The Photosheric Poynting Flux and Coronal Heating

Abstract:

Some models of coronal heating suppose that convective motions at the
photosphere shuffle the footpoints of coronal magnetic fields and
thereby inject sufficient magnetic energy upward to account for
observed coronal and chromospheric energy losses in active regions.
Using high-resolution observations of plage magnetic fields made with
the Solar Optical Telescope aboard the Hinode satellite, we
investigate this idea by estimating the upward transport of magnetic
energy --- the vertical Poynting flux, S_z --- across the photosphere
in a plage region. To do so, we combine: (i) estimates of
photospheric horizontal velocities, v_h, determined by local
correlation tracking applied to a sequence of line-of-sight magnetic
field maps from the Narrowband Filter Imager, with (ii) a vector
magnetic field measurement from the SpectroPolarimeter. Plage fields
are ideal observational targets for estimating energy injection by
convection, because they are: (i) strong enough to be measured with
relatively small uncertainties; (ii) not so strong that convection is
heavily suppressed (as within umbrae); and (iii) unipolar, so S_z in
plage is not influenced by mixed-polarity processes (e.g., flux
emergence) unrelated to heating in stable, active-region fields. In
this plage region, we found that the average S_z varied in space, but
was positive (upward) and sufficient to explain coronal heating, with
values near (5 +/- 1) x 10⁷ erg / cm² / s. We find the energy input
per unit magnetic flux to be on the order of 10⁵ erg / s / Mx. A
comparison of intensity in a Ca II image co-registered with one plage
magnetogram shows stronger spatial correlations with both total field
strength and unsigned vertical field, |B_z|, than either S_z or
horizontal flux density, B_h. The observed Ca II brightness
enhancement, however, probably contains a strong contribution from a
near-photosphere hot-wall effect, which is unrelated to heating in the
solar atmosphere.