Fine structure and dynamic heating from temporal and spatial analysis of a solar active region observed with Solar Dynamics Observatory (SDO)

An outstanding issue in current solar and astrophysical research is that of the heating of the solar corona. How is the corona heated to temperatures of greater than 1 MK when the photosphere below is only 6000 K? One observational approach to addressing this important question is to focus on particular areas in the corona such as active regions (ARs). In a scenario that heating is impulsive and the cross-field spatial scale of the heating is so small, under the resolution of the current instruments, we attempted to narrow the question further to discrete bright magnetic flux tubes, the coronal loops, inside active regions. We investigate the emission variability, heating and substructure of coronal loops in the core of one such active region, observed in high spatial and temporal detail by the Solar Dynamics Observatory (SDO) in 2010. Widespread in that active region, previous works had detected small amounts of very hot plasma (> 4 MK), much hotter than the typical plasma temperature of coronal plasma in active regions (~ 3 MK), outside of major flares. Most probably, storms of fast and intense heat pulses bring some plasma to such high temperature for a short time, and the work in this thesis develops under this scenario of highly intermittent heating, and is divided into two parts. In the first part, our approach is to analyze single light curves in the smallest resolution elements (0.6”) of the images taken in two EUV channels (94 A and 335 A) with a high cadence (~ 12 s) from the Atmospheric Imaging Assembly on-board SDO. We compare the observed light curves with those obtained from a specific loop model. According to the model, a loop is made up of a bundle of thinner strands, each heated impulsively and independently of the others. The frequency of the pulses depends on their energy as a power-law, more intense ones being also less frequent. The pulses occur at random times. We use a 0D strand hydrodynamic model, which describes the evolution of the space-averaged physical quantities, in particular density and temperature, and from them we derive the EUV light curves in a single strand. We then combine the light curves of many single strands that we would intercept along the line of sight inside a pixel. The next step is to compare the resulting simulated light curves with the observed light curves. We use two independent methods: an artificial intelligent system (Probabilistic Neural Network, PNN) and a simple cross-correlation technique. We make some exploration of the space of the parameters to constrain the distribution of the heat pulses, their duration and their spatial size, and, as a feedback on the data, their signatures on the light curves. From both methods the best agreement is obtained for a relatively large population of events (1000) with a short duration (less than 1 minute) and a relatively shallow distribution (power law with index 1.5) in a limited energy range (1.5 decades). The feedback on the data indicates that bumps in the light curves, especially in the 94 A channel, are signatures of a heating excess that occurred a few minutes before. i In the second part of the work we extend the analysis of time resolved emission of single pixels by including spatially resolved strand modeling and by studying the evolution of emission along the loops in the EUV 94 A and 335 A channels. We replicate the modeling using a 1D hydrodynamic code that describes the evolution of the physical quantities distributed along the loop strand. We use exactly the same parameters which labeled the best absolute match in the first part, as the input of the space-resolved analysis. We find that the amplitude of the random fluctuations driven by the random heat pulses increases from the bottom to the top of the loop in the 94 A channel and, viceversa, from the top to the bottom in the 335 A channel. This prediction is confirmed by the observation of a set of aligned neighbouring pixels along a bright arc of an active region core. Maps of pixel fluctuations may therefore provide easy diagnostics of nano-flaring regions.