Spontaneous transition to a fast 3D turbulent reconnection regime

We show how the conversion of magnetic field energy via magnetic reconnection can progress in a fully three-dimensional, fast, volume-filling regime. An initial configuration representative of many laboratory, space and astrophysical plasmas spontaneously evolves from the well-known regime of slow, resistive reconnection to a new regime that allows to explain the rates of energy transfer observed in jets emitted from accretion disks, in stellar/solar flare processes as well as in laboratory plasmas. This process does not require any pre-existing turbulence seed which often is not observed in the host systems prior to the onset of the energy conversion. The dynamics critically depends on the interplay of perturbations developing along the magnetic field lines and across them, a process possible only in three-dimensions. The simulations presented here are the first able to show this transition in a fully three-dimensional configuration.Comment: 6 pages, 6 figure

Characteristics of polar coronal hole jets

High spatial- and temporal-resolution images of coronal hole regions show a dynamical environment where mass flows and jets are frequently observed. These jets are believed to be important for the coronal heating and the acceleration of the fast solar wind. We studied the dynamics of two jets seen in a polar coronal hole with a combination of imaging from EIS and XRT onboard Hinode. We observed drift motions related to the evolution and formation of these small-scale jets, which we tried to model as well. We found observational evidence that supports the idea that polar jets are very likely produced by multiple small-scale reconnections occurring at different times in different locations. These eject plasma blobs that flow up and down with a motion very similar to a simple ballistic motion. The associated drift speed of the first jet is estimated to be $\approx$ 27 km s$^{-1}$. The average outward speed of the first jet is $\approx 171$ km s$^{-1}$, well below the escape speed, hence if simple ballistic motion is considered, the plasma will not escape the Sun. The second jet was observed in the south polar coronal hole with three XRT filters, namely, C$_{-}$poly, Al$_{-}$poly, and Al$_{-}$mesh filters. We observed that the second jet drifted at all altitudes along the jet with the same drift speed of $\simeq$ 7 km s$^{-1}$. The enhancement in the light curves of low-temperature EIS lines in the later phase of the jet lifetime and the shape of the jet's stack plots suggests that the jet material is falls back, and most likely cools down. To support this conclusion, the observed drifts were interpreted within a scenario where reconnection progressively shifts along a magnetic structure, leading to the sequential appearance of jets of about the same size and physical characteristics. On this basis, we also propose a simple qualitative model that mimics the observations.Comment: Accepted Astronomy and Astrophysic

Exploring the mechanism of formation of native-like and precursor amyloid oligomers for the native acylphosphatase from Sulfolobus solfataricus

Over 40 human diseases are associated with the formation of well-defined proteinaceous fibrillar aggregates. Since the oligomers precursors to the fibrils are increasingly recognized to be the causative agents of such diseases, it is important to elucidate the mechanism of formation of these early species. The acylphosphatase from Sulfolobus solfataricus is an ideal system as it was found to form, under conditions in which it is initially native, two types of prefibrillar aggregates: (1) initial enzymatically active aggregates and (2) oligomers with characteristics reminiscent of amyloid protofibrils, with the latter originating from the structural reorganization of the initial assemblies. By studying a number of protein variants with a variety of biophysical techniques, we have identified the regions of the sequence and the driving forces that promote the first aggregation phase and show that the second phase consists in a cooperative conversion involving the entire globular fol

Reconnection in a slow Coronal Mass Ejection

This paper aims at studying reconnection occurring in the aftermath of the 28 May 2004, CME, first imaged by the LASCO (Large Angle and Spectrometric Coronagraph) C2 at 11:06 UT. The CME was observed in White Light and UV radiation: images acquired by the LASCO C2 and C3 coronagraphs and spectra acquired by UVCS (Ultraviolet Coronagraph Spectrometer) allowed us to identify the level at which field lines, stretched outwards by the CME ejection, reconnect below the CME bubble. As the CME propagates outwards, reconnection occurs at increasingly higher levels. The process goes on at a low pace for several hours: here we give the profile of the reconnection rate vs. heliocentric distance over a time interval of &asymp;14 h after the CME onset, extending estimates of the reconnection rate to larger distances than previously inferred by other authors. The reconnection rate appears to decrease with time/altitude. We also calculate upper and lower limits to the density in the diffusion region between 4 and 7 <I>R</I>_&#x2299; and conclude by comparing estimates of the classical and anomalous resistivity in the diffusion region with the value inferred from the data. The latter turns out to be &ge;5 order of magnitudes larger than predicted by classical or anomalous theories, pointing to the need of identifying the process responsible for the observed value

Spontaneous non-steady magnetic reconnection within the solar environment

This work presents a 2.5-dimensional simulation study of the instability of current-sheets located in a medium with a strong density variation along the current layer. The initial force-free configuration is observed to undergo a two-stage evolution consisting of an abrupt regime transition from a slow to a fast reconnection process leading the system to a final chaotic configuration. Yet, the onset of the fast phase is not determined by the presence of any anomalous enhancement in plasma's local resistivity, but rather is the result of a new mechanism discovered in Lapenta (2008)* and captured only when sufficient resolution is present. Hence, the effects of the global resistivity, the global viscosity and the plasma beta on the overall dynamics are considered. This mechanism allowing the transition from slow to fast reconnection provides a simple but effective model of several processes taking place within the solar atmosphere from the high chromosphere up to the low corona. In fact, the understanding of a spontaneous transition to a self-feeding fast reconnection regime as well as its macroscopic evolution is the first and fundamental step to produce realistic models of all those phenomena requiring fast (and high power) triggering events (* Lapenta G. 2008, Phys. Rev. Lett., 100, 235001).Comment: 29 pages, 10 figure

Multispacecraft observations of a prominence eruption

On 9 May 2007 a prominence eruption occurred at the West limb. Remarkably, the event was observed by the STEREO/EUVI telescopes and by the HINODE/EIS and SOHO/UVCS spectrometers. We present results from all these instruments. High-cadence (~37 s) data from STEREO/EUVI A and B in the He II λ304 line were used to study the 3-D shape and expansion of the prominence. The high spatial resolution EUVI images (~1.5"/pixel) have been used to infer via triangulation the 3-D shape and orientation of the prominence 12 min after the eruption onset. At this time the prominence has mainly the shape of a "hook" highly inclined southward, has an average thickness of 0.068 R⊙, a length of 0.43 R⊙ and lies, in first approximation, on a plane. Hence, the prominence is mainly a 2-D structure and there is no evidence for a twisted flux rope configuration. HINODE/EIS was scanning with the 2" slit the region where the filament erupted. The EIS spectra show during the eruption remarkable non-thermal broadening (up to ~100 km s−1) in the region crossed by the filament in spectral lines emitted at different temperatures, possibly with differences among lines from higher Fe ionization stages. The CME was also observed by the SOHO/UVCS instrument: the spectrograph slit was centered at 1.7 R⊙, at a latitude of 5° SW and recorded a sudden increase in the O VI λλ1032–1037 and Si XII λ520 spectral line intensities, representative of the CME front transit

Compression algorithm for Multi Element Telescope for Imaging and Spectroscopy (METIS)

The compression algorithm defined for METIS (Multi Element Telescope for Imaging and Spectroscopy) arises from the standard CCSDS 123.0-r-1, that has been modified and adapted to the mission purposes, and integrated with other pieces of software to let the compressor work in the most efficient way with the expected acquisitions of the sensor. The major modification is the insertion in the prediction loop of a uniform scalar quantizer, extending the standard to a near-lossless version; in addition a local decoder has been added as well, in order to keep a local copy of the dequantized residuals to allow a correct reconstruction at the decoder side. A lossy compression can even be executed in a variable-quality way, meaning that it is possible to change the quantization step size among successive image lines. The ability of the original software to process three-dimensional images has been kept but adapted to the mission needs: instead of considering wavelength, consecutive acquisitions are collected together to build up the 3D cube, so that time becomes the third dimension; and since solar acquisitions change really slowly in time, the effectiveness of this adjustment works very well and prediction of the current pixels becomes much more accurate if considering the previous acquisitions ones. Further, a pre-processing routine has been developed to exploit the geometry of the images; it consists in a re-mapping of the pixels in order to take advantage of the radial structure of solar acquisitions, through a function that has been named “radialization”. It receives the standard image as input, and computes for every pixel the distance and the angle with respect to the center; these become the two new coordinates, as it happens when switching from a Cartesian system to a polar one. The triangular-shaped output is then centered and padded in order to keep a rectangular structure, and matrices for the two dimensions are kept, so that the whole piece of code can be executed only once, and the “radialized” image can be then obtained by a simple mapping using these structures, resulting in a really light operation from a computational point of view; a further advantage can be identified in the lack of interpolation among pixels, so that eventually, the compression of the image, or better of a section of it, can occur losslessly. Radialization also simplifies a possible selection of areas of interest of the image: for example it would be possible to keep the nearest solar corona area coded losslessly, and decreasing linearly the quality of the reconstruction in a radial sense by successive circular corona-shaped structures, by using variable lossy compression for consecutive radialized image lines

FIB/SEM and SEM/EDS microstructural analysis of metal-ceramic and zirconia-ceramic interfaces

Recently introduced FIB/SEM analysis in microscopy seems to provide a high-resolution characterization of the samples by 3D (FIB) cross-sectioning and (SEM) high resolution imaging. The aim of this study was to apply the FIB/SEM and SEM/EDS analysis to the interfaces of a metal-ceramic vs. two zirconiaceramic systems. Plate samples of three different prosthetic systems were prepared in the dental lab following the manufacturers’ instructions, where metal-ceramic was the result of a ceramic veneering (porcelain-fused-tometal) and the two zirconia- ceramic systems were produced by the dedicated CAD-CAM procedures of the zirconia cores (both with final sintering) and then veneered by layered or heat pressed ceramics. In a FIB/SEM equipment (also called DualBeam), a thin layer of platinum (1μm) was deposited on samples surface crossing the interfaces, in order to protect them during milling. Then, increasingly deeper trenches were milled by a focused ion beam, first using a relatively higher and later using a lower ion current (from 9 nA to 0.28 nA, 30KV). Finally, FEG-SEM (5KV) micrographs (1000–50,000X) were acquired. In a SEM the analysis of the morphology and internal microstructure was performed by 13KV secondary and backscattered electrons signals (in all the samples). The compositional maps were then performed by EDS probe only in the metal-ceramic system (20kV). Despite the presence of many voids in all the ceramic layers, it was possible to identify: (1) the grain structures of the metallic and zirconia substrates, (2) the thin oxide layer at the metalceramic interface and its interactions with the first ceramic layer (wash technique), (3) the roughness of the two different zirconia cores and their interactions with the ceramic interface, where the presence of zirconia grains in the ceramic layer was reported in two system possibly due to sandblasting before ceramic firing