The IVM measures the line-of-sight Doppler velocity in the process of computing the magnetic field. This note describes the situation for the IVM_TRIPLET_GRAM code.
The Stokes spectra at each spatial pixel are processed to determine the Doppler shifts relative to some reference position, generally the center of the field of view. The Fabry-Perot etalon causes a quadratic variation of the line position in instrumental units over the field. That quadratic variation is removed at the end, with an amplitude based on the known properties of the etalon and with a centroid based on a fit to the surface.
In the following, the actual measured value is the Doppler shift in nm, but the conversion to velocity is straightforward.
1) The Stokes I profile is fit to determine the wavelength of line center. Call this Vi. This is actually a weighted sum,
If we assume that the intensities Iq and Im of the quiet (unmagnetized) and magnetic areas are equal, then this just becomes
This assumption fails in sunspots, but f ~ 1 there, so that Vi = Vm.
2) The Stokes Q, U, and V profiles are fit for the magnetic parameters, which include the filling factor f and the Difference in wavelength from the Stokes I solution, dVm.
The actual velocity in the magnetic field should be
Solving (2) for Vq gives
Wherever f = 1, Vi = Vm and Vq is undefined. In practice, noise guarantees that f > 0, but may be small.
3) The attached image shows the three velocities for AR9845 on 28-Feb-2002.
All the images are scaled uniformly to +/- 1.2 km/sec = white/black.
Last modified: Wed Oct 20 05:10:11 HST 2004