614 research outputs found
Surveillance and outbreak reports Outbreak of Shigella sonnei infections in the Orthodox Jewish community of Antwerp, Belgium, April to
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Continuous upflows and sporadic downflows observed in active regions
We present a study of the temporal evolution of coronal loops in active
regions and its implications for the dynamics in coronal loops. We analyzed
images of the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics
Observatory (SDO) at multiple temperatures to detect apparent motions in the
coronal loops. Quasi-periodic brightness fluctuations propagate upwards from
the loop footpoint in hot emission at 1MK, while sporadic downflows are seen in
cool emission below 1MK. The upward motion in hot emission increases just after
the cool downflows. The apparent propagating pattern suggests a hot upflow from
the loop footpoints, and is considered to supply hot plasma into the coronal
loop, but a wavelike phenomenon cannot be ruled out. Coronal condensation
occasionally happens in the coronal loop, and the cool material flows down to
the footpoint. Emission from cool plasma could have a significant contribution
to hot AIA channels in the event of coronal condensation.Comment: 5 pages, 6 figures, A&A in pres
A Tale Of Two Spicules: The Impact of Spicules on the Magnetic Chromosphere
We use high-resolution observations of the Sun in Ca II H 3968 A from the
Solar Optical Telescope on Hinode to show that there are at least two types of
spicules that dominate the structure of the magnetic solar chromosphere. Both
types are tied to the relentless magnetoconvective driving in the photosphere,
but have very different dynamic properties. ``Type-I'' spicules are driven by
shock waves that form when global oscillations and convective flows leak into
the upper atmosphere along magnetic field lines on 3-7 minute timescales.
``Type-II'' spicules are much more dynamic: they form rapidly (in ~10s), are
very thin (<200km wide), have lifetimes of 10-150s (at any one height) and seem
to be rapidly heated to (at least) transition region temperatures, sending
material through the chromosphere at speeds of order 50-150 km/s. The
properties of Type II spicules suggest a formation process that is a
consequence of magnetic reconnection, typically in the vicinity of magnetic
flux concentrations in plage and network. Both types of spicules are observed
to carry Alfven waves with significant amplitudes of order 20 km/s.Comment: 8 pages, 5 figures, accepted for Hinode special issue of PAS
A polynomial-time algorithm for optimizing over N-fold 4-block decomposable integer programs
In this paper we generalize N-fold integer programs and two-stage integer
programs with N scenarios to N-fold 4-block decomposable integer programs. We
show that for fixed blocks but variable N, these integer programs are
polynomial-time solvable for any linear objective. Moreover, we present a
polynomial-time computable optimality certificate for the case of fixed blocks,
variable N and any convex separable objective function. We conclude with two
sample applications, stochastic integer programs with second-order dominance
constraints and stochastic integer multi-commodity flows, which (for fixed
blocks) can be solved in polynomial time in the number of scenarios and
commodities and in the binary encoding length of the input data. In the proof
of our main theorem we combine several non-trivial constructions from the
theory of Graver bases. We are confident that our approach paves the way for
further extensions
Identification of different types of kink modes in coronal loops: principles and application to TRACE results
We explore the possible observational signatures of different types of kink
modes (horizontal and vertical oscillations in their fundamental mode and
second harmonic) that may arise in coronal loops, with the aim of determining
how well the individual modes can be uniquely identified from time series of
images. A simple, purely geometrical model is constructed to describe the
different types of kink-mode oscillations. These are then `observed' from a
given direction. In particular, we employ the 3D geometrical parameters of 14
TRACE loops of transverse oscillations to try to identify the correct observed
wave mode. We find that for many combinations of viewing and loop geometry it
is not straightforward to distinguish between at least two types of kink modes
just using time series of images. We also considered Doppler signatures and
find that these can help obtain unique identifications of the oscillation modes
when employed in combination with imaging. We then compare the modeled spatial
signatures with the observations of 14 TRACE loops. We find that out of three
oscillations previously identified as fundamental horizontal mode oscillations,
two cases appear to be fundamental vertical mode oscillations (but possibly
combined with the fundamental horizontal mode), and one case appears to be a
combination of the fundamental vertical and horizontal modes, while in three
cases it is not possible to clearly distinguish between the fundamental mode
and the second-harmonic of the horizontal oscillation. In five other cases it
is not possible to clearly distinguish between a fundamental horizontal mode
and the second-harmonic of a vertical mode.Comment: 12 pages, 10 figures, 2 tables. A&A in pres
Sparse Sums of Positive Semidefinite Matrices
Recently there has been much interest in "sparsifying" sums of rank one
matrices: modifying the coefficients such that only a few are nonzero, while
approximately preserving the matrix that results from the sum. Results of this
sort have found applications in many different areas, including sparsifying
graphs. In this paper we consider the more general problem of sparsifying sums
of positive semidefinite matrices that have arbitrary rank.
We give several algorithms for solving this problem. The first algorithm is
based on the method of Batson, Spielman and Srivastava (2009). The second
algorithm is based on the matrix multiplicative weights update method of Arora
and Kale (2007). We also highlight an interesting connection between these two
algorithms.
Our algorithms have numerous applications. We show how they can be used to
construct graph sparsifiers with auxiliary constraints, sparsifiers of
hypergraphs, and sparse solutions to semidefinite programs
Observations of quasi-periodic phenomena associated with a large blowout solar jet
A variety of periodic phenomena have been observed in conjunction with large
solar jets. We aim to find further evidence for {(quasi-)}periodic behaviour in
solar jets and determine what the periodic behaviour can tell us about the
excitation mechanism and formation process of the large solar jet. Using the
304 {\AA} (He-II), 171 {\AA} (Fe IX), 193 {\AA} (Fe XII/XXIV) and 131 {\AA} (Fe
VIII/XXI) filters on-board the Solar Dynamic Observatory (SDO) Atmospheric
Imaging Assembly (AIA), we investigate the intensity oscillations associated
with a solar jet. Evidence is provided for multiple magnetic reconnection
events occurring between a pre-twisted, closed field and open field lines.
Components of the jet are seen in multiple SDO/AIA filters covering a wide
range of temperatures, suggesting the jet can be classified as a blowout jet.
Two bright, elongated features are observed to be co-spatial with the large
jet, appearing at the jet's footpoints. Investigation of these features reveal
they are defined by multiple plasma ejections. The ejecta display
(quasi-)periodic behaviour on timescales of 50 s and have rise velocities of
40-150 km\,s along the open field lines. Due to the suggestion that the
large jet is reconnection-driven and the observed properties of the ejecta, we
further propose that these ejecta events are similar to type-II spicules. The
bright features also display (quasi)-periodic intensity perturbations on the
timescale of 300 s. Possible explanations for the existence of the
(quasi-)periodic perturbations in terms of jet dynamics and the response of the
transition region are discussed.Comment: Astronomy and Astrophysics - In Prin
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