424 research outputs found
Pulse-beam heating of deep atmospheric layers triggering their oscillations and upwards moving shocks that can modulate the reconnection in solar flares
We study processes occurring after a sudden heating of the chromosphere at
the flare arcade footpoints which is assumed to be caused by particle beams.
For the numerical simulations we adopt a 2-D magnetohydrodynamic (MHD) model,
in which we solve a full set of the time-dependent MHD equations by means of
the FLASH code, using the Adaptive Mesh Refinement (AMR) method. In the initial
state we consider a model of the solar atmosphere with densities according to
the VAL-C model and the magnetic field arcade having the X-point structure
above, where the magnetic reconnection is assumed. We found that the sudden
pulse-beam heating of the chromosphere at the flare arcade footpoints generates
magnetohydrodynamic shocks, one propagating upwards and the second one
propagating downwards in the solar atmosphere. The downward moving shock is
reflected at deep and dense atmospheric layers and triggers oscillations of
these layers. These oscillations generate the upwards moving
magnetohydrodynamic waves that can influence the above located magnetic
reconnection in a quasi-periodic way. Because these processes require a sudden
heating in very localized regions in the chromosphere therefore they can be
also associated with seismic waves
Radio fiber bursts and fast magnetoacoustic wave trains
We present a model for dm-fiber bursts that is based on assuming fast sausage
magnetoacoustic wave trains that propagate along a dense vertical filament or
current sheet. Eight groups of dm-fiber bursts that were observed during solar
flares were selected and analyzed by the wavelet analysis method. To model
these fiber bursts we built a semi-empirical model. We also did
magnetohydrodynamic simulations of a propagation of the magnetoacoustic wave
train in a vertical and gravitationally stratified current sheet. In the
wavelet spectra of the fiber bursts computed at different radio frequencies we
found the wavelet tadpoles, whose head maxima have the same frequency drift as
the drift of fiber bursts. It indicates that the drift of these fiber bursts
can be explained by the propagating fast sausage magnetoacoustic wave train.
Using new semi-empirical and magnetohydrodynamic models with a simple radio
emission model we generated the artificial radio spectra of the fiber bursts,
which are similar to the observed ones.Comment: 7 pages, 10 figure
Spectroscopic Observations and Modelling of Impulsive Alfv\'en Waves Along a Polar Coronal Jet
Using the Hinode/EIS 2 spectroscopic observations, we study the intensity,
velocity, and FWHM variations of the strongest Fe XII 195.12 \AA\ line along
the jet to find the signature of Alfv\'en waves. We simulate numerically the
impulsively generated Alfv\'en waves within the vertical Harris current-sheet,
forming the jet plasma flows, and mimicking their observational signatures.
Using the FLASH code and the atmospheric model with embedded weakly expanding
magnetic field configuration within a vertical Harris current-sheet, we solve
the two and half-dimensional (2.5-D) ideal magnetohydrodynamic (MHD) equations
to study the evolution of Alfv\'en waves and vertical flows forming the plasma
jet. At a height of from the base of the jet, the
red-shifted velocity component of Fe XII 195.12 \AA\ line attains its maximum
() which converts into a blue-shifted one between the
altitude of . The spectral intensity continously increases up
to , while FWHM still exhibits the low values with almost
constant trend. This indicates that the reconnection point within the jet's
magnetic field topology lies in the corona from its
footpoint anchored in the Sun's surface. Beyond this height, FWHM shows a
growing trend. This may be the signature of Alfv\'en waves that impulsively
evolve due to reconnection and propagate along the jet. From our numerical
data, we evaluate space- and time- averaged Alfv\'en waves velocity amplitudes
at different heights in the jet's current-sheet, which contribute to the
non-thermal motions and spectral line broadening. The synthetic width of Fe XII
line exhibits similar trend of increment as in the
observational data, possibly proving the existence of impulsively generated (by
reconnection) Alfv\'en waves which propagate along the jet
BART 2001–2004: An intelligent robotic observatory
BART is a robotic observatory located at the Astronomical Institute in Ondrejov, Czech Republic. It is a relatively low-cost (25 cm + two wide-field lenses) device developed for rapid follow-ups of GRBs. Since 2001 when it started
to observe, it has done several such observations. Meanwhile, photometric monitoring tasks are performed, using an intelligent selection algorithm. Not only the
telescope is automatic, the entire observatory does not require human presence: system prepares the schedule, observes, registers the images and stores them into
database without human intervention: human assistance is reduced to maintenance and weather checking. BART is a primary developing platform for RTS2, the robotic
telescope operation software, which allows for unattended observation as well as control of the entire observatory
Advances on Testing C-Planarity of Embedded Flat Clustered Graphs
We show a polynomial-time algorithm for testing c-planarity of embedded flat
clustered graphs with at most two vertices per cluster on each face.Comment: Accepted at GD '1
Searching for galactic sources in the Swift GRB catalog
Since the early 1990s Gamma Ray Bursts have been accepted to be of
extra-galactic origin due to the isotropic distribution observed by BATSE and
the redshifts observed via absorption line spectroscopy. Nevertheless, upon
further examination at least one case turned out to be of galactic origin. This
particular event presented a Fast Rise, Exponential Decay (FRED) structure
which leads us to believe that other FRED sources might also be Galactic. This
study was set out to estimate the most probable degree of contamination by
galactic sources that certain samples of FREDs have. In order to quantify the
degree of anisotropy the average dipolar and quadripolar moments of each sample
of GRBs with respect to the galactic plane were calculated. This was then
compared to the probability distribution of simulated samples comprised of a
combination of isotropically generated sources and galactic sources. We observe
that the dipolar and quadripolar moments of the selected subsamples of FREDs
are found more than two standard deviations outside those of random
isotropically generated samples.The most probable degree of contamination by
galactic sources for the FRED GRBs of the Swift catalog detected until February
2011 that do not have a known redshift is about 21 out of 77 sources which is
roughly equal to 27%. Furthermore we observe, that by removing from this sample
those bursts that may have any type of indirect redshift indicator and multiple
peaks gives the most probable contamination increases up to 34% (17 out of 49
sources). It is probable that a high degree of contamination by galactic
sources occurs among the single peak FREDs observed by Swift.Comment: Published to A&A, 4 pages, 5 figures, this arXiv version includes
appended table with all the bursts considered in this stud
- …