34th Solar Physics Division Meeting, June 2003
Session 8 Helioseismology
Poster, Monday, June 16, 2003, 3:30-5:00pm, Mezzanine

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[8.03] GONG Magnetogram Zero-Point Correction Status

R. Clark, J. Harvey, F. Hill, C. Toner (NSO)

Line-of-sight component magnetograms from GONG+ are produced every minute at every site. The noise level is about 3 G per pixel but the zero point is incorrect by as much as 10 G. This zero point error varies across the solar image and with time. This error precludes using the magnetograms for meaningful extrapolations of weak photospheric fields into the corona.

Experiments show the cause is slow, asymmetric, locally varying switching of the LCD modulator from one retardation state to the other, generating a false magnetic field pattern (zero error). The mirrors directing sunlight into the instrument produce slight (~1 varying during the day which interacts with modulator imperfections to make a complicated correction problem. Atmospheric variations during the one-minute integration period can also cause trouble.

The zero point error should affect the daily calibration in virtually the same way as a regular magnetogram taken at the same time. The daily calibration is used to create a 'magnetic flat field' to correct a nearly simultaneous regular magnetogram. The result should be a nearly rror-free magnetogram that can then be used to determine the zero point error during the rest of the day by subtracting suitably rotated and masked versions of the data. This error is then fit with a suitable function (e.g. Zernike polynomials) and the coefficients used to synthesize a correction at any time. The coefficients are smoothed and averaged over several days to reduce instrumental and atmospheric noise, and real solar changes that might leak through the masks.

Present performance, determined by comparison among different sites, is good to about 1 G. A limitation appears to be seeing effects causing rapidly changing, small fluctuations of the zero point error. This is being investigated.

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Bulletin of the American Astronomical Society, 35 #3
© 2003. The American Astronomical Soceity.