A MAGNETIC CALIBRATION OF PHOTOSPHERIC DOPPLER VELOCITIES

摘要: The zero point of measured photospheric Doppler shifts is uncertain for at least two reasons: instrumental variations (from, e.g., thermal drifts); and the convective blueshift, a known correlation between intensity upflows. Accurate knowledge is, however, useful (1) improving estimates Poynting flux magnetic energy across photosphere, (2) constraining processes underlying cancellation, mutual apparent loss in closely spaced, opposite-polarity magnetogram features. We present method to absolutely calibrate line-of-sight (LOS) velocities solar active regions (ARs) near disk center using three successive vector magnetograms one Dopplergram coincident with central magnetogram. It exploits fact that along polarity inversion lines (PILs) LOS field determine component velocity perpendicular field, optimizes consistency changes PILs transport transverse by velocities, assuming ideal electric fields govern evolution. Previous calibrations fitted center-to-limb variation but this approach cannot, itself, account residual limb. apply our AR 11158, observed themore » Helioseismic Magnetic Imager aboard Solar Dynamics Observatory, find clear evidence offsets range 50-550 m s{sup -1}. In addition, we note simpler calibration can be determined from an pair median among all near-disk-center PIL pixels. briefly discuss shortcomings initial implementation, suggest ways address these. as step data reduction, use temporal continuity direction correct apparently spurious fluctuations resolution 180 Degree-Sign ambiguity.« less