The pre-penumbral magnetic canopy in the solar atmosphere

Penumbrae are the manifestation of magnetoconvection in highly inclined (to the vertical direction) magnetic field. The penumbra of a sunspot tends to form, initially, along the arc of the umbra antipodal to the main region of flux emergence. The question of how highly inclined magnetic field can concentrate along the antipodal curves of umbrae, at least initially, remains to be answered. Previous observational studies have suggested the existence of some form of overlying magnetic canopy which acts as the progenitor for penumbrae. We propose that such overlying magnetic canopies are a consequence of how the magnetic field emerges into the atmosphere and are, therefore, part of the emerging region. We show, through simulations of twisted flux tube emergence, that canopies of highly inclined magnetic field form preferentially at the required locations above the photosphere

Critical decay index at the onset of solar eruptions

Magnetic flux ropes are topological structures consisting of twisted magnetic field lines that globally wrap around an axis. The torus instability model predicts that a magnetic flux rope of major radius $R$ undergoes an eruption when its axis reaches a location where the decay index $-d(\ln B_{ex})/d(\ln R)$ of the ambient magnetic field $B_{ex}$ is larger than a critical value. In the current-wire model, the critical value depends on the thickness and time-evolution of the current channel. We use magneto-hydrodynamic (MHD) simulations to investigate if the critical value of the decay index at the onset of the eruption is affected by the magnetic flux rope's internal current profile and/or by the particular pre-eruptive photospheric dynamics. The evolution of an asymmetric, bipolar active region is driven by applying different classes of photospheric motions. We find that the critical value of the decay index at the onset of the eruption is not significantly affected by either the pre-eruptive photospheric evolution of the active region or by the resulting different magnetic flux ropes. As in the case of the current-wire model, we find that there is a `critical range' $[1.3-1.5]$, rather than a `critical value' for the onset of the torus instability. This range is in good agreement with the predictions of the current-wire model, despite the inclusion of line-tying effects and the occurrence of tether-cutting magnetic reconnection.Comment: 15 pages, 9 figures. To appear in The Astrophysical Journa

IRIS observations of magnetic interactions in the solar atmosphere between pre-existing and emerging magnetic fields. II. UV emission properties

Multi-wavelength ultraviolet (UV) observations by the IRIS satellite in active region NOAA 12529 have recently pointed out the presence of long-lasting brightenings, akin to UV bursts, and simultaneous plasma ejections occurring in the upper chromosphere and transition region during secondary flux emergence. These signatures have been interpreted as evidence of small-scale, recurrent magnetic reconnection episodes between the emerging flux region (EFR) and the pre-existing plage field. Here, we characterize the UV emission of these strong, intermittent brightenings and we study the surge activity above the chromospheric arch filament system (AFS) overlying the EFR. We analyze the surges and the cospatial brightenings observed at different wavelengths. We find an asymmetry in the emission between the blue and red wings of the Si IV 1402 \AA{} and Mg II k 2796.3 \AA{} lines, which clearly outlines the dynamics of the structures above the AFS that form during the small-scale eruptive phenomena. We also detect a correlation between the Doppler velocity and skewness of the Si IV 1394 \AA{} and 1402 \AA{} line profiles in the UV burst pixels. Finally, we show that genuine emission in the Fe XII 1349.4 \AA{} line is cospatial to the Si IV brightenings. This definitely reveals a pure coronal counterpart to the reconnection event.Comment: 19 pages, 8 figures + 3 figures in the Appendix; accepted in Ap

Enantioselective Pd(0)-Catalyzed Activation of Secondary C(sp3)−H Bonds and Mechanistic Studies

Direct transition metal catalyzed C−H bond functionalization has emerged as a powerful strategy to form new C−C or C−X bonds over the past decades. Advantages such as no requirement for pre-functionalization of the reagents and therefore the reduced metal waste makes it an attractive alternative to the well-established cross-coupling reactions. In this regard, Pd(0)-catalyzed C(sp3)−H activation has been applied in the past for the construction of complex molecules and in numerous total syntheses of natural products. In addition, elegant enantioselective methodologies have granted access to important scalemic products for the pharmaceutical and agrochemical industry. However, most of these methods rely on the desymmetrization of prochiral alkyl groups leading to the formation of the stereogenic center distal to the activation site. The activation of enantiotopic secondary C−H bonds remains underdeveloped, mainly attributed to their low intrinsic reactivity, with only one report on the synthesis of β-lactams. To address this long-standing challenge, a highly reactive Pd/NHC catalytic system was design and optimized for the synthesis of chiral indanes. A variety of products were obtained in good to high yields and high enantioselectivities using IBioxR NHC ligands developed by the Glorius group. Furthermore, the first synthesis of chiral 3ary amides by Pd(0)-catalyzed C(sp3)−H activation was described in consistent high enantioselectivites. Moreover, a stereochemical analysis provided valuable insights on the relationship between ligand structure and enantioinduction. Mechanistic studies were performed on the newly developed Pd/NHC catalyzed activation of methylene C−H bonds to examine the selectivity trends observed in the course of the development of the Pd(0)-catalyzed C(sp3)−H activation reaction. For instance, primary C−H bonds are preferentially activated over secondary C−H bonds or the selectivity between C−H bonds is altered by changing the substituent in α-position. In contrast to C(sp2)−H bond activation, where the use of Hammet plots has helped to quantify the influence of substituents with different electronic properties on the C−H activation process, the design and development of new C(sp3)−H activation methods has been mainly guided by chemical intuition. In this context, a reactivity scale was constructed putting C(sp3)−H bonds differing in α-substitution on a series of most to least reactive by performing initial rate experiments. Prior kinetic isotope studies and kinetic analysis of the reaction to obtain the orders in reaction components confirmed the C−H activation step to be rate-limiting, thus suggesting that the comparison of the different C−H bonds is significant. The activation of remote C(sp3)−H bonds enabled by 1,4-Pd shift has experienced significant growth in the past 5 years. With this strategy new type of complex molecules, lacking structural motifs promoting the ring closure otherwise required for direct C−H activation, could be synthesized. Nevertheless, no enantioselective methods are reported to this date. To this end, new substrates were designed and synthesized and preliminary screening of chiral ligand families previously applied in direct enantioselective Pd(0)-catalyzed C−H activation was undertaken to contribute to the progress of asymmetric remote C(sp3)−H activation

Information retrieval in multimedia databases using relevance feedback algorithms. Applying logistic regression to relevance feedback in image retrieval systems

This master tesis deals with the problem of image retrieval from large image databases. A particularly interesting problem is the retrieval of all images which are similar to one in the user's mind, taking into account his/her feedback which is expressed as positive or negative preferences for the images that the system progressively shows during the search. Here, a novel algorithm is presented for the incorporation of user preferences in an image retrieval system based exclusively on the visual content of the image, which is stored as a vector of low-level features. The algorithm considers the probability of an image belonging to the set of those sought by the user, and models the logit of this probability as the output of a linear model whose inputs are the low level image features. The image database is ranked by the output of the model and shown to the user, who selects a few positive and negative samples, repeating the process in an iterative way until he/she is satisfied. The problem of the small sample size with respect to the number of features is solved by adjusting several partial linear models and combining their relevance probabilities by means of an ordered weighted averaged (OWA) operator. Experiments were made with 40 users and they exhibited good performance in finding a target image (4 iterations on average) in a database of about 4700 imagesZuccarello, PD. (2007). Information retrieval in multimedia databases using relevance feedback algorithms. Applying logistic regression to relevance feedback in image retrieval systems. http://hdl.handle.net/10251/12196Archivo delegad

Mark Sandman vs. Triumph Group and Liberty Mutal Insurance Company: Reply Brief

Petition for Review from the Board of Review of the Industrial Commission of Utah Benjamin A. Sims Administrative Law Judg

Satellite observations of reconnection between emerging and pre-existing small-scale magnetic fields

We report multi-wavelength ultraviolet observations taken with the IRIS satellite, concerning the emergence phase in the upper chromosphere and transition region of an emerging flux region (EFR) embedded in the unipolar plage of active region NOAA 12529. The photospheric configuration of the EFR is analyzed in detail benefitting from measurements taken with the spectropolarimeter aboard the Hinode satellite, when the EFR was fully developed. In addition, these data are complemented by full-disk, simultaneous observations of the SDO satellite, relevant to the photosphere and the corona. In the photosphere, magnetic flux emergence signatures are recognized in the fuzzy granulation, with dark alignments between the emerging polarities, cospatial with highly inclined fields. In the upper atmospheric layers, we identify recurrent brightenings that resemble UV bursts, with counterparts in all coronal passbands. These occur at the edges of the EFR and in the region of the arch filament system (AFS) cospatial to the EFR. Jet activity is also found at chromospheric and coronal levels, near the AFS and the observed brightness enhancement sites. The analysis of the IRIS line profiles reveals the heating of dense plasma in the low solar atmosphere and the driving of bi-directional high-velocity flows with speeds up to 100 km/s at the same locations. Furthermore, we detect a correlation between the Doppler velocity and line width of the Si IV 1394 and 1402 \AA{} line profiles in the UV burst pixels and their skewness. Comparing these findings with previous observations and numerical models, we suggest evidence of several long-lasting, small-scale magnetic reconnection episodes between the emerging bipole and the ambient field. This process leads to the cancellation of a pre-existing photospheric flux concentration of the plage with the opposite polarity flux patch of the EFR. [...]Comment: 4 pages, 2 figures, to be published in "Nuovo Cimento C" as proceeding of the Third Meeting of the Italian Solar and Heliospheric Communit