489 research outputs found
Nonequilibrium relaxation analysis of a quasi-one-dimensional frustrated XY model for charge-density waves in ring-shaped crystals
We propose a model for charge density waves in ring shaped crystals, which
depicts frustration between intra- and inter-chain couplings coming from
cylindrical bending. It is then mapped to a three dimensional uniformly
frustrated XY model with one dimensional anisotropy in connectivity. The
nonequilibrium relaxation dynamics is investigated by Monte Carlo simulations
to find a phase transition which is quite different from that of usual whisker
crystal. We also find that the low temperature state is a three dimensional
phase vortex lattice with a two dimensional phase coherence in a cylindrical
shell and the system shows power law relaxation in the ordered phase.Comment: 6 pages, 6 epsfiles, revised versio
Quenching of phase coherence in quasi-one dimensional ring crystals
The comparison of the single-particle (SP) dynamics between the whisker and
ring NbSe crystals provides new insight into the phase transition
properties in quasi-one-dimensional charge density wave (CDW) systems.Comment: 9 pages, 4 figure
Direct observation of high-speed plasma outflows produced by magnetic reconnection in solar impulsive events
Spectroscopic observations of a solar limb flare recorded by SUMER on SOHO
reveal, for the first time, hot fast magnetic reconnection outflows in the
corona. As the reconnection site rises across the SUMER spectrometer slit,
significant blue- and red-shift signatures are observed in sequence in the Fe
XIX line, reflecting upflows and downflows of hot plasma jets, respectively.
With the projection effect corrected, the measured outflow speed is between
900-3500 km/s, consistent with theoretical predictions of the Alfvenic outflows
in magnetic reconnection region in solar impulsive events. Based on theoretic
models, the magnetic field strength near the reconnection region is estimated
to be 19-37 Gauss.Comment: 5 pages, 6 color figures, 1 animation onlin
Comparison of transient horizontal magnetic fields in a plage region and in the quiet Sun
Properties of transient horizontal magnetic fields (THMFs) in both plage and
quiet Sun regions are obtained and compared. Spectro-polarimetric observations
with the Solar Optical Telescope (SOT) on the Hinode satellite were carried out
with a cadence of about 30 seconds for both plage and quiet regions located
near disk center. We select THMFs that have net linear polarization (LP) higher
than 0.22%, and an area larger than or equal to 3 pixels, and compare their
occurrence rates and distribution of magnetic field azimuth. We obtain
probability density functions (PDFs) of magnetic field strength and inclination
for both regions.The occurrence rate in the plage region is the same as for the
quiet Sun. The vertical magnetic flux in the plage region is ~8 times larger
than in the quiet Sun. There is essentially no preferred orientation for the
THMFs in either region. However, THMFs in the plage region with higher LP have
a preferred direction consistent with that of the plage-region's large-scale
vertical field pattern. PDFs show that there is no difference in the
distribution of field strength of horizontal fields between the quiet Sun and
the plage regions when we avoid the persistent large vertical flux
concentrations for the plage region. The similarity of the PDFs and of the
occurrence rates in plage and quiet regions suggests that a local dynamo
process due to the granular motion may generate THMFs all over the sun. The
preferred orientation for higher LP in the plage indicates that the THMFs are
somewhat influenced by the larger-scale magnetic field pattern of the plage.Comment: 11 pages, 7 figures, A&A accepte
Nanoflare Evidence from Analysis of the X-Ray Variability of an Active Region Observed with Hinode/XRT
The heating of the solar corona is one of the big questions in astrophysics.
Rapid pulses called nanoflares are among the best candidate mechanisms. The
analysis of the time variability of coronal X-ray emission is potentially a
very useful tool to detect impulsive events. We analyze the small-scale
variability of a solar active region in a high cadence Hinode/XRT observation.
The dataset allows us to detect very small deviations of emission fluctuations
from the distribution expected for a constant rate. We discuss the deviations
in the light of the pulsed-heating scenario.Comment: 6 pages, 4 figure
Polar Field Reversal Observations with Hinode
We have been monitoring yearly variation in the Sun's polar magnetic fields
with the Solar Optical Telescope aboard {\it Hinode} to record their evolution
and expected reversal near the solar maximum. All magnetic patches in the
magnetic flux maps are automatically identified to obtain the number density
and magnetic flux density as a function of th total magnetic flux per patch.
The detected magnetic flux per patch ranges over four orders of magnitude
( -- Mx). The higher end of the magnetic flux in the polar
regions is about one order of magnitude larger than that of the quiet Sun, and
nearly that of pores. Almost all large patches ( Mx) have the
same polarity, while smaller patches have a fair balance of both polarities.
The polarity of the polar region as a whole is consequently determined only by
the large magnetic concentrations. A clear decrease in the net flux of the
polar region is detected in the slow rising phase of the current solar cycle.
The decrease is more rapid in the north polar region than in the south. The
decrease in the net flux is caused by a decrease in the number and size of the
large flux concentrations as well as the appearance of patches with opposite
polarity at lower latitudes. In contrast, we do not see temporal change in the
magnetic flux associated with the smaller patches ( Mx) and that of
the horizontal magnetic fields during the years 2008--2012.Comment: 21 pages, 7 figures. Accepted for publication in Ap
Drift-Kinetic Modeling of Particle Acceleration and Transport in Solar Flares
Based on the drift-kinetic theory, we develop a model for particle
acceleration and transport in solar flares. The model describes the evolution
of the particle distribution function by means of a numerical simulation of the
drift-kinetic Vlasov equation, which allows us to directly compare simulation
results with observations within an actual parameter range of the solar corona.
Using this model, we investigate the time evolution of the electron
distribution in a flaring region. The simulation identifies two dominant
mechanisms of electron acceleration. One is the betatron acceleration at the
top of closed loops, which enhances the electron velocity perpendicular to the
magnetic field line. The other is the inertia drift acceleration in open
magnetic field lines, which produces antisunward electrons. The resulting
velocity space distribution significantly deviates from an isotropic
distribution. The former acceleration can be a generation mechanism of
electrons that radiate loop-top nonthermal emissions, and the latter be of
escaping electrons from the Sun that should be observed by in-situ measurements
in interplanetary space and resulting radio bursts through plasma
instabilities.Comment: 32 Pages, 11 figures, accepted by Ap
RHESSI Observations of a Simple Large X-ray Flare on 11-03-2003
We present data analysis and interpretation of a simple X-class flare
observed with RHESSI on November 3, 2003. In contrast to other X-class flares
observed previously, this flare shows a very simple morphology with well
defined looptop (LT) and footpoint (FP) sources. The almost monotonic upward
motion of the LT source and increase in separation of the two FP sources are
consistent with magnetic reconnection models proposed for solar flares. In
addition, we find that the source motions are relatively slower during the more
active phases of hard X-ray emission; the emission centroid of the LT source
shifts toward higher altitudes with the increase of energy; the separation
between the LT emission centroids at two different photon energies is
anti-correlated with the FP flux. Non-uniformity of the reconnecting magnetic
fields could be a possible explanation of these features.Comment: To appear in the Astrophysical Journal Letters (12 pages, 4 figures
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