170 research outputs found
Impulsive Heating of Solar Flare Ribbons Above 10 MK
The chromospheric response to the input of flare energy is marked by extended
extreme ultraviolet (EUV) ribbons and hard X-ray (HXR) footpoints. These are
usually explained as the result of heating and bremsstrahlung emission from
accelerated electrons colliding in the dense chromospheric plasma. We present
evidence of impulsive heating of flare ribbons above 10 MK in a two-ribbon
flare. We analyse the impulsive phase of SOL2013-11-09T06:38, a C2.6 class
event using data from Atmospheric Imaging Assembly (AIA) on board of Solar
Dynamics Observatory (SDO) and the Reuven Ramaty High Energy Solar
Spectroscopic Imager (RHESSI) to derive the temperature, emission measure and
differential emission measure of the flaring regions and investigate the
evolution of the plasma in the flaring ribbons. The ribbons were visible at all
SDO/AIA EUV/UV wavelengths, in particular, at 94 and 131 \AA\ filters,
sensitive to temperatures of 8 MK and 12 MK. Time evolution of the emission
measure of the plasma above 10 MK at the ribbons has a peak near the HXR peak
time. The presence of hot plasma in the lower atmosphere is further confirmed
by RHESSI imaging spectroscopy analysis, which shows resolved sources at 11-13
MK associated with at least one ribbon. We found that collisional beam heating
can only marginally explain the necessary power to heat the 10 MK plasma at the
ribbons.Comment: 21 pages, 15 figure
IRIS Observations of the Mg II h & k Lines During a Solar Flare
The bulk of the radiative output of a solar flare is emitted from the
chromosphere, which produces enhancements in the optical and UV continuum, and
in many lines, both optically thick and thin. We have, until very recently,
lacked observations of two of the strongest of these lines: the Mg II h & k
resonance lines. We present a detailed study of the response of these lines to
a solar flare. The spatial and temporal behaviour of the integrated
intensities, k/h line ratios, line of sight velocities, line widths and line
asymmetries were investigated during an M class flare (SOL2014-02-13T01:40).
Very intense, spatially localised energy input at the outer edge of the ribbon
is observed, resulting in redshifts equivalent to velocities of ~15-26km/s,
line broadenings, and a blue asymmetry in the most intense sources. The
characteristic central reversal feature that is ubiquitous in quiet Sun
observations is absent in flaring profiles, indicating that the source function
increases with height during the flare. Despite the absence of the central
reversal feature, the k/h line ratio indicates that the lines remain optically
thick during the flare. Subordinate lines in the Mg II passband are observed to
be in emission in flaring sources, brightening and cooling with similar
timescales to the resonance lines. This work represents a first analysis of
potential diagnostic information of the flaring atmosphere using these lines,
and provides observations to which synthetic spectra from advanced radiative
transfer codes can be compared.Comment: 12 pages, 14 figures, Accepted for publication in Astronomy and
Astrophysic
The spectral content of SDO/AIA 1600 and 1700 \AA\ filters from flare and plage observations
The strong enhancement of the ultraviolet emission during solar flares is
usually taken as an indication of plasma heating in the lower solar atmosphere
caused by the deposition of the energy released during these events. Images
taken with broadband ultraviolet filters by the {\em Transition Region and
Coronal Explorer} (TRACE) and {\em Atmospheric Imaging Assembly} (AIA 1600 and
1700~\AA) have revealed the morphology and evolution of flare ribbons in great
detail. However, the spectral content of these images is still largely unknown.
Without the knowledge of the spectral contribution to these UV filters, the use
of these rich imaging datasets is severely limited. Aiming to solve this issue,
we estimate the spectral contributions of the AIA UV flare and plage images
using high-resolution spectra in the range 1300 to 1900~\AA\ from the Skylab
NRL SO82B spectrograph. We find that the flare excess emission in AIA 1600~\AA\
is { dominated by} the \ion{C}{4} 1550~\AA\ doublet (26\%), \ion{Si}{1}
continua (20\%), with smaller contributions from many other chromospheric lines
such as \ion{C}{1} 1561 and 1656~\AA\ multiplets, \ion{He}{2} 1640~\AA,
\ion{Si}{2} 1526 and 1533~\AA. For the AIA 1700~\AA\ band, \ion{C}{1} 1656~\AA\
multiplet is the main contributor (38\%), followed by \ion{He}{2} 1640 (17\%),
and accompanied by a multitude of other, { weaker} chromospheric lines, with
minimal contribution from the continuum. Our results can be generalized to
state that the AIA UV flare excess emission is of chromospheric origin, while
plage emission is dominated by photospheric continuum emission in both
channels.Comment: Accepted for publication in ApJ Skylab NRL SO82B data used in this
work available at http://dx.doi.org/10.5525/gla.researchdata.68
Soft X-ray Pulsations in Solar Flares
The soft X-ray emissions of solar flares come mainly from the bright coronal
loops at the highest temperatures normally achieved in the flare process. Their
ubiquity has led to their use as a standard measure of flare occurrence and
energy, although the overwhelming bulk of the total flare energy goes
elsewhere. Recently Dolla et al. (2012) noted quasi-periodic pulsations (QPP)
in the soft X-ray signature of the X-class flare SOL2011-02-15, as observed by
the standard photometric data from the GOES (Geostationary Operational
Environmental Satellite) spacecraft. In this paper we analyze the suitability
of the GOES data for this kind of analysis and find them to be generally
valuable after September, 2010 (GOES-15). We then extend the Dolla et al.
result to a complete list of X-class flares from Cycle 24, and show that most
of them (80%) display QPPs in the impulsive phase. The pulsations show up
cleanly in both channels of the GOES data, making use of time-series of
irradiance differences (the digital time derivative on the 2-s sampling). We
deploy different techniques to characterize the periodicity of GOES pulsations,
considering the red-noise properties of the flare signals, finding a range of
chracteristic time scales of the QPPs for each event, but usually with no
strong signature of a single period dominating in the power spectrum. The QPP
may also appear on somewhat longer time scales during the later gradual phase,
possibly with a greater tendency towards coherence, but the sampling noise in
GOES difference data for large irradiance values (X-class flares) makes these
more uncertain. We show that there is minimal phase difference between the
differenced GOES energy channels, or between them and the hard X-ray variations
on short time scales. During the impulsive phase the footpoints of the
newly-forming flare loops may also contribute to the observed soft X-ray
variations.Comment: 15 pages, 7 figure
Prediction of the mechanical properties of granites by ultrasonic pulse velocity and Schmidt hammer hardness
The present work deals with the use of simple and economical non destructive
techniques, ultrasonic pulse velocity and Schmidt hammer to predict the strength and elastic
properties of granitic stones that are characteristic in ancient masonry constructions. Good
correlations between NDTs and strength and modulus of elasticity were found, which indicate
them as appropriate techniques for estimating the mechanical properties
Experimental properties of granites
In the present work, a study of the tensile mechanical behavior of a set of twelve selected
granite lithotypes is carried out. Some petrographic aspects such as grain size and presence of
planar anisotropy explain the variation on the parameters that characterize the tensile
behavior, like the strength and fracture energy. Other factors that influence the tensile
mechanical behavior, such as weathering degree and physical properties like porosity and
density, are also analyzed. Statistical correlations between the ultrasonic pulse velocity and
the mechanical and physical properties are proposed
Analysis of weathering and internal texture on the engineering properties of granites
The research significance of the present paper concerns the need of knowing the mechanical
properties of stone existing in the ancient constructions, when a suitable methodology for their rehabilitation or
strengthening is followed. The rehabilitation of ancient constructions, mainly the ones pertaining to the architectural
heritage is a demand of modern societies. The deep knowledge of mechanical and fracture properties of
the stone is also an important advantage, when structural analysis is needed for the evaluation of the safety conditions
of ancient constructions. Thus, this paper aims at providing large information about the influence of factors
such as weathering state and planar anisotropy on the engineering properties of Portuguese granites. It was
found that both weathering and internal structure play a major role on the mechanical tensile and compressive
behavior of granites. Besides, the non-destructive evaluation reveal as possible technique to predict the weathering
and anisotropy of granites
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