531 research outputs found
Markov Chain Monitoring
In networking applications, one often wishes to obtain estimates about the
number of objects at different parts of the network (e.g., the number of cars
at an intersection of a road network or the number of packets expected to reach
a node in a computer network) by monitoring the traffic in a small number of
network nodes or edges. We formalize this task by defining the 'Markov Chain
Monitoring' problem.
Given an initial distribution of items over the nodes of a Markov chain, we
wish to estimate the distribution of items at subsequent times. We do this by
asking a limited number of queries that retrieve, for example, how many items
transitioned to a specific node or over a specific edge at a particular time.
We consider different types of queries, each defining a different variant of
the Markov chain monitoring. For each variant, we design efficient algorithms
for choosing the queries that make our estimates as accurate as possible. In
our experiments with synthetic and real datasets we demonstrate the efficiency
and the efficacy of our algorithms in a variety of settings.Comment: 13 pages, 10 figures, 1 tabl
Alfv\'en waves in simulations of solar photospheric vortices
Using advanced numerical magneto-hydrodynamic simulations of the magnetised
solar photosphere, including non-grey radiative transport and a non-ideal
equation of state, we analyse plasma motions in photospheric magnetic vortices.
We demonstrate that apparent vortex-like motions in photospheric magnetic field
concentrations do not exhibit "tornado"-like behaviour or a "bath-tub" effect.
While at each time instance the velocity field lines in the upper layers of the
solar photosphere show swirls, the test particles moving with the
time-dependent velocity field do not demonstrate such structures. Instead, they
move in a wave-like fashion with rapidly changing and oscillating velocity
field, determined mainly by magnetic tension in the magnetised intergranular
downflows. Using time-distance diagrams, we identify horizontal motions in the
magnetic flux tubes as torsional Alfv\'en perturbations propagating along the
nearly vertical magnetic field lines with local Alfv\'en speed.Comment: 5 pages, 4 figures, accepted to ApJ
Understanding Astrophysical Noise from Stellar Surface Magneto-Convection
To obtain cm/s precision, stellar surface magneto-convection must be
disentangled from observed radial velocities (RVs). In order to understand and
remove the convective signature, we create Sun-as-a-star model observations
based on a 3D magnetohydrodynamic solar simulation. From these Sun-as-a-star
model observations, we find several line characteristics are correlated with
the induced RV shifts. The aim of this campaign is to feed directly into future
high precision RV studies, such as the search for habitable, rocky worlds, with
forthcoming spectrographs such as ESPRESSO.Comment: 6 pages, 3 figures; presented at the 18th Cambridge Workshop on Cool
Stars, Stellar Systems, and the Sun (CoolStars18); to appear in the
proceedings of Lowell Observatory (9-13 June 2014), edited by G. van Belle &
H. Harris. Updated with correct y-axis units on righthand plot in figure
Chromospheric Inversions of a Micro-flaring Region
We use spectropolarimetric observations of the Ca II 8542~\AA\ line, taken
from the Swedish 1-m Solar Telescope (SST), in an attempt to recover dynamic
activity in a micro-flaring region near a sunspot via inversions. These
inversions show localized mean temperature enhancements of 1000~K in the
chromosphere and upper photosphere, along with co-spatial bi-directional
Doppler shifting of 5 - 10 km s. This heating also extends along a
nearby chromospheric fibril, co-spatial to 10 - 15 km s down-flows.
Strong magnetic flux cancellation is also apparent in one of the footpoints,
concentrated in the chromosphere. This event more closely resembles that of an
Ellerman Bomb (EB), though placed slightly higher in the atmosphere than is
typically observed.Comment: 9 pages, 9 figures, accepted in ApJ. Movies are stored here:
https://star.pst.qub.ac.uk/webdav/public/areid/Microflare
Analysis of roles and groups in blogosphere
In the paper different roles of users in social media, taking into
consideration their strength of influence and different degrees of
cooperativeness, are introduced. Such identified roles are used for the
analysis of characteristics of groups of strongly connected entities. The
different classes of groups, considering the distribution of roles of users
belonging to them, are presented and discussed.Comment: 8th International Conference on Computer Recognition Systems, CORES
201
RHESSI and SOHO/CDS Observations of Explosive Chromospheric Evaporation
Simultaneous observations of explosive chromospheric evaporation are
presented using data from the Reuven Ramaty High Energy Solar Spectroscopic
Imager (RHESSI) and the Coronal Diagnostic Spectrometer (CDS) onboard SOHO. For
the first time, co-spatial imaging and spectroscopy have been used to observe
explosive evaporation within a hard X-ray emitting region. RHESSI X-ray images
and spectra were used to determine the flux of non-thermal electrons
accelerated during the impulsive phase of an M2.2 flare. Assuming a
thick-target model, the injected electron spectrum was found to have a spectral
index of ~7.3, a low energy cut-off of ~20 keV, and a resulting flux of
>4x10^10 ergs cm^-2 s^-1. The dynamic response of the atmosphere was determined
using CDS spectra, finding a mean upflow velocity of 230+/-38 km s^-1 in Fe XIX
(592.23A), and associated downflows of 36+/-16 km s^-1 and 43+/-22 km s^-1 at
chromospheric and transition region temperatures, respectively, relative to an
averaged quiet-Sun spectra. The errors represent a 1 sigma dispersion. The
properties of the accelerated electron spectrum and the corresponding
evaporative velocities were found to be consistent with the predictions of
theory.Comment: 5 pages, 4 figures, ApJL (In Press
Discovery of close companions to the nearby young stars HD 199143 and HD 358623
Young stellar systems in the solar neighborhood provide valuable laboratories
for detailed studies of star and planet formation. The bright F8V star HD
199143 and the Li-rich late-type emission line star HD 358623 are among the
nearest young stars identified to date, and may be members of a young
association in Capricornus. We present high-resolution near-infrared images of
these two sources, obtained using the adaptive optics system on the 3.6-meter
telescope at the European Southern Observatory in La Silla, Chile. Our
observations reveal that both are in fact close binary systems. The newly
discovered companion at a separation of 1'' may account for the unusual
characteristics of HD 199143 --rapid rotation, emission lines, ultraviolet
variability, and excess infrared emission-- recently discussed by van den
Ancker and co-workers. HD 199143 may be a rare example of a close binary with
only a circum{\it secondary} disk. With the detection of a 2'' companion,
HD 358623 is now possibly one of the closest known T Tauri binaries. Both
binary systems are prime targets for follow-up spectroscopic and astrometric
observations.Comment: 9 pages, 1 PostScript figure, to appear in The Astrophysical Journal
Letter
A New Multiple Stellar System in the Solar Neighborhood
Adaptive optics corrected images obtained with the CIAO instrument at the
Subaru 8.2-meter telescope show the presence of two subarsecond companions to
the nearby (d=19.3 pc) young star GJ 900, which was previously classified as a
single member of the IC 2391 supercluster. The two companions share the same
proper motion as the primary and are redder. Their projected separations from
the primary are 10 AU and 14.5 AU for B and C, respectively. The estimated
masses for the two new companions depend strongly on the age of the system. For
the range of ages found in the literature for IC 2391 supercluster members
(from 35 Myr to 200 Myr), the expected masses range from 0.2 M to 0.4
M for the B component, and from 0.09 M to 0.22 M for
the C component. The determination of the dynamical mass of the faintest
component of GJ 900 will yield the age of the system using theoretical
evolutionary tracks. The apparent separations of the GJ 900 system components
meet the observational criterion for an unstable Trapezium-type system, but
this could be a projection effect. Further observations are needed to establish
the nature of this interesting low-mass multiple system.Comment: Scheduled for publication in the Astronomical Journal (August 2003
Analytical determination of coronal parameters using the period ratio P<sub>1</sub>/2P<sub>2</sub>
<p>Context. In transverse coronal loop oscillations, two periodicities have been measured simultaneously and are interpreted as the fundamental
kink mode (with period P1) and the first harmonic (with period P2). Deviations of the period ratio P1/2P2 from unity provide
information about the extent of longitudinal structuring within the loop.</p>
<p>Aims. Here we develop an analytical approximation that describes the shift in P1/2P2 in terms of the ratio L/Λc of the length 2L of a
coronal loop and the density scale height Λc.</p>
<p>Methods. We study the MHD wave equations in a low β plasma using the thin tube approximation. Disturbances are described by a
differential equation which may be solved for various equilibrium density profiles, obtaining dispersion relations in terms of Bessel
functions. These dispersion relations may be used to obtain analytical approximations to the periods P1 and P2. We also present a
variational approach to determining the period ratio and show how the WKB method may be used.</p>
<p>Results. Analytical approximations to the period ratio P1/2P2 are used to shed light on the magnitude of longitudinal structuring in
a loop, leading to a determination of the density scale height. We apply our formula to the observations in Verwichte et al. (2004) and
Van Doorsselaere et al. (2007), obtaining the coronal density scale height.</p>
<p>Conclusions. Our simple formula and approximate approaches highlight a useful analytical tool for coronal seismology. We demonstrate
that P1/2P2 is linked to the density scale height, with no need for estimates of other external parameters. Given the accuracy of
current observations, our formula provides a convenient means of determining density scale heights.</p>
Intensity enhancement of O VI ultraviolet emission lines in solar spectra due to opacity
Opacity is a property of many plasmas, and it is normally expected that if an
emission line in a plasma becomes optically thick, its intensity ratio to that
of another transition that remains optically thin should decrease. However,
radiative transfer calculations undertaken both by ourselves and others predict
that under certain conditions the intensity ratio of an optically thick to thin
line can show an increase over the optically thin value, indicating an
enhancement in the former. These conditions include the geometry of the
emitting plasma and its orientation to the observer. A similar effect can take
place between lines of differing optical depth. Previous observational studies
have focused on stellar point sources, and here we investigate the
spatially-resolved solar atmosphere using measurements of the I(1032 A)/I(1038
A) intensity ratio of O VI in several regions obtained with the Solar
Ultraviolet Measurements of Emitted Radiation (SUMER) instrument on board the
Solar and Heliospheric Observatory (SoHO) satellite. We find several I(1032
A)/I(1038 A) ratios observed on the disk to be significantly larger than the
optically thin value of 2.0, providing the first detection (to our knowledge)
of intensity enhancement in the ratio arising from opacity effects in the solar
atmosphere. Agreement between observation and theory is excellent, and confirms
that the O VI emission originates from a slab-like geometry in the solar
atmosphere, rather than from cylindrical structures.Comment: 17 pages, 4 figures, ApJ Letters, in pres
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