Characteristics and Evolution of the Magnetic Field and Chromospheric Emission in an Active Region Core Observed by Hinode

We describe the characteristics and evolution of the magnetic field and chromospheric emission in an active region core observed by the Solar Optical Telescope (SOT) on Hinode. Consistent with previous studies, we find that the moss is unipolar, the spatial distribution of magnetic flux evolves slowly, and that the magnetic field is only moderately inclined. We also show that the field line inclination and horizontal component are coherent, and that the magnetic field is mostly sheared in the inter-moss regions where the highest magnetic flux variability is seen. Using extrapolations from Spectropolarimeter (SP) magnetograms we show that the magnetic connectivity in the moss is different than in the quiet Sun because most of the magnetic field extends to significant coronal heights. The magnetic flux, field vector, and chromospheric emission in the moss also appear highly dynamic, but actually show only small scale variations in magnitude on time-scales longer than the cooling times for hydrodynamic loops computed from our extrapolations, suggesting highfrequency (continuous) heating events. Some evidence is found for flux (Ca ii intensity) changes on the order of 100–200 G (DN) on time-scales of 20–30 mins that could be taken as indicative of low-frequency heating. We find, however, that only a small fraction (10%) of our simulated loops would be expected to cool on these time-scales, and we do not find clear evidence that the flux changes consistently produce intensity changes in the chromosphere. Using observations from the EUV Imaging Spectrometer (EIS) we also determine that the filling factor in the moss is ∼ 16%, consistent with previous studies and larger than the size of an SOT pixel. The magnetic flux and chromospheric intensity in most individual SOT pixels in the moss vary by less than ∼ 20% and ∼10%, respectively, on loop cooling time-scales. In view of the high energy requirements of the chromosphere, we suggest that these variations could be sufficient for the heating of ‘warm’ EUV loops, but that the high basal levels may be more important for powering the hot core loops rooted in the moss. The magnetic field and chromospheric emission appear to evolve gradually on spatial scales comparable to the cross-field scale of the fundamental coronal structures inferred from EIS measurements. Subject headings: Sun: activity— Sun: magnetic topology—Sun: photosphere—Sun: chromosphere—Sun: corona