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

摘要

An outstanding issue in current solar and astrophysical research is that of the heating ofudthe solar corona. How is the corona heated to temperatures of greater than 1 MK whenudthe photosphere below is only 6000 K? One observational approach to addressing thisudimportant question is to focus on particular areas in the corona such as active regionsud(ARs). In a scenario that heating is impulsive and the cross-field spatial scale of theudheating is so small, under the resolution of the current instruments, we attempted toudnarrow the question further to discrete bright magnetic flux tubes, the coronal loops,udinside active regions. We investigate the emission variability, heating and substructureudof coronal loops in the core of one such active region, observed in high spatial andudtemporal detail by the Solar Dynamics Observatory (SDO) in 2010. Widespread inudthat active region, previous works had detected small amounts of very hot plasma (> 4udMK), much hotter than the typical plasma temperature of coronal plasma in activeudregions (~ 3 MK), outside of major flares. Most probably, storms of fast and intenseudheat pulses bring some plasma to such high temperature for a short time, and the workudin this thesis develops under this scenario of highly intermittent heating, and is dividedudinto two parts.udIn the first part, our approach is to analyze single light curves in the smallest resolutionudelements (0.6”) of the images taken in two EUV channels (94 A and 335 A)udwith a high cadence (~ 12 s) from the Atmospheric Imaging Assembly on-board SDO.udWe compare the observed light curves with those obtained from a specific loop model.udAccording to the model, a loop is made up of a bundle of thinner strands, each heatedudimpulsively and independently of the others. The frequency of the pulses depends onudtheir energy as a power-law, more intense ones being also less frequent. The pulsesudoccur at random times. We use a 0D strand hydrodynamic model, which describesudthe evolution of the space-averaged physical quantities, in particular density and temperature,udand from them we derive the EUV light curves in a single strand. We thenudcombine the light curves of many single strands that we would intercept along the lineudof sight inside a pixel.udThe next step is to compare the resulting simulated light curves with the observedudlight curves. We use two independent methods: an artificial intelligent system (ProbabilisticudNeural Network, PNN) and a simple cross-correlation technique. We makeudsome exploration of the space of the parameters to constrain the distribution of theudheat pulses, their duration and their spatial size, and, as a feedback on the data, theirudsignatures on the light curves. From both methods the best agreement is obtained for audrelatively large population of events (1000) with a short duration (less than 1 minute)udand a relatively shallow distribution (power law with index 1.5) in a limited energyudrange (1.5 decades). The feedback on the data indicates that bumps in the light curves,udespecially in the 94 A channel, are signatures of a heating excess that occurred a fewudminutes before.udiudIn the second part of the work we extend the analysis of time resolved emissionudof single pixels by including spatially resolved strand modeling and by studying theudevolution of emission along the loops in the EUV 94 A and 335 A channels. Weudreplicate the modeling using a 1D hydrodynamic code that describes the evolution ofudthe physical quantities distributed along the loop strand. We use exactly the sameudparameters which labeled the best absolute match in the first part, as the input of theudspace-resolved analysis. We find that the amplitude of the random fluctuations drivenudby the random heat pulses increases from the bottom to the top of the loop in theud94 A channel and, viceversa, from the top to the bottom in the 335 A channel. Thisudprediction is confirmed by the observation of a set of aligned neighbouring pixels alonguda bright arc of an active region core. Maps of pixel fluctuations may therefore provideudeasy diagnostics of nano-flaring regions.