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Thesis (doctoral)--Georg-August-Universität zu Göttingen, 1983. / Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (p. 238-242).

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Abstract This thesis studies the long-term evolution of the photospheric magnetic field using surface flux transport simulations. The photospheric magnetic field and magnetic activity are tightly connected to space weather, and affect the whole heliosphere including the Earth. However, due to a lack of reliable observations our understanding of the long-term evolution of the photospheric magnetic field is still poor. Surface flux transport models, which are capable of simulating the evolution of the whole surface field from observations of solar activity, can be used to study the field in times when direct observations are not available. In this thesis we validate our surface flux transport model, optimize its parameters and test its sensitivity to uncertainties in parameter values and input data. We find a need to extend the model with a decay term to properly model the deep and long minimum between solar cycles 23 and 24, and simulate the photospheric magnetic field of cycles 21–24 using magnetographic observations as input. We also study consequences of hemispherically asymmetric activity, and show that activity in one hemisphere is enough to maintain polar fields in both hemispheres through cross-equatorial flow of magnetic flux. We develop a new method to reconstruct active regions from calcium K line and sunspot polarity observations. We show that this reconstruction is able to accurately capture the correct axial dipole moment of active regions. We study the axial dipole moments of observed active regions and find that a significant fraction of them have a sign opposite to the sign expected from Hale’s and Joy’s laws, proving that the new reconstruction method has an advantage over existing methods that rely on Hale’s and Joy’s laws to define polarities. We show one example of a long simulation covering solar cycles 15–21, demonstrating that using the active region reconstruction and surface flux transport model presented in this thesis it is possible to simulate the large-scale evolution of the photospheric magnetic field over the past century. / Original papers The original publications are not included in the electronic version of the dissertation. Virtanen, I. O. I., Virtanen, I. I., Pevtsov, A. A., Yeates, A., & Mursula, K. (2017). Reconstructing solar magnetic fields from historical observations. II. Testing the surface flux transport model. Astronomy & Astrophysics, 604, A8. https://doi.org/10.1051/0004-6361/201730415 http://jultika.oulu.fi/Record/nbnfi-fe2017103050356 Virtanen, I. O. I., Virtanen, I. I., Pevtsov, A. A., & Mursula, K. (2018). Reconstructing solar magnetic fields from historical observations. III. Activity in one hemisphere is sufficient to cause polar field reversals in both hemispheres. Astronomy & Astrophysics, 616, A134. https://doi.org/10.1051/0004-6361/201732323 http://jultika.oulu.fi/Record/nbnfi-fe201902205813 Virtanen, I. O. I., Virtanen, I. I., Pevtsov, A. A., Bertello, L., Yeates, A., & Mursula, K. (2019). Reconstructing solar magnetic fields from historical observations. IV. Testing the reconstruction method. Astronomy & Astrophysics, 627, A11. https://doi.org/10.1051/0004-6361/201935606 http://jultika.oulu.fi/Record/nbnfi-fe2019091828628 Virtanen, I. O. I., Virtanen, I. I., Pevtsov, A. A., & Mursula, K. (2019) Axial dipole moment of solar active regions in cycles 21-24. Manuscript.

The large flashes of radiation that are observed in GRBs are generally believed to arise in a relativistic jetted outflow. This thesis addresses the question of how and where in the jet this radiation is produced. It further explores the jet properties that can be inferred from the observations made by the Fermi GST that regularly observes GRBs in the range 8 keV - 300 GeV.  In my analysis I focus on the observational effects of the emission from the jet photosphere. I show that the photosphere has an important role in shaping the observed radiation spectrum and that its manifestations can significantly vary between bursts. For bursts in which the photospheric  emission component can be identified, the dynamics of the flow can be explored by determining the  jet Lorentz factor and the position of the jet nozzle. I also develop the theory of how to derive the properties of the outflow for general cases. The spectral analysis of the strong burst GRB110721A reveals a two-peaked spectrum, with the peaks evolving differently. I conclude that three main flow quantities can describe the observed spectral behaviour in bursts:  the luminosity, the Lorentz factor, and the nozzle radius. While the photosphere can appear like a pure blackbody it can also be substantially broadened, due to dissipation of the jet energy below the photosphere. I show that Comptonisation of the blackbody can shape the observed spectra and describe its evolution. In particular this model can very well explain GRB110920A which has two prominent breaks in its spectra.  Alternative models including synchrotron emission leads to severe physical constraints, such as the need for very high electron Lorentz factors, which are not expected in internal shocks. Even though different manifestations of the photospheric emission can explain the data, and lead to ambiguous interpretations, I argue that dissipation below the photosphere is the most important process in shaping the observed spectral shapes and evolutions. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 4: In press. Paper 5: Submitted.</p>

The surface (photosphere) of the Sun is threaded throughout by magnetic fields. Groups of magnetic fields form magnetic features (of a wide range of sizes in flux and area) on the surface where the fields are directed into or out of the Sun. The aim of this thesis is to examine in detail the four key processes, emergence, cancellation, fragmentation and coalescence, which determine the behaviour of small-scale magnetic features, in the Sun's photosphere. I identify features in both Hinode/NFI and SOHO/MDI full-disk to enable these processes to be examined at the currently smallest observable scales and over an entire solar cycle. The emerging event frequency versus flux distribution, for intranetwork emerging regions to active regions, is found to follow a power-law distribution with index -2.50, which spans nearly 7 orders of magnitude in flux (10¹⁶ - 10²³ Mx) and 18 orders of magnitude in frequency. The global rate of flux emergence is found to be 3.9 x 10²⁴ Mx day⁻¹. Since the slope of all emerged fluxes is less than -2 this implies that most of the new flux that is fed into the solar atmosphere is from small-scale emerging events. This single power-law distribution over all emerged fluxes suggest a scale-free dynamo, therefore indicating that in addition to dynamo actions in the tachocline producing sunspots, a turbulent dynamo may act throughout the convection zone. Similarly for cancellations I find a power-law relationship between the frequency of cancellation and the peak flux lost per cancelling event (for events detected in both Hinode/NFI and SOHO/MDI full-disk), with slope -2.10. Again, the process of cancellation appears to be scale free and the slope is less than -2 indicating that numerous small-scale features are cancelling the majority of flux on the Sun. I also estimate the frequency of all surface processes at solar maximum and find, 1.3 x 10⁸, 4.5 x 10⁷, 4.0 x 10⁷ and 3.6 x 10⁶ events per day over the whole surface for emergence, cancellation, fragmentation and coalescence events, respectively. All the surface processes are found to behave in a similar manner over all flux scales. The majority of events for all processes occur in features with flux below 10²º Mx, which highlights the dynamic nature of the magnetic carpet. Using SOHO/MDI full-disk data I investigate the cyclic variation of the 4 key processes throughout cycle 23. It is found that the rate of emerging events, cancellations, fragmentations and coalescences varied in anti-phase with the solar cycle by factors of 3.4, 3.1, 2.4 and 2.2, respectively over the cycle. Not surprisingly, therefore, the number of network features detected throughout the cycle also exhibits an anti-phase variation over the solar cycle by a factor of 1.9. The mean peak flux of tracked small-scale network, fragmenting, coalescing and cancelling features showed in-phase relationships with the solar cycle by factors of 1.4, 1.7, 2.4 and 1.2, respectively. The total flux which is emerged and cancelled by small-scale events, varied in anti-phase with the solar cycle, by factors of 1.9 and 3.2. This is clearly due to the variation in the number of emerging and cancelling events and the fact that the flux of individual emerging events showed no cyclic variation. The results in this thesis show that the large-scale solar cycle plays a complex role in the surface processes features undergo. The fact that the number of ephemeral regions emerging has an anti-phase variation to the solar cycle has a knock-on effect in the number of features which are available to undergo surface processes. Also decaying active regions, during more active periods, contribute more small-scale features, with high flux density, into the network which has an effect on the surface processes. This work has revealed the significant importance of small-scale features in the flux budget through continual emergence and cancellation, plus highlighted how through dynamic surface motions, small-scale features form the fundamental components with which the network is developed.

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京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第19359号 / 理博第4121号 / 新制||理||1593(附属図書館) / 32373 / 新制||理||1593 / 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 一本 潔, 教授 柴田 一成, 准教授 野上 大作 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

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