Brian T. Welsch, George H. Fisher
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.
View original:
http://arxiv.org/abs/1201.2451
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