A Test of a Moment Technique for Vector Field Calculations

Details A Test of a Moment Technique for Vector Field Calculations A Test of a Moment Technique for Vector Field Calculations

Dec 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...181.9404w&link_type=abstract

American Astronomical Society, 181st AAS Meeting, #94.04; Bulletin of the American Astronomical Society, Vol. 24, p.1269

Astronomy and Astrophysics

Astronomy

Scientific paper

Since last year, we have been using a moments technique to calculate the solar vector magnetic field from measurements of the Stokes profiles from the San Fernando Observatory Video Spectra-Spectroheliograph (SFO VSSHG). As part of our evaluation of the accuracy of this technique, I have carried out simulations using analytic profiles. Let S_n be the nth moment of Stokes profile S(lambda ). That is: $ S_n = int lambda (n) S(lambda )dlambda where \lambda = 0 is taken at line center and the integral is done over the line profile. We use the following approximations: {Q_2 / I_0} = {1 / 2} B(2) sin (2gamma cos ) 2 chi {U_2 / I_0} = {1 / 2} B(2) sin (2gamma sin ) 2 chi {V_1 / I_0} = B cos gamma where B is the magnitude of the field, \gamma is its inclination to the line of sight, and \chi is its azimuth angle. These approximations are exact if the Stokes profiles are given by the Seares formulae. We have applied them to profiles generated in the Unno-Rachkovsky solution to the transfer equation, and show that they are a useful approximation for more realistic line profiles as well. The largest systematic errors are in the azimuth angle \chi$, with smaller errors in determination of the longitudinal component of the field and essentially no error in determining the magnitude of the transverse component. This work was supported in part by NASA grant NAGW-2453 and NSF grant ATM-9115111.

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