Astronomy and Astrophysics – Astrophysics – Solar and Stellar Astrophysics
Scientific paper
2012-01-12
Astronomy and Astrophysics
Astrophysics
Solar and Stellar Astrophysics
35 pages, 10 figures, to be submitted to ApJ
Scientific paper
In solar active regions (ARs), Doppler shifts measured along polarity inversion lines (PILs) of the line-of-sight (LOS) magnetic field determine one component of the velocity perpendicular to the magnetic field. Along PILs, these velocities can be used to: (i) improve estimates of photospheric electric fields, which can, in turn, be used to derive the Poynting flux of magnetic energy across the photosphere; and (ii) constrain the physical processes underlying flux cancellation, the mutual apparent loss of magnetic flux as closely spaced, opposite-polarity magnetogram features approach each other. Unfortunately, at least two factors introduce uncertainties into the zero point of measured Doppler velocities. First, instrumental effects (e.g., thermal variations in instrument components) can cause drifts in calibrations. Second, the convective blueshift, a well-known correlation between intensity and upflows, can bias estimates of the plasma's rest wavelength. Here, we present a method to absolutely calibrate LOS velocities using three successive vector magnetograms and one Dopplergram coincident with the central magnetogram. The method assumes ideal electric fields govern magnetic field evolution along PILs, and enforces consistency between changes in LOS flux near PILs and the transport of transverse magnetic flux by LOS velocities. For a subset of the initial vector magnetograms released by the SDO/HMI Team, we find clear evidence of offsets in the Doppler zero point, of 150 -- 300 m s$^{-1}$, which exhibit variations on the timescale of the SDO orbit. Estimation of the Doppler zero point from modeling the center-to-limb variation of convective blueshifts cannot account for such instrumental biases. Our method could also be used to identify episodes of flux cancellation or flux emergence in which non-ideal electric fields are present, and can characterize those fields.
Fisher George H.
Welsch Brian Thomas
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