Transient x-ray and ultraviolet phenomena in solar polar coronal holes

In the last few years several new missions have been developed and launched to observe our star in order to understand the physical processes that dom- inate the plasma dynamics of the solar atmosphere. These missions have provided solar scientists with unprecedented high temporal and spatial reso- lution images of the Extreme UltraViolet (EUV) and X-rays corona. In these thesis we have focussed on the study of transient phenomena within coronal holes (CH) that can be observed with the imaging instruments on board of these new missions. In the first three chapters of this thesis we give an introduction to these transient phenomena and to their studies by means of observations of their EUV and X-rays emissions, we describe the main characteristics of the mis- sions and of the instruments whose data have been analysed in this work and we review the diagnostics techniques that have been adopted to derive the plasma parameters of the observed features. In the second part of this thesis we present in detail the observations, the methods and the results of the three main works that have been carried out during the PhD. Three main publications, that are here presented, resulted from these works. In the first work we present an analysis of X-ray Bright Points (BPs) and X-ray jets observed by Hinode/X-Ray Telescope (XRT) on November 2–4 2007, within the solar northern polar CH. After selecting small subregions that include several BPs, we followed their brightness evolution over a time interval of a few hours, when several jets were observed. We find that most of the jets occurred in close temporal association with brightness maxima in multiple BPs: more precisely, most jets are closely correlated with the brightening of at least 2 BPs. We suggest that the jets result from magnetic connectivity changes that also induce the BP variability. We surmise that the jets and implied magnetic connectivity we describe are small-scale ver- sions of the active-region-scale phenomenon whereby flares and eruptions are triggered by interacting bipoles. The second work is still based on data from the Hinode XRT which re- i ii vealed the occurrence, in polar coronal holes, of much more numerous jets than previously indicated by the Yohkoh/Soft X-ray Telescope. These plasma ejections can be of two types, depending on whether they fit the standard reconnection scenario for coronal jets or if they include a blowout-like erup- tion. In this second work we analyze two jets, one standard and one blowout, that have been observed by Hinode and STEREO experiments, aiming at inferring differences in physical parameters that correspond to the different morphology of the events. To this end we adopt spectroscopic techniques and give the profiles of plasma temperature, density and outflow speed vs. time and position along the jets. It turns out that the blowout jet has a higher outflow speed, marginally higher temperature and is rooted in a stronger magnetic field region than the standard event. Our data provide evidence for recursively occurring reconnection episodes within both the standard and the blowout jet, pointing either to bursty reconnection or to reconnection occurring at different locations over the jet lifetimes. We make a crude esti- mate of the energy budget of the two jets and show how energy is partitioned among different forms. Also, we show that the magnetic energy that feeds the blowout jet is a factor 10 higher than the magnetic energy that fuels the standard event. In the third work we analyze a solar polar-coronal-hole plume over its entire ≈ 40 h lifetime, using high resolution Solar Dynamic Observatory (SDO) Atmospheric Imaging Assembly (AIA) data. We examine (1) the plume’s relationship to a bright point (BP) that persists at its base, (2) plume outflows and their possible contribution to solar wind mass supply, and (3) physical properties of the plume. We find that the plume started ≈2 h after the BP first appeared and became undetectable ≈1 h after the BP disappeared. We detected radially-moving radiance variations from both the plume and from interplume regions, corresponding to apparent outflow speeds ranging over ≈(30 to 300) km s−1 with outflow velocities being higher in the “cooler” AIA 171 ̊A channel than in the “hotter” 193 ̊A and 211 ̊A channels, which is inconsistent with wave motions; therefore we conclude that the observed radiance variations represent material outflows. If they persist into the heliosphere and plumes cover ≈10 % of a typical coronal hole area, these flows could account for ≈50 % of the solar wind mass. From a Differential Emission Measure analysis of the AIA images, we find that the average electron temperature of the plume remained approximately constant over its lifetime, at Te ≈ 8.5 × 105 K. Its density however decreased with the age of the plume, being about a factor-of-three lower when the plume faded compared to when it was born. We conclude that the plume died due to a density reduction rather than to a temperature decrease.