Magnetic and Velocity Field of Sunspots in the Photosphere and Upper Chromosphere
Creators: name: Joshi, Jayant
Publisher: name: Zenodo
Year: 2014
Subjects: Sun: sunspot
Sun: photosphere
Sun: chromosphere
Polarimetry

description: Sunspots, the dark blemishes on the solar surface have been widely studied for the past 400 years. Sunspots are the most readily identifiable manifestation of magnetic field concentrations on the solar surface. Interaction of the sunspot magnetic field with the plasma makes them one of the most interesting objects for research in solar physics. This thesis presents a study of the photospheric and upper chromospheric velocity and magnetic field structure of sunspots by analyzing spectro-polarimetric observations. These observations comprise different spectral lines obtained with two ground based telescopes and a space borne telescope. The lower brightness of sunspots on the solar surface is due to the presence of strong magnetic fields (up to 4 kG in the umbra), which makes the overturning convection inefficient. Convection is the main heat transport mechanism in the quiet Sun. The Penumbra, the annular part around the umbra has a brightness of about 75% of that in the quiet Sun. At the same time it has an average magnetic field strength of around 1.5 kG. The brightness of penumbrae has been an enigma for solar physicists for a long time. Theoretical models like the gappy penumbra model and the convective roll model as well as magnetohydrodynamic (MHD) simulations suggest that the heat transport in penumbrae is based on the presence of overturning convection. Direct observational evidence for the presence of convective flows in penumbral filaments was missing so far. In Chapter 3 we present observations of a penumbra in the C i 5380 Å spectral line formed in the deep photosphere. These high spatial resolution observations (0. ′′ 14) are obtained with the Swedish 1-m Solar Telescope (SST). Doppler map clearly shows the presence of several dark downflow lanes at the edges of the penumbral filaments which surround the bright upflows at the center of the filaments, supporting overturning convection as a mechanism of heat transport in penumbrae. Chapter 4 analyses the vertical gradient of the magnetic field strength in sunspots in the photosphere. This study includes two different sets of observations: (1) Observations with the Solar Optical Telescope (SOT) onboard the Hinode satellite. (2) Observations with the Tenerife Infrared Polarimeter-2 (TIP-2) mounted at the German Vacuum Tower Telescope (VTT). The spatially coupled inversions (using the SPINOR code) of the Zeeman sensitive Fe i 6301.5 Å and Fe i 6302.5 Å spectral lines observed with the SOT show local patches in the penumbra where the magnetic field decreases with optical depth. Such detection of local magnetic “canopies” have not been reported earlier in the literature. These patches coincide with the bright inner parts of penumbral filaments and indicates that the stronger fields from the dark spines expand and cover i.e. overlie the weaker and<br> more horizontal fields. The radial profile of the vertical gradient obtained with azimuthal averages shows that in the inner penumbra the magnetic field decreases with depth in the lower photosphere whereas in the outer penumbra it has the opposite trend. This result is consistent with low spatial resolution (1. ′′ 0) spectro-polarimetric observations obtained with VTT/TIP-2. These particular observations include the photospheric Si i 10827.1 Å and Ca i 10833.4 Å spectral lines. These observational results are compared with MHD simulations of a sunspot. These comparisons show that the decreasing magnetic field strength with optical depth can be attributed to highly corrugated iso-optical depth surfaces. The 3D structure of a sunspot’s magnetic field is studied in Chapter 5 by comparing the measurements of photospheric and the upper chromospheric magnetic field. Here the photospheric magnetic field is obtained by inversion (using the SPINOR code) of the Si i 10827.1 Å and Ca i 10833.4 Å spectral lines together, whereas inversion of the He i triplet at 10830 Å using the HeLIx + code provided the upper chromospheric magnetic field. These observations were recorded with VTT/TIP-2. The umbral magnetic field strength in the upper chromosphere is found to be lower by a factor 1.4-1.6 compared to the photosphere. The vertical gradient of the magnetic field strength between the pho-<br> tosphere and the upper chromosphere is in a range between 0.5 G km<sup>−1 </sup>and 1.0 G km<sup>-1 </sup>in the umbra. The difference of the magnetic field strength between both atmospheric layers steadily decreases from the sunspot center to the outer boundary of the sunspot. The magnetic canopy at the upper chromospheric heights is found just outside the visible boundary of the sunspot. The most striking result found is that the magnetic field vector is less inclined in the upper chromosphere compared to the photosphere. A higher magnetic field strength is observed in a light-bridge in the upper chromosphere compared to that in the photosphere.
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