http://arxiv.org/abs/1402.4794
Some models of coronal heating suppose that random (cf., coherent) 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 observationally test 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) that cannot explain steady heating in stable, active-region fields. In this and a previously analyzed plage region, we found that the average S_z varied between the regions, but was positive (upward) and sufficient to explain coronal heating, with values near 2 x 10^7 erg / cm^2 / s. We find the energy input per unit magnetic flux to be on the order of 10^4 erg / s / Mx. A comparison of intensity in a Ca II image co-registered with one plage magnetogram shows stronger spatial correlation with unsigned vertical field, |B_z|, than either S_z or horizontal flux density, B_h.
B. Welsch
Thu, 20 Feb 14
48/52
You must be logged in to post a comment.