Helioseismology of sunspots - on the detection of subsurface structure and evolution

Abstract: Sunspots have been a central topic of Solar Physics for more than four decades. Even though much has been discovered concerning their atmospheric properties since, little is known about subsurface structure, flows and magnetic field topology.
Helioseismology is the scientific study of solar waves and offers the sole opportunity within Solar Physics to look into deeper layers of the Sun. Thereby sunspot seismology summarizes all helioseismic studies concerning subsurface properties of sunspots.
Although helioseismic inversions yield insightful and reliable results about both large- and small scale structures of the Sun, they have shown a tendency to fail in the presence of a strong magnetic field.

Finding reliable procedures to reveal subsurface properties of sunspots will contribute to many fields of Solar physics. Almost all aspects of the solar cycle are related to sunspots: Both their total number and average size are related to the cycle strength, their emergence as a function of latitude indicates cycle progression and their polarity is related to the global magnetic field constellation and many more.

The present work aims to derive and inspect new methods applicable in sunspot seismology and their capability of detecting subsurface structures, including emerging active regions (EARs). For this purpose, unexplored observations in sunspot seismology using established methods such as the Fourier-Hankel decomposition (FHD) or the time-distance analysis are investigated. As basis for these studies, both observational data recorded with the Helioseismic and Magnetic Imager (HMI) and artificial data simulated with the Seismic-Propagation-through-Active-Regions-and-Convection-Code (SPARC) are used.

Numerical simulations of waves interacting with magnetic flux tubes reveal a specific type of wave, that re-emerges around the flux tube after undergoing mode conversion within the subsurface magnetic field. It is shown that this wave travels trough most of the interior of the flux tube and is later detectable at the surface, thus representing an interesting object for sunspot seismology. Characterizing this wave using time-distance analysis shows that its group travel time is delayed by $\varDelta t=282.6\,$s compared to the incident wave, thus creating the possibility to discern it from the original (not converted) wave. A follow up analysis shows that the temporal pattern of this wave is sensitive to subsurface properties of the magnetic flux tube model used for the simulation. Finally, the possibility of detecting such a signal in real data is explored and a method of doing so is presented.

Observational studies use HMI data and the FHD to investigate the absorption spectra of sunspots at high frequencies and emerging active regions that have been reported to exhibit f-mode power strengthening up to $2\,$days prior to full emergence. An absorption signature of unknown origin at high frequencies unique to sunspots is reported. Explorations of this signal show that it is likely related to the reduced emissivity inside a sunspot, that arises from magnetic fields inhibiting convection and thus the excitation of acoustic waves. A hypothesis is derived that yields an analytical expression of the emissivity reduction $\gamma$ as a function of absorption $\alpha$. A comparison to earlier (observational and theoretical) studies shows little agreement, although the utility of the analytical expression shown outperforms those of empirical nature in some regards. At any rate, further analysis of the high-frequency absorption signature may reveal other aspects of the sunspot interior that then can be related to the absorption spectrum. Lastly, a phenomenon known as f-mode strengthening of EARs is investigated using the FHD in this work and its observations can be confirmed, although here it appears to be less significant than originally claimed. Interestingly, the strengthening is not accompanied by a decrease (thus an emission) in the respective absorption spectrum, which indicates that the nature of this phenomenon is related to that of the acoustic halo

Object Details

• Location: Deutsche Nationalbibliothek Frankfurt am Main
• Extent: Online-Ressource
• Language: Englisch
• Notes: Universität Freiburg, Dissertation, 2021

Classification and Topics

• Keyword: Sunspots ; Helioseismology ; Evolution ; Sonnenfleck ; Sonnenfleckenzyklus ; Helioseismologie ; Sonnenplasma

Contributors, Places and Time

• Event: Veröffentlichung
• (where): Freiburg
• (who): Universität
• (when): 2022
• Creator: Waidele, Matthias
• Contributor: Roth, Markus ; Buchleitner, Andreas ; Schätz, Tobias ; Kiepenheuer-Institut für Sonnenphysik ; Albert-Ludwigs-Universität Freiburg. Fakultät für Mathematik und Physik

Further information

• DOI: 10.6094/UNIFR/225779
• URN: urn:nbn:de:bsz:25-freidok-2257791
• Rights: Open Access; Der Zugriff auf das Objekt ist unbeschränkt möglich.
• Last update: 25.03.2025, 1:44 PM CET