Analysis of photospheric magnetic fields in AR 12546: a case study

Abstract

We investigate high-resolution observations with the spectropolarimeter (SP) aboard the Hinode satellite of the Solar Optical Telescope (SOT) of a positive polarity sunspot of an active region (AR) (NOAA 12546). We present a case study for the properties of the thermal, magnetic field, and plasma flows as a function of the optical depth from the inversion of the observed Stokes profiles, covering a wide field of view area. Particular attention is paid to the examination of the net circular polarization (NCP) and area asymmetry of spectral lines in sunspots. We detect a large red-shifted velocity of 10 km sec⁻¹ localized with the presence of a strong magnetic field corresponding to the NCP best fit of the inverted profiles. In addition, the comparison between the observed and calculated NCPs or Stokes V area asymmetries of spectral lines fitted well for most pixels in the field of view region, with a significant indication of a single-component inversion. We study the vertical gradients of temperature, magnetic strength, inclination field, and LOS velocity in the deeper and higher layers of the photosphere. The difference in the penumbral temperature between the two atmospheric layers is −130 K. The penumbral regions reveal a magnetic field gradient of \(\Delta B \sim -0.5\) to −0.9 G km⁻¹. The inclination gradient in the limb-side penumbra is between \((\Delta \gamma /\Delta z) = -9 \times 10^{-3}\) deg km⁻¹ to \(-3.5 \times 10^{-2}\) deg km⁻¹. In particular, we find a higher positive inclination gradient in the disk center-side penumbra \((\Delta \gamma /\Delta z)= 0.2 - 1.5 \deg \text{ km}^{-1}\). The velocity gradient is \((\Delta V_{LOS}/\Delta \log \tau )= -0.13\) to −0.30 km sec⁻¹ in the limb-side penumbra, and the disk center-side penumbra is given by \((\Delta V_{LOS}/\Delta \log \tau )= 0.11\) to 0.29 km sec⁻¹.