No description available.

Thesis (Ph. D.)--University of California, San Diego, 1984. / Vita. Includes bibliographical references (leaves 141-142).

Thesis (Ph. D.)--University of California, San Diego, 1984. / Vita. Includes bibliographical references (leaves 141-142).

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第22253号 / 理博第4567号 / 新制||理||1656(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 柴田 一成, 准教授 浅井 歩, 教授 一本 潔 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

No description available.

One of the earliest indications of magnetic fields acting in the solar atmosphere came at the beginning of the 20th century when George Hale noted a "decided definiteness of structure" in photographs within the Hydrogen Balmer-alpha line core. Fine structure both in the chromosphere and in the corona result from processes that are not well understood but accepted as a consequence of the solar magnetic field. Our knowledge of this field is lacking, and until recently, the assumed relationship between fine thermal structure and the magnetic field remained untested. Here, spectropolarimetric diagnostics of fine structures in the solar chromosphere and cool corona are advanced using the infrared He I triplet at 1083 nm. Precise calibration procedures are developed for the Facility Infrared Spectropolarimeter (FIRS), recently commissioned at the Dunn Solar Telescope. Together with high-order adaptive optics, we simultaneously map fine structures while obtaining a polarimetric sensitivity of up to 2 x 10 ⁻⁴ of the incoming intensity. These instrument improvements result in the first maps of the He I polarized signatures within an active region superpenumbra, where Hale first recognized fine-structuring. Selective absorption and emission processes due to non-equilibrium optical pumping are recognized. Our interpretation, using advanced inversions of the He I triplet, provides confirmation of Hale's initial suspicion--the fine structures of the solar chromosphere are visual markers for the magnetic field. Yet, the fine chromospheric thermal structure is not matched by an equivalently fine magnetic structure. Our ability to measure this field suggests the utility of the He I triplet as an inner boundary condition for the inner heliospheric magnetic field. In the corona itself, we infer the vector properties of a catastrophically-cooled coronal loop, uniting space-based and ground-based instrumentation. We determine how fine loops are anchored in the photosphere via a narrow umbral flare, the consequence of a supersonic downflow of cooled material. A stereoscopic reconstruction as well as full-Stokes inversions of the He I measurements provide the first comparison of the 3D thermal structure and 3D magnetic structure of a fine-scaled coronal loop.

Several techniques exist for retrieving and studying information about the properties of the Solar atmosphere from the polarization state of spectral lines. These are commonly called spectral diagnostics. Among the current problems to which these are applied, one is to understand the interconnection between the solar magnetic field and chromospheric heating. Non-LTE inversion has so far been the most reliable method for inferring chromospheric magnetic fields from high-resolution spectropolarimetric observations. However, if the magnetic field is sufficiently low that the line is in the weak field regime, the weak field approximation is often used as a complement. The latter allows for rapid analysis of large datasets and can be used to infer the vector components of the magnetic field. The reliability of the approximation in highly dynamic and stratified atmospheres has however not been well studied. The purpose of this project is to study and assess the reliability, the validity conditions, and the origin of possible breakdowns of the weak field approximation. This is done by computing the magnetic field of a model chromosphere, performed with realistic three-dimensional magnetohydrodynamics, from synthetic Ca II 8542 polarization profiles. Real magnetic fields are further on inferred from sunspot and plage observations of the same line with the intention to test the method under observational constraints. Stokes I profiles with peculiar shapes are seen in both sunspots and plage, apart from the common quiet profiles. The effect of two such types on the inferred field is studied more closely in this project; raised core profiles, which exhibit a flat core and are common in plage and in the vicinity of bright points, and umbral flashes, which exhibit core emission and are seen in sunspots. The shape of the former is directly connected to presence of steep vertical temperature gradients arising from chromospheric heating, while that of the latter is due to oscillatory motions of the plasma. The weak field approximation works well for observations with high S/N ratio and where quiet profiles are abundant, such as in sunspots. It is vulnerable in plage regions where there is an abundance of raised core (RC) or umbral flash (UF) profiles which lead to failed estimations. Profiles with low S/N tend to yield failed estimations as well. This is common where there is plage, whereby clusters of failed inversions tend to arise in such regions. The vertical component and inclination are well determined, while the horizontal component and azimuth are less well determined. The approximation works well in general, and allows for a rapid and efficient inference of the magnetic field vector as long as the wavelength range is chosen wisely.

The chromosphere and transition region of the solar atmosphere provide an interface between the cool photosphere (6000 K) and the hot corona (1 million K). Both layers exhibit dramatic deviations from thermal and hydrostatic equilibrium in the form of intense plasma heating and mass transfer. The exact mechanisms responsible for transporting energy to the upper atmosphere remain unknown, but these must include a variety of energetic processes operating across many spatial and temporal scales. This dissertation comprises three studies of possible mechanisms for plasma heating and energy transport in the solar chromosphere and transition region. The first study establishes the theoretical framework for a collisional, two-stream plasma instability in the quiet-Sun chromosphere similar to the Farley-Buneman instability which actively heats the E-region of Earth's ionosphere. After deriving a linear dispersion relationship and employing a semi-empirical model of the chromosphere along with carefully computed collision frequencies, this analysis shows that the threshold electron drift velocity for triggering the instability is remarkably low near the temperature minimum where convective overshoots could continuously trigger the instability. The second study investigates simultaneous Interface Region Imaging Spectrograph (IRIS) observations of magnetohydrodynamic (MHD) waves in the chromospheres and transition regions of sunspots. By measuring the dominant wave periods, apparent phase velocities, and spatial and temporal separations between appearances of two observationally distinct oscillatory phenomena, the data show that these are consistent with upward-propagating slow magnetoacoustic modes tied to inclined magnetic field lines in the sunspot, providing a conduit for photospheric seismic energy to transfer upward. The third and final study focuses on intense, small-scale (1 arcsec) active region brightenings known as IRIS UV bursts. These exhibit dramatic FUV/NUV emission line splitting and deep absorption features, suggesting that they result from reconnection events embedded deep in the cool lower chromosphere. IRIS FUV spectral observations and Solar Dynamics Obser- vatory/Helioseismic and Magnetic Imager (SDO/HMI) magnetograms of a single evolving active region reveal that bursts prefer to form during the active region's emerging phase. These bursts tend to be spatially coincident with small-scale, photospheric, bipolar regions of upward and downward magnetic flux that dissipate as the active region matures.

In this Licentiate thesis I review the properties of the solar atmosphere and the diagnostic value ofdifferent spectral lines in the visible and ultraviolet (UV) along with the millimeter (mm) continua in theelectromagnetic spectrum of the Sun.While the solar atmosphere has been routinely observed in high-resolution from ground-based opti-cal telescopes such as the Swedish Solar Telescope (SST), and more recently in the UV from space tele-scopes such as the Interface Region Imaging Spectrograph (IRIS), radio observations lag behind despitetheir great usefulness. This is likely to change thanks to the Atacama Large Millimeter Array (ALMA)that only started observing the Sun in 2016 with a few limitations, but the first results are promising.ALMA observations probe the solar chromosphere at different heights by tuning into slightly differentfrequencies at potentially milliarcsecond scales if the full array is able to operate with the longest base-lines. This new spectral window onto the Sun is expected to advance various fields of research suchas wave propagation and oscillations in the chromosphere, thermal structure of filaments/prominences,triggering of flares and microflares, and more generally chromospheric and coronal heating, because themm-intensities can be modelled by simply assuming local-thermodynamic equilibrium.In da Silva Santos et al. (2018) we find that coordinated observations from SST, IRIS and ALMA willpermit us to estimate with greater accuracy the full thermodynamical state of the plasma as a functionof optical depth based on experiments with a snapshot of a three-dimensional magnetohydrodynamicsimulation of the Sun’s atmosphere. Particularly, the mm-continuum improves the accuracy of inferredtemperatures in the chromosphere. Here we expand on the Why and How this can be done. The goal isto better constrain the temperature stratification in the solar atmosphere in order to understand chromo-spheric heating.

Solar plage has been the topic of many studies since its initial description in the mid 19th century, but as of yet it has not been understood to the point where we can reproduce all aspects of these active regions in quasi-realistic numerical models. To a large extent, this is caused by an incomplete understanding of the magnetic structure that drives the activity in these areas. Detailed measurements have been done of the magnetic field configuration of plage in the photosphere since the late 20th century, but only a handful of papers have managed to make any measurements at all in the higher situated chromosphere, despite the fact that the magnetic field vector of plage is important in understanding chromospheric magnetic fields in general, as well as the heating processes of the higher atmosphere. In Pietrow et al. (2020) we add to these measurements by introducing what is to our knowledge the first full Stokes inversion of chromospheric plage, which allowed us to estimate the magnetic field vector at an optical depth of logτ = -3.5. The obtained value is |B| = 440 ± 90 G in the plage with an inclination of 10° ± 16° with respect to the local vertical. Our reported magnetic field strength matches with a recent study by Morosin et al. (2020), but is higher by a factor of two or more compared to previous studies that measured the field using other methods. Additionally we measure an average magnetic field strength of |B| = 300 ± 50 G in a fibrillar region close to the plage. In this thesis we explore the difficulties of measuring this magnetic field vector. Since plage exists in a complex environment, we will begin with a general description of the structure and properties of the solar atmosphere and the layers from which it is composed, as well as review the types of active regions that can be found in the solar atmosphere. Our focus then narrows to the chromosphere, the diagnostic properties of spectral lines that are sensitive to this layer (mainly the \cair line), plage regions, and plage chromospheric magnetic fields. Additionally, we touch upon the theory of radiative transfer and how physical characteristics of the atmosphere can be inferred from polarised light. We also give attention to the observing process with the Swedish 1-m Solar Telescope (SST) and the workings of the reduction pipeline and post-reduction methods as well as the process spectropolarimetric inversions. Finally, once we have understood why and how this project has been done, we summarize our findings and compare them to current literature.