309 research outputs found
Temporal evolution of multiple evaporating ribbon sources in a solar flare
We present new results from the Interface Region Imaging Spectrograph showing
the dynamic evolution of chromospheric evaporation and condensation in a flare
ribbon, with the highest temporal and spatial resolution to date. IRIS observed
the entire impulsive phase of the X-class flare SOL2014-09-10T17:45 using a 9.4
second cadence `sit-and-stare' mode. As the ribbon brightened successively at
new positions along the slit, a unique impulsive phase evolution was observed
for many tens of individual pixels in both coronal and chromospheric lines.
Each activation of a new footpoint displays the same initial coronal up-flows
of up to ~300 km/s, and chromospheric downflows up to 40 km/s. Although the
coronal flows can be delayed by over 1 minute with respect to those in the
chromosphere, the temporal evolution of flows is strikingly similar between all
pixels, and consistent with predictions from hydrodynamic flare models. Given
the large sample of independent footpoints, we conclude that each flaring pixel
can be considered a prototypical, `elementary' flare kernel.Comment: Accepted for publication - ApJ letter
New technique to measure the cavity defects of Fabry-Perot interferometers
(Abridged):
We define and test a new technique to accurately measure the cavity defects
of air-spaced FPIs, including distortions due to the spectral tuning process
typical of astronomical observations. We further develop a correction technique
to maintain the shape of the cavity as constant as possible during the spectral
scan. These are necessary steps to optimize the spectral transmission profile
of a two-dimensional spectrograph using one or more FPIs.
We devise a generalization of the techniques developed for the so-called
phase-shifting interferometry to the case of FPIs. The technique is applicable
to any FPI that can be tuned via changing the cavity spacing (-axis), and
can be used for any etalon regardless of the coating' reflectivity. The major
strength of our method is the ability to fully characterize the cavity during a
spectral scan, allowing for the determination of scan-dependent modifications
of the plates. As a test, we have applied this technique to three 50 mm
diameter interferometers, with cavity gaps ranging between 600 micron and 3 mm,
coated for use in the visible range.
We obtain accurate and reliable measures of the cavity defects of air-spaced
FPIs, and of their evolution during the entire spectral scan. Our main, and
unexpected, result is that the relative tilt between the two FPI plates varies
significantly during the spectral scan, and can dominate the cavity defects; in
particular, we observe that the tilt component at the extremes of the scan is
sensibly larger than at the center of the scan. Exploiting the capability of
the electronic controllers to set the reference plane at any given spectral
step, we develop a correction technique that allows the minimization of the
tilt during a complete spectral scan. The correction remains highly stable over
long periods, well beyond the typical duration of astronomical observations.Comment: 15 pages, 20+ figures, accepted for publication in A&A. Two
additional movies are available in the online version of the pape
TRACE and ground-based observations of microflares
We present high temporal and spatial resolution, multiwavelength observations of small-scale activity phenomena. The data were obtained during a coordinated campaign between ground-based and space observatories. Our analysis supports the concept that small flares are just miniature flares, i.e. that the same physical processes are at work, but that it is mandatory to have an excellent resolution in order to clearly identify them. In particular, we find that chromospheric downward motions are a very distinctive characteristic of the flare phenomenon even in tiny events
Anchoring selenido-carbonyl ruthenium clusters to functionalised silica xerogels
Silica Xerogels containing carbonyl Ru3Se2 nido clusters were prepared in three different ways. The simple dispersion of [Ru3(mu3-Se)2(CO)7(PPh 3)2] via sol gel process produces an inhomogeneous material; by contrast, homogeneous xerogels were obtained by reaction of [Ru3(mu3-Se)2(CO)8(PPh 3)] with functionalised xerogels containing grafted diphenylphosphine moieties and by reaction of [Ru3(CO)12] with a xerogel containing grafted phosphine-selenide groups. The reaction between [Ru3(CO)12] and dodecyldiphenylphosphine selenide led to the formation of four selenido carbonyl clusters, which are soluble in hydrocarbon solvents and can be deposited as thin films from their solution by slow evaporation. Xerogéis de silica contendo carbonilas metálicas polinucleares (clusters) com estrutura nido Ru3Se2 foram preparadas através de três rotas diferentes. A simples dispersão do cluster [Ru3(mi3-Se)2(CO)7(PPh 3)2] pelo processo sol gel produz um material não homogêneo. Xerogéis homogêneos foram obtidos através da reação de [Ru3(mi3-Se)2(CO)8(PPh 3)] com xerogéis funcionalizados contendo fragmentos difenilfosfino enxertados, e pela reação do [Ru3(CO)12] com um xerogel contendo grupos fosfinoseleneto enxertados. A reação entre [Ru3(CO)12] e seleneto de dodecildifenilfosfinoseleneto resultou na formação de quatro clusters contendo ligantes carbonil e seleneto, solúveis em solventes apolares e que podem ser empregados na confecção de filmes
Design of functionalized polysiloxanes: synthesis and study on sulfur-containing xerogels with mono- and bifunctional surface layer
Urea- and thiourea-containing trialkoxysilanes were prepared with a high yield and purity. By sol-gel processing of these precursors the new functionalized xerogels were synthesized and characterized by FTIR, elemental analysis, thermal analysis and adsorption method. Mono- and bifunctionalized xerogels were easily prepared by mixing of precursors in the desired ratios together with tetraethyl orthosilicate. Obtained functional polysiloxanes are perspective materials for metal sorption and hybrid catalysis applications
The Three-dimensional Evolution of Rising, Twisted Magnetic Flux Tubes in a Gravitationally Stratified Model Convection Zone
We present three-dimensional numerical simulations of the rise and
fragmentation of twisted, initially horizontal magnetic flux tubes which evolve
into emerging Omega-loops. The flux tubes rise buoyantly through an
adiabatically stratified plasma that represents the solar convection zone. The
MHD equations are solved in the anelastic approximation, and the results are
compared with studies of flux tube fragmentation in two dimensions. We find
that if the initial amount of field line twist is below a critical value, the
degree of fragmentation at the apex of a rising Omega-loop depends on its
three-dimensional geometry: the greater the apex curvature of a given
Omega-loop, the lesser the degree of fragmentation of the loop as it approaches
the photosphere. Thus, the amount of initial twist necessary for the loop to
retain its cohesion can be reduced substantially from the two-dimensional
limit. The simulations also suggest that as a fragmented flux tube emerges
through a relatively quiet portion of the solar disk, extended crescent-shaped
magnetic features of opposite polarity should form and steadily recede from one
another. These features eventually coalesce after the fragmented portion of the
Omega-loop emerges through the photosphere.Comment: 17 pages, 17 figures, uses AAS LaTeX macros v5.0. ApJ, in pres
Record Processing in ITACA, the New Italian Strong-Motion Database
The development of the new Italian strong-motion database ITACA
(ITalian AC-celerometric Archive, http://itaca.mi.ingv.it) is in progress under the
sponsorship of the National Department of Civil Protection (DPC) within Project
S4, in the framework of DPC-INGV 2007–2009 research agreement. This work
started from the alpha version of ITACA [8], where 2,182 3-component records
from 1,004 earthquakes, mainly recorded by the National Accelerometric Network,
RAN, operated by DPC, were processed and included in the database. Earthquake
metadata, recording station information and reports on the available geologicalgeophysical
information of 452 recording sites, corresponding to about 70% of the
total, were also included. Subsequently, ITACA has been updated and will reach
its final stage by the end of Project S4, around mid-2010, with additional features,
improved information about recording stations, and updated records, including the
Mw6.3 L’Aquila earthquake. All records were re-processed with respect to the alpha
version [9], with a special care to preserve information about late-triggered events
and to ensure compatibility of corrected records, i.e., velocity and displacement
traces obtained by the first and second integral of the corrected acceleration should
not be affected by unrealistic trends. After a short introduction of ITACA and its
most relevant features and statistics, this paper mainly deals with the newly adopted
processing scheme, with reference to the problems encountered and the solutions
that have been devised
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