Abstract I have made a detailed study of the fundamental properties of the solar photospheric magnetic field, which helps in better understanding the Sun’s radiative and particle outputs that affect the Earth’s near-space environment, as well as the entire heliosphere. Photospheric magnetic field is an essential parameter for space weather and space climate. The photospheric magnetic field includes a wide range of large-scale and small-scale structures, but the contribution of weak, small-scale fields to the total flux on the solar surface is dominant. This thesis discusses the spatial-temporal structure and long-term evolution of the solar photospheric magnetic field. Particularly, the thesis presents, for the first time, the spatial distribution of the asymmetry of weak field values and its evolution in solar cycles 21–24. I found that the asymmetry (also called shift) of the distribution of positive and negative weak-field values is a real physical phenomenon. I also found that the shifts are most effectively produced at the supergranulation scale. I studied the asymmetry of the distribution of weak field values separately in the two solar hemispheres. My results show that the shifts of weak-field field distributions in the two solar hemispheres have always the same sign as the new polarity of the polar field in the respective hemisphere and solar cycle. I also found that the hemispheric shifts change their sign in the late ascending to maximum phase of the solar cycle and attain their maximum in the early to mid-declining phase. This evolution of the hemispheric weak-field gives a new signal of the solar magnetic cycle. We also studied the long-term spatial-temporal evolution of the weak-field shift and skewness of the distribution of photospheric magnetic field values during solar cycles 21–24 in order to clarify the role and relation of the weak field values to the overall magnetic field evolution. Our results give evidence for the preference of even the weakest field elements toward the prevailing magnetic polarity since the emergence of an active region, and for a systematic coalescence of stronger magnetic fields of opposite to produce weak fields during the poleward drift of the surge. / Original papers Original papers are not included in the electronic version of the dissertation. Getachew, T., Virtanen, I., & Mursula, K. (2017). Structure of the Photospheric Magnetic Field During Sector Crossings of the Heliospheric Magnetic Field. Solar Physics, 292(11). https://doi.org/10.1007/s11207-017-1198-9 http://jultika.oulu.fi/Record/nbnfi-fe201802083259 Getachew, T., Virtanen, I., & Mursula, K. (2019). Asymmetric Distribution of Weak Photospheric Magnetic Field Values. The Astrophysical Journal, 874(2), 116. https://doi.org/10.3847/1538-4357/ab0749 http://jultika.oulu.fi/Record/nbnfi-fe2019061320447 Getachew, T., Virtanen, I., & Mursula, K. (2019). A New Signal of the Solar Magnetic Cycle: Opposite Shifts of Weak Magnetic Field Distributions in the Two Hemispheres. Geophysical Research Letters, 46(16), 9327–9333. https://doi.org/10.1029/2019gl083339 Mursula, K., Getachew, T., & Virtanen, I. (2019). Spatial-temporal evolution of photospheric weak-field shifts in solar cycles 21-24. Astron. Astrophys., submitted.

The radiative mechanism responsible for the prompt gamma-ray burst (GRB) emission remains elusive. For the last decade, optically thin synchrotron emission from shocks internal to the GRB jet appeared to be the most plausible explanation. However, the synchrotron interpretation is incompatible with a significant fraction of GRB observations, highlighting the need for new ideas. In this thesis, it is shown that the narrow, dominating component of the prompt emission from the bright GRB090902B is initially consistent only with emission released at the optically thick jet photosphere. However, this emission component then broadens in time into a more typical GRB spectrum, which calls for an explanation. In this thesis, a previously unconsidered way of broadening the spectrum of photospheric emission, based on considerations of the lateral jet structure, is presented and explored. Expressions for the spectral features, as well as polarization properties, of the photospheric emission observed from structured, relativistic jets are derived analytically under simplifying assumptions on the radiative transfer close to the photosphere. The full, polarized radiative transfer is solved through Monte Carlo simulations, using a code which has been constructed for this unique purpose. It is shown that the typical observed GRB spectrum can be obtained from the photosphere, without the need for additional, commonly assumed, physical processes (e.g. energy dissipation, particle acceleration, or additional radiative processes). Furthermore, contrary to common expectations, it is found that the observed photospheric emission can be highly linearly polarized (up to $\sim 40 \, \%$). In particular, it is shown that a shift of $\pi/2$ of the angle of polarization is the only shift allowed by the proposed model, consistent with the only measurement preformed to date. A number of ways to test the theory is proposed, mainly involving simultaneous spectral and polarization measurements. The simplest measurement, which tests not only the proposed theory but also common assumptions on the jet structure, involves only two consecutive measurements of the angle of polarization during the prompt emission. / <p>QC 20131204</p>

Gamma-ray bursts (GRBs) are the most energetic electromagnetic events in the universe, but there are still unanswered questions about them, like the underlying radiation mechanisms that cause the different parts of their light curves. Given that Wolf-Rayet (WR) stars with circumburst rings could be the progenitor of GRBs with late flares \cite{complex}, the purpose of this thesis was to determine if the precursor and the main emission of GRBs with late time flares might originate due to different radiation mechanisms, and thereby if WR stars could be the progenitors. 271 of the longest GRBs with flux above 10 photons/cm$^2$/s were studied and all GRBs with a precursor and a defined quiescent period were chosen for further studies. The chosen 39 GRBs were divided into different categories depending on the appearance of their light curves. A gold sample with $R_{P, max}/R_{D, max} &lt; 0.4$ and $T_Q/T_{tot} &gt; 0.5$, for the maximum count rate of the precursor $R_{P, max}$, the dominant emission $R_{D, max}$, and for the normalised quiescent period $T_Q/T_{tot}$ was concluded to have similar characteristics and to fit what would be expected if WR stars would be the progenitors. This group of GRBs all have a short and less bright precursor, a long quiescent period and a main emission which is brighter and longer than the precursor. The distributions of the photon index $\alpha$ for the precursor and the dominant emission for the gold sample indicate that the precursor is due to photospheric emission and the dominant emission is due to synchrotron emission. This is consistent with the interpretation that the precursor is a result of the jet interacting with the photosphere and the dominant emission is a result of interactions with the circumburst ring of a star like the WR stars. The next step in this investigation would be to study GRBs with more than one precursor that otherwise fit the description of the gold sample, to determine if these fit into the gold sample as well. / Gammablixtrar (GRB) är de mest kraftfulla elektromagnetiska eventen i universum men det finns fortfarande obesvarade frågor om dem, som de underliggande strålningsmekanismerna som orsakar de olika delarna av deras ljuskurvor. För en del av alla GRBs tar det upp till eller mer än 100 sekunder från utlösningstiden till det att en topp ses i ljuskurvan. För dessa finns då ofta en liten svag topp, som följs av en lång lugn period och sedan den dominant, starkare utstrålningen. GRBs tros kunna härstamma från Wolf-Rayet-stjärnor (WR-stjärnor), som är massiva, döende stjärnor som kan vara omringade av bubblor, nebulosor, och ringar. Om GRBs härstammar från dessa förväntas den första mindre toppen och den andra större toppen uppkomma på grund av olika strålningsprocesser. Syftet med detta projekt var därför att undersöka huruvida dessa toppar uppkommer på grund av olika processer eller ej. Sammanfattningsvis så hittades en distinkt och homogen grupp av GRBs med likande egenskaper. Resultaten påvisar att den första svaga toppen är fotosfärisk strålning, så att den uppkommer på grund av att jetstrålen från GRBn interagerar med fotosfären. Därtill tyder resultaten på att den dominanta starkare toppen är synkrotronstrålning, som kan uppkomma när jetstrålen interagerar med en ring runt en WR-stjärna. Nästa steg i detta projekt skulle vara att studera GRBs med fler än en mindre topp innan den dominant utstrålningen, för att se om dessa också har liknande egenskaper som de som hittades i den homogena gruppen.

À ce jour, les différentes méthodes de reconstruction des mouvements du plasma à la surface du Soleil qui ont été proposées présupposent une MHD idéale (Welsch et al., 2007). Cependant, Chae & Sakurai (2008) ont montré l’existence d’une diffusivité magnétique turbulente à la photosphère. Nous introduisons une généralisation de la méthode du Minimum Energy Fit (MEF ; Longcope, 2004) pour les plasmas résistifs. Le Resistive Minimum Energy Fit (MEF-R ; Tremblay & Vincent, 2014) reconstruit les champs de vitesse du plasma et la diffusivité magnétique turbulente qui satisfont à l’équation d’induction magnétique résistive et qui minimisent une fonctionnelle analogue à l’énergie cinétique totale. Une séquence de magnétogrammes et de Dopplergrammes sur les régions actives AR 9077 et AR 12158 ayant chacune produit une éruption de classe X a été utilisée dans MEF-R pour reconstruire les mouvements du plasma à la surface du Soleil. Les séquences temporelles des vitesses et des diffusivités magnétiques turbulentes calculées par MEF-R sont comparées au flux en rayons X mous enregistré par le satellite GOES-15 avant, pendant et après l’éruption. Pour AR 12158, nous observons une corrélation entre les valeurs significatives de la diffusivité magnétique turbulente et de la vitesse microturbulente pour les champs magnétiques faibles. / To this day, the various methods proposed for the reconstruction of plasma motions at the Sun’s surface are all based on ideal MHD (Welsch et al., 2007). However, Chae & Sakurai (2008) have shown the existence of an eddy magnetic diffusivity at the photosphere. We introduce a generalization of the Minimum Energy Fit (MEF; Longcope, 2004) for resistive plasmas. The Resistive Minimum Energy Fit (MEF-R; Tremblay & Vincent, 2014) infers velocity fields and an eddy magnetic diffusivity which solve the resistive magnetic induction equation and minimize an energy-like functional. A sequence of magnetograms and Dopplergrams documenting the active regions AR 9077 and AR 12158 are used as input in MEF-R to reconstruct plasma motions at the Sun’s surface. Time series of the inferred velocities and eddy magnetic diffusivities are compared to the soft X-ray flux observed by GOES-15. We find a positive correlation between significant eddy magnetic diffusivities and microturbulent velocities for weak magnetic fields in AR 12158.