Investigating small-scale dynamics: Analysis and Interpretation of Solar Orbiter observations

The solar atmosphere is a highly dynamic and magnetically complex environment, where two of the key open questions are how the corona is heated to over a million kelvin and how the solar wind is generated. Small-scale activities, such as bright points, small jets, and spicules, are believed to play a fundamental role in supplying energy and material to the corona. Although these energy releases are subtle compared to large-scale events, they may be pervasive enough to contribute significantly. Due to their transient and small-scale nature, observations in high resolution, both spatially and temporally, are essential for understanding the mechanisms underlying these small-scale energy releases and assessing their role in sustaining the coronal temperature and driving the solar wind. Along with the advancements in observational techniques, our understanding of small-scale structures in the corona has significantly improved over the past few decades. The launch of Solar Orbiter in February 2020 has further expanded these opportunities. It follows a unique highly elliptical orbit, allowing it to observe the Sun from as close as 0.28 au at perihelia and to capture the high-latitude views of the polar regions (starting from 2025). Solar Orbiter carries ten instruments, both in-situ and remote-sensing, including the Extreme Ultraviolet Imager (EUI) for high-resolution imaging, the Spectral Imaging of the Coronal Environment (SPICE) for spectroscopic analysis, and the Polarimetric and Helioseismic Imager (SO/PHI) for magnetic field measurement, offering valuable data for studying the small-scale dynamics in the solar atmosphere. This thesis presents the results of two main investigations. The first study is focused on the EUI brightenings in the quiet sun, the smallest transient EUV brightenings that have been so far reported, based on the high-resolution observations from EUI. Three EUI brightenings are captured by both the High Resolution EUV telescope (HRIEUV, part of EUI) and SPICE in this study. The detection of the EUI brightenings is almost at the limit of SPICE's capability, which means that the identification of these structures is only possible with the assistance of HRIEUV images. On the other hand, SPICE can provide the multi thermal information of the brightenings. By combining both data sets, two of these EUI brightenings with longer duration are found to be detectable at Ne VIII temperature (0.6 MK). The signatures of all three brightenings are detected in an O VI line (0.3 MK). It is also possible to investigate the thermal evolution of one of these brightenings, where double peaks are observed in the light curves of C III (0.06 MK) and O VI, while the only peak in the light curve of Ne VIII is found between the two peak times of the intensities of the two transition region lines. This suggests that the temperature of this brightening could increase from the formation temperature of the C III line to that of the O VI line and then to the Ne VIII temperature (from upper transition region to low corona), and then it cools down. These findings also lead to the conclusion that some EUI brightenings could barely reach coronal temperatures. As part of this study I also derive a model of the noise of the SPICE detectors. As these detectors are conceptually very similar to those that will be used on future missions such as the Extreme Ultraviolet Spectroscopic Telescope (EUVST) under construction for the forthcoming Solar-C mission, these results will be applicable with only minor adaptation. The second part of this work is about the small-scale transients in the coronal hole plumes. Plumes are the largely ray-like structures that could channel magnetohydrodynamic waves and solar wind and are observed to host high-speed propagating disturbances (PDs) and small-scale transients at their bases. Three plumes are detected within an equatorial coronal hole by HRIEUV. At these plume bases, from tens to hundreds of base brightenings are observed in the 30-min observation. Their properties (intensity, size, lifetime, shape and apparent speed) are studied statistically. The results show that most of the base brightenings are small-scale, short-lived and appear slightly elongated. These brightenings exhibit complex motion, with most moving at speeds below 10 km s-1 in the plane of sky. Potential field extrapolation, based on magnetic field data obtained by SO/PHI, is applied to de-project the apparent speed to the real velocity along the magnetic field, which is still found to be significantly lower than, and hard to reconcile with, the higher speeds observed in PDs at greater heights within the plumes. Since a direct connection between base brightenings and PDs remains uncertain, two possible explanations are proposed for the base brightenings: they could be associated with wave-driven Type I spicules or result from interchange reconnection events. In summary, this study utilizes a diverse set of high-resolution observations from Solar Orbiter to investigate small-scale structures in both the quiet Sun and coronal hole regions. These data provide valuable insights into the characteristics of these structures and the physical mechanisms behind them. Due to the limitations of the data sets, certain aspects remain inconclusive in this work and require further investigation with observations from multiple instruments. In the last chapter I finally provide some considerations of how observations from forthcoming missions such as EUVST and Multi-slit Solar Explorer (MUSE) will be able to improve our knowledge of the events studied in this thesis.