14,555 research outputs found
RADYN simulations of non-thermal and thermal models of Ellerman bombs
Ellerman bombs (EBs) are brightenings in the H line wings that are
believed to be caused by magnetic reconnection in the lower atmosphere. To
study the response and evolution of the chromospheric line profiles, we perform
radiative hydrodynamic simulations of EBs using both non-thermal and thermal
models. Overall, these models can generate line profiles that are similar to
observations. However, in non-thermal models we find dimming in the H
line wings and continuum when the heating begins, while for the thermal models
dimming occurs only in the H line core, and with a longer lifetime.
This difference in line profiles can be used to determine whether an EB is
dominated by non-thermal heating or thermal heating. In our simulations, if a
higher heating rate is applied, the H line will be unrealistically
strong, while there are still no clear UV burst signatures.Comment: 20 pages, 9 figures, accepted for publication in Ap
"Virus hunting" using radial distance weighted discrimination
Motivated by the challenge of using DNA-seq data to identify viruses in human
blood samples, we propose a novel classification algorithm called "Radial
Distance Weighted Discrimination" (or Radial DWD). This classifier is designed
for binary classification, assuming one class is surrounded by the other class
in very diverse radial directions, which is seen to be typical for our virus
detection data. This separation of the 2 classes in multiple radial directions
naturally motivates the development of Radial DWD. While classical machine
learning methods such as the Support Vector Machine and linear Distance
Weighted Discrimination can sometimes give reasonable answers for a given data
set, their generalizability is severely compromised because of the linear
separating boundary. Radial DWD addresses this challenge by using a more
appropriate (in this particular case) spherical separating boundary.
Simulations show that for appropriate radial contexts, this gives much better
generalizability than linear methods, and also much better than conventional
kernel based (nonlinear) Support Vector Machines, because the latter methods
essentially use much of the information in the data for determining the shape
of the separating boundary. The effectiveness of Radial DWD is demonstrated for
real virus detection.Comment: Published at http://dx.doi.org/10.1214/15-AOAS869 in the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Non-LTE Calculations of the Fe I 6173 {\AA} Line in a Flaring Atmosphere
The Fe I 6173 {\AA} line is widely used in the measurements of vector
magnetic fields by instruments including the Helioseismic and Magnetic Imager
(HMI). We perform non-local thermodynamic equilibrium calculations of this line
based on radiative hydrodynamic simulations in a flaring atmosphere. We employ
both a quiet-Sun atmosphere and a penumbral atmosphere as the initial one in
our simulations. We find that, in the quiet-Sun atmosphere, the line center is
obviously enhanced during an intermediate flare. The enhanced emission is
contributed from both radiative backwarming in the photosphere and particle
beam heating in the lower chromosphere. A blue asymmetry of the line profile
also appears due to an upward mass motion in the lower chromosphere. If we take
a penumbral atmosphere as the initial atmosphere, the line has a more
significant response to the flare heating, showing a central emission and an
obvious asymmetry. The low spectral resolution of HMI would indicate some loss
of information but the enhancement and line asymmetry are still kept. By
calculating polarized line profiles, we find that the Stokes I and V profiles
can be altered as a result of flare heating. Thus the distortion of this line
has a crucial influence on the magnetic field measured from this line, and one
should be cautious in interpreting the magnetic transients observed frequently
in solar flares.Comment: 12 pages, 5 figures, accepted by ApJ
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