167 research outputs found
Imaging Spectroscopy of a White-Light Solar Flare
We report observations of a white-light solar flare (SOL2010-06-12T00:57,
M2.0) observed by the Helioseismic Magnetic Imager (HMI) on the Solar Dynamics
Observatory (SDO) and the Reuven Ramaty High-Energy Solar Spectroscopic Imager
(RHESSI). The HMI data give us the first space-based high-resolution imaging
spectroscopy of a white-light flare, including continuum, Doppler, and magnetic
signatures for the photospheric FeI line at 6173.34{\AA} and its neighboring
continuum. In the impulsive phase of the flare, a bright white-light kernel
appears in each of the two magnetic footpoints. When the flare occurred, the
spectral coverage of the HMI filtergrams (six equidistant samples spanning
\pm172m{\AA} around nominal line center) encompassed the line core and the blue
continuum sufficiently far from the core to eliminate significant Doppler
crosstalk in the latter, which is otherwise a possibility for the extreme
conditions in a white-light flare. RHESSI obtained complete hard X-ray and
\Upsilon-ray spectra (this was the first \Upsilon-ray flare of Cycle 24). The
FeI line appears to be shifted to the blue during the flare but does not go
into emission; the contrast is nearly constant across the line profile. We did
not detect a seismic wave from this event. The HMI data suggest stepwise
changes of the line-of-sight magnetic field in the white-light footpoints.Comment: 14 pages, 7 figures, Accepted by Solar Physic
The Influence of Solar Flares on the Lower Solar Atmosphere: Evidence from the Na D Absorption Line Measured by GOLF/SOHO
Solar flares presumably have an impact on the deepest layers of the solar
atmosphere and yet the observational evidence for such an impact is scarce.
Using ten years of measurements of the Na D and Na D Fraunhofer
lines, measured by GOLF onboard SOHO, we show that this photospheric line is
indeed affected by flares. The effect of individual flares is hidden by solar
oscillations, but a statistical analysis based on conditional averaging reveals
a clear signature. Although GOLF can only probe one single wavelength at a
time, we show that both wings of the Na line can nevertheless be compared. The
varying line asymmetry can be interpreted as an upward plasma motion from the
lower solar atmosphere during the peak of the flare, followed by a downward
motion.Comment: 13 pages, 7 figure
Eruptions of Magnetic Ropes in Two Homologous Solar Events on 2002 June 1 and 2: a Key to Understanding of an Enigmatic Flare
The goal of this paper is to understand the drivers, configurations, and
scenarios of two similar eruptive events, which occurred in the same solar
active region 9973 on 2002 June 1 and 2. The June 2 event was previously
studied by Sui, Holman, and Dennis (2006, 2008), who concluded that it was
challenging for popular flare models. Using multi-spectral data, we analyze a
combination of the two events. Each of the events exhibited an evolving
cusp-like feature. We have revealed that these apparent ``cusps'' were most
likely mimicked by twisted magnetic flux ropes, but unlikely to be related to
the inverted Y-like magnetic configuration in the standard flare model. The
ropes originated inside a funnel-like magnetic domain whose base was bounded by
an EUV ring structure, and the top was associated with a coronal null point.
The ropes appear to be the major drivers for the events, but their rise was not
triggered by reconnection in the coronal null point. We propose a scenario and
a three-dimensional scheme for these events in which the filament eruptions and
flares were caused by interaction of the ropes.Comment: 22 pages, 11 figure
Surface and capillary transitions in an associating binary mixture model
We investigate the phase diagram of a two-component associating fluid mixture
in the presence of selectively adsorbing substrates. The mixture is
characterized by a bulk phase diagram which displays peculiar features such as
closed loops of immiscibility. The presence of the substrates may interfere the
physical mechanism involved in the appearance of these phase diagrams, leading
to an enhanced tendency to phase separate below the lower critical solution
point. Three different cases are considered: a planar solid surface in contact
with a bulk fluid, while the other two represent two models of porous systems,
namely a slit and an array on infinitely long parallel cylinders. We confirm
that surface transitions, as well as capillary transitions for a large
area/volume ratio, are stabilized in the one-phase region. Applicability of our
results to experiments reported in the literature is discussed.Comment: 12 two-column pages, 12 figures, accepted for publication in Physical
Review E; corrected versio
Image Registration and Fusion for Interventional MRI Guided Thermal Ablation of the Prostate Cancer
Abstract. We are investigating interventional MRI (iMRI) guided thermal ablation treatment of the prostate cancer. Functional images such as SPECT can detect and localize tumor in the prostate not reliably seen in MRI. We intend to combine the advantages of SPECT with iMRI-guided treatments. Our concept is to first register the low-resolution SPECT with a high resolution MRI volume. Then by registering the high-resolution MR image with iMRI acquisitions, we can, in turn, map the functional data and high-resolution anatomic information to iMRI images for improved tumor targeting. For the first step, we used a mutual information registration method. For the latter, we developed a robust slice to volume (SV) registration algorithm. Image data were acquired from patients and volunteers. Compared to our volume-to-volume registration that was previously evaluated to be quite accurate, the SV registration accuracy is about 0.5 mm for transverse images covering the prostate. With our image registration and fusion software, simulation experiments show that it is feasible to incorporate SPECT and high resolution MRI into the iMRI-guided treatment.
The Flare-energy Distributions Generated by Kink-unstable Ensembles of Zero-net-current Coronal Loops
It has been proposed that the million degree temperature of the corona is due
to the combined effect of barely-detectable energy releases, so called
nanoflares, that occur throughout the solar atmosphere. Alas, the nanoflare
density and brightness implied by this hypothesis means that conclusive
verification is beyond present observational abilities. Nevertheless, we
investigate the plausibility of the nanoflare hypothesis by constructing a
magnetohydrodynamic (MHD) model that can derive the energy of a nanoflare from
the nature of an ideal kink instability. The set of energy-releasing
instabilities is captured by an instability threshold for linear kink modes.
Each point on the threshold is associated with a unique energy release and so
we can predict a distribution of nanoflare energies. When the linear
instability threshold is crossed, the instability enters a nonlinear phase as
it is driven by current sheet reconnection. As the ensuing flare erupts and
declines, the field transitions to a lower energy state, which is modelled by
relaxation theory, i.e., helicity is conserved and the ratio of current to
field becomes invariant within the loop. We apply the model so that all the
loops within an ensemble achieve instability followed by energy-releasing
relaxation. The result is a nanoflare energy distribution. Furthermore, we
produce different distributions by varying the loop aspect ratio, the nature of
the path to instability taken by each loop and also the level of radial
expansion that may accompany loop relaxation. The heating rate obtained is just
sufficient for coronal heating. In addition, we also show that kink instability
cannot be associated with a critical magnetic twist value for every point along
the instability threshold
Excavator Wheel Drive Reconstruction.
Import 05/08/2014V této práci je popsán pomocný pohon kolesa rypadla a možnost jeho rozběhu softstartérem a frekvenčním měničem. V první části je probrán asynchronní motor s kotvou nakrátko, jeho popis a princip funkce. Další části se věnují softstartéru a frekvenčnímu měniči, jejich popisu, funkcím a možnostem řízení. Dále je zde provedeno měření jejich vlivu na síť. Poslední část je věnována návrhu rekonstrukce pohonu pro napájení s frekvenčním měničem.This thesis describes the auxiliary drive wheel excavators and the possibility of starting the softstarter and frequency converter. In the first part of the document the asynchronous motor with squirrel cage is discussed, its description and function. Other parts of the document are devoted to the soft starter and frequency converter, their description, function and management options. Furthermore, there is performed measurement of their effect on the network. The last section is devoted to the design of the reconstruction drive with the frequency converter.410 - Katedra elektroenergetikyvýborn
Recent Advances in Understanding Particle Acceleration Processes in Solar Flares
We review basic theoretical concepts in particle acceleration, with
particular emphasis on processes likely to occur in regions of magnetic
reconnection. Several new developments are discussed, including detailed
studies of reconnection in three-dimensional magnetic field configurations
(e.g., current sheets, collapsing traps, separatrix regions) and stochastic
acceleration in a turbulent environment. Fluid, test-particle, and
particle-in-cell approaches are used and results compared. While these studies
show considerable promise in accounting for the various observational
manifestations of solar flares, they are limited by a number of factors, mostly
relating to available computational power. Not the least of these issues is the
need to explicitly incorporate the electrodynamic feedback of the accelerated
particles themselves on the environment in which they are accelerated. A brief
prognosis for future advancement is offered.Comment: This is a chapter in a monograph on the physics of solar flares,
inspired by RHESSI observations. The individual articles are to appear in
Space Science Reviews (2011
- …