A Hot Downflowing Model Atmosphere For Umbral Flashes And The Physical Properties Of Their Dark Fibrils

We perform NLTE inversions in a large set of umbral flashes, including the dark fibrils visible within them, and in the quiescent umbra by using the inversion code NICOLE on a set of full Stokes high-resolution Ca II 8542 A observations of a sunspot at disk center. We find that the dark structures have Stokes profiles that are distinct from those of the quiescent and flashed regions. They are best reproduced by atmospheres that are more similar to the flashed atmosphere in terms of velocities, even if with reduced amplitudes. We also find two sets of solutions that finely fit the flashed profiles: a set that is upflowing, featuring a transition region that is deeper than in the quiescent case and preceded by a slight dip in temperature, and a second solution with a hotter atmosphere in the chromosphere but featuring downflows close to the speed of sound at such heights. Such downflows may be related, or even dependent, on the presence of coronal loops, rooted in the umbra of sunspots, as is the case in the region analyzed. Similar loops have been recently observed to have supersonic downflows in the transition region and are consistent with the earlier "sunspot plumes" which were invariably found to display strong downflows in sunspots. Finally we find, on average, a magnetic field reduction in the flashed areas, suggesting that the shock pressure is moving field lines in the upper layers.Comment: Accepted in June for publication at ApJ. Comments to [email protected] or [email protected]

Suppression of Hydrogen Emission in an X-Class White-Light Solar Flare

We present unique NUV observations of a well-observed X-class flare from NOAA 12087 obtained at Ond\v{r}ejov Observatory. The flare shows a strong white-light continuum but no detectable emission in the higher Balmer and Lyman lines. RHESSI and Fermi observations indicate an extremely hard X-ray spectrum and gamma-ray emission. We use the RADYN radiative hydrodynamic code to perform two type of simulations. One where an energy of 3 x 10^11 erg/cm^2/s is deposited by an electron beam with a spectral index of ~3 and a second where the same energy is applied directly to the photosphere. The combination of observations and simulations allow us to conclude that the white-light emission and the suppression or complete lack of hydrogen emission lines is best explained by a model where the dominant energy deposition layer is located in the lower layers of the solar atmosphere rather than the chromosphere.Comment: 13 page

Statistical Signatures of Nanoflare Activity. I. Monte Carlo Simulations and Parameter Space Exploration

Small-scale magnetic reconnection processes in the form of nanoflares have become increasingly hypothesized as important mechanisms for the heating of the solar atmosphere, driving propagating disturbances along magnetic field lines in the Sun\u2019s corona, and instigating rapid jetlike bursts in the chromosphere. Unfortunately, the relatively weak signatures associated with nanoflares place them below the sensitivities of current observational instrumentation. Here we employ Monte Carlo techniques to synthesize realistic nanoflare intensity time series from a dense grid of power-law indices and decay timescales. Employing statistical techniques, which examine the modeled intensity fluctuations with more than 107 discrete measurements, we show how it is possible to extract and quantify nanoflare characteristics throughout the solar atmosphere, even in the presence of significant photon noise. A comparison between the statistical parameters (derived through examination of the associated intensity fluctuation histograms) extracted from the Monte Carlo simulations and Solar Dynamics Observatory (SDO)/ Atmospheric Imaging Assembly (AIA) 171 and 94 \uc5 observations of active region NOAA\uf0a011366 reveals evidence for a flaring power-law index within the range of 1.82\uf0a0\u84\uf0a0\u3b1\uf0a0\u84\uf0a01.90, combined with e-folding timescales of 385\uf0a0\ub1\uf0a026and 262\uf0a0\ub1\uf0a017s for the SDO/AIA 171 and 94\uc5 channels, respectively. These results suggest that nanoflare activity is not the dominant heating source for the active region under investigation. This opens the door for future dedicated observational campaigns to not only unequivocally search for the presence of small-scale reconnection in solar and stellar environments but also quantify key characteristics related to such nanoflare activity

The magnetic properties of photospheric magnetic bright points with high-resolution spectropolarimetry

Magnetic bright points (MBPs) are small-scale magnetic elements ubiquitous across the solar disc, with the prevailing theory suggesting that they form due to the process of convective collapse. Employing a unique full Stokes spectropolarimetric data set of a quiet Sun region close to disc centre obtained with the Swedish Solar Telescope, we look at general trends in the properties of magnetic bright points. In total we track 300 MBPs in the data set and we employ NICOLE inversions to ascertain various parameters for the bright points such as line-of-sight magnetic field strength and line-of-sight velocity, for comparison. We observe a bimodal distribution in terms of maximum magnetic field strength in the bright points with peaks at ∼480 G and ∼1700 G, although we cannot attribute the kilogauss fields in this distribution solely to the process of convective collapse. Analysis of muram simulations does not return the same bimodal distribution. However, the simulations provide strong evidence that the emergence of new flux and diffusion of this new flux play a significant role in generating the weak bright point distribution seen in our observations

Detecting Prominent Patterns of Activity in Social Media

A large part of the Web, today, consists of online platforms that allow their users to generate digital content. They include online social networks, multimedia-sharing websites, blogging platforms, and online discussion boards, to name a few examples. Users of those platforms generate content in the form of digital items (e.g. documents, images, or videos), inspect content generated by others, and, finally, interact with each other (e.g. by commenting on each other's generated items). For the social process of information exchange they enable, such platforms are customarily referred to as `social media'. Activity on social media is largely spontaneous and uncoordinated, but it is not random; users choose the discussions they engage in and who they interact with, and their choices and actions reflect what they find important. In this thesis, we define and quantify notions of importance for items, users, and social connections between users, and, based on those definitions, propose efficient algorithms to detect important instances of social media activity. Our description of the algorithms is accompanied with experimental studies that showcase their performance on real datasets in terms of efficiency and effectiveness.Ph

Αριθμητική μοντελοποίηση εργαστηριακών δοκιμών κοπής πετρωμάτων με κοπτικά δίσκου

Εθνικό Μετσόβιο Πολυτεχνείο--Μεταπτυχιακή Εργασία. Διεπιστημονικό-Διατμηματικό Πρόγραμμα Μεταπτυχιακών Σπουδών (Δ.Π.Μ.Σ.) “Σχεδιασμός και Κατασκευή Υπόγειων Έργων

Hydrogen Emission in Type II White-light Solar Flares

Type II WLFs have weak Balmer line emission and no Balmer jump. We carried out a set of radiative hydrodynamic simulations to understand how the hydrogen radiative losses vary with the electron beam parameters and more specifically with the low energy cutoff. Our results have revealed that for low energy beams, the excess flare Lyman emission diminishes with increasing low energy cutoff as the energy deposited into the top chromosphere is low compared to the energy deposited into the deeper layers. Some Balmer excess emission is always present and is driven primarily by direct heating from the beam with a minor contribution from Lyman continuum backwarming. The absence of Lyman excess emission in electron beam models with high low energy cutoff is a prominent spectral signature of type II WLFs.Comment: 7 page