Small-scale structures in the upper atmosphere of the Sun

Abstract

<b>Context:</b> Numerous small-scale structures with sizes of the order of megameters (Mm), constitute the background for the large-scale structures in the solar atmosphere. Although these features are small, their large number suggests that they play an important role in the energy transport and the magnetic structuring in the solar atmosphere. <br> <b>Aims:</b> The aim of this work is to investigate the physical properties of the small-scale structures in the solar atmosphere. Particular attention is given to miniature loops (with a length of approximately 1Mm) observed for the first time at coronal temperature (> 1 MK), and their relation between the emission of the small-scale structures and the underlying magnetic field. Many of the small-scale structures in the solar atmosphere remain unresolved with current telescopes. Still, one can investigate the relation of the small-scale structures to the underlying magnetic field in a statistical fashion. This will be the final topic of this thesis. <br><b>Methods:</b> We focus here on UV and EUV observations, in combination with photospheric magnetic field maps (magnetograms) from space-based telescopes. The corresponding images, magnetograms and spectroscopic raster maps are aligned to sub-pixel accuracy. The spectral data are used to derive various plasma parameters (e.g. intensity, Doppler shift) by line-fitting techniques, and create the respective 2D maps. Further more, we employ a Differential Emission Measure (DEM) technique to estimate coronal temperatures from the EUV images. Finally, the spatial and temporal evolution of structures is compared with changes of the underlying photospheric magnetic field. <br><b>Results:</b> The small-scale structures that are analysed in this thesis have a broad range of lifetimes, from few minutes to several tens of minutes. In particular, the miniature loops have a lifetime compatible to that of granules ( ~ 5 minutes), whereas the magnetic field patches exist for about 1 hr. We argue that the small-scale short-living structures can reach coronal temperatures. Further more, the structures are closely related to the photospheric magnetic field. A statistical analysis of small scale upper atmospheric structures in various magnetically active regions shows that the emission from these structures can be used as a proxy for the magnetic field. By assuming a power-law relation of the emission to the magnetic field, we show that this power-law varies smoothly from the lower chromosphere to the transition region. <br><b>Conclusion:</b> The result of this thesis reveals a new view of the small-scale structures in the solar atmosphere. We suggest that small-scale loop-like structures observed at coronal temperatures are miniature versions of hot coronal loops. Based on the statistical studies we conclude that the emission originating from higher temperatures is more sensitive to the magnetic field. The decrease of the correlation between magnetic field and intensity with temperature is probably caused by the spatial expansion of the magnetic structures. <br><br>Chapter 5 is published as a journal article A&A, 599, A137 (2017) and reproduced with permission &copy; ESO.