|dc.description.abstracteng||This thesis presents a study of sunspots, which are the most prominent features observed on the solar surface. Their structure is governed by the magnetic field. The umbra and penumbra of sunspots harbor different strengths and inclinations of the magnetic field. In sunspot penumbrae, inclined fields of ∼1–2 kG are traditionally observed, while the strongest magnetic fields are usually measured in the umbra. Inside sunspot umbrae, magnetic fields are vertical and their typical value is ∼3 kG. In addition to the magnetic field, plasma flows are an integral part of penumbral filaments. Flows are ubiquitously observed running along filaments. This highly dynamic phenomenon is known as the Evershed flow. The Evershed flow is typically observed to run mostly parallel to the solar surface radially outwards in sunspot penumbrae. Chapter 1 provides an introduction to the observational aspects of sunspots.
Spectropolarimetric data taken from space by the Japanese Hinode spacecraft and the Solar Dynamic Observatory (SDO) were analyzed. These datasets covering photospheric heights complement each other. Data taken by the Spectro-Polarimeter (SP) part of the Solar Optical Telescope (SOT) onboard Hinode have excellent spectral and spatial resolution. However, they have poor temporal and spatial coverage. In contrast, the Helioseismic and Magnetic Imager onboard SDO takes full-disk datasets every 45 seconds, albeit at lower spatial resolution and much lower spectral resolution. The combination of both types of spectropolarimetric data was explored within this thesis. In addition, the
chromospheric data also taken by SOT were analyzed to look for the response of the layer above the photosphere.
To obtain depth-dependent atmospheric conditions when analyzing the Hinode/SOT-SP data, state-of-the-art inversions of the radiative transfer equations of polarized light were used. These types of inversions account for the smearing of the information carried by each pixel among its surroundings, due to the point-spread function of the SOT. The data
and the physics behind procedures applied in this thesis are described in Chapter 2.
The second part of this work concentrates on the study of Evershed flows. In particular, it focuses on the study of the so-called counter Evershed flow (CEF). CEFs are penumbral flows that run toward the umbra, i.e., in the opposite direction to the (normal) Evershed
Chapter 3 explores the ejection of three CEFs from the sunspot penumbra. Two CEFs were found to move radially outwards with speeds ranging from 65 to 117 m s −1. The third CEF appeared to be dragged by the rotation of a satellite spot. Superstrong fields were found at the umbra-penumbra boundary where the CEFs hit the umbra and they weakened when the decay of the CEFs started and moved away from this boundary. Chromospheric brightenings were found to be associated with the CEFs.
Chapter 4 dives into a simple question: how rare are CEFs? Prior to this work, just a handful of CEFs were found in the literature. We searched for CEFs in 97 sunspot groups appearing on the Sun over a span of 6 years. We followed the sunspots for 9.6 ± 1.4 days on average and found 384 CEFs in total. CEFs appeared in 83.5% of the sunspot groups, with a median value of six CEFs per sunspot group over the observed time. Contrary to previous works, we concluded that CEFs are rather common features, occurring in all sunspot groups, regardless of the magnetic complexity of the group. They are, however, rather short-lived. Hence, CEFs were only observed on average during 5.9% of the mean total duration of all the observations analyzed here, which explains why CEFs have rarely been reported in the past.
The third part of this thesis explores the magnetic field in sunspot groups with special emphasis on superstrong fields. Three independent observations in the last three decades have found strong magnetic fields that are at least twice as high as the typical umbral values (B ≥ 5 kG, where B stands for the magnetic field strength). These reports have found superstrong magnetic fields at the end-points of penumbral filaments, in a bipolar light bridge, and close to the umbra-penumbra boundary. Furthermore, as of now, there have been few statistical analyzes reproducing the search for superstrong magnetic fields in a large sample of sunspots.
Chapter 5 provides a study on superstrong fields found in a bipolar light bridge part of AR 11967. A previous report used a simplistic Milne-Eddington atmospheric model to fit the spectropolarimetric observations. We obtained substantially better fits to the observations when using depth-dependent coupled inversions. Our results corroborated that the bipolar light bridge harbored magnetic fields larger than 5 kG over an area of 32.7 arcsec^2. We also reported the strongest magnetic field so far measured in a light bridge with an amplitude of 8.2 kG.
To examine how often and where superstrong fields appear in sunspots, Chapter 6 describes the largest solar catalog, at present, of the depth-dependent atmospheric conditions of sunspot groups. We named this catalog MODEST. This catalog currently has 869 spatial scans of 84 individual sunspot groups. The scans cover 8.1×10^7 spatial pixels, which is equivalent to ∼89% of the solar surface. MODEST captures all types of solar features observed in the solar photosphere.
In Chapter 7 we apply the MODEST catalog to search for superstrong magnetic fields in bright regions within sunspots. We confirm the appearance of superstrong magnetic fields in 47 bipolar light bridges, along with their highly sheared structure and bi-directional velocity field patterns. In this Chapter, we also confirm the existence of the other two places where strong magnetic fields in sunspots appear and we report a new additional one. These three locations are: (1) near the umbra-penumbra boundary when CEFs reach the umbra. (2) At the ends of penumbral filaments, in association with the strong downflows present there. (3) At the outer penumbra at the boundary of small intruding pores. The amplification of the field occurs where the normal Evershed flow coming from the main spot hits the intruding pore. These four places are located in bright regions within sunspots and are added to the (dark) umbral location where superstrong magnetic fields are traditionally observed (Livingston et al. 2006). The Chapter concludes with a discussion of the possible mechanisms that could lead to such large magnetic fields.
Finally, Chapter 8 summarizes the main results of this work, and Chapter 9 provides perspectives that can be developed in the future.||de