly