28,489 research outputs found
Hard X-ray and UV Observations of the 2005 January 15 Two-ribbon Flare
In this paper, we present comprehensive analysis of a two-ribbon flare
observed in UV 1600{\AA} by Transition Region and Coronal Explorer and in HXRs
by Reuven Ramaty High Energy Solar Spectroscopic Imager. HXR (25-100 keV)
imaging observations show two kernels of size (FWHM) 15?? moving along the two
UV ribbons. We find the following results. (1) UV brightening is substantially
enhanced wherever and whenever the compact HXR kernel is passing, and during
the HXR transit across a certain region, the UV count light curve in that
region is temporally correlated with the HXR total flux light curve. After the
passage of the HXR kernel, the UV light curve exhibits smooth monotonical
decay. (2)We measure the apparent motion speed of the HXR sources and UV ribbon
fronts, and decompose the motion into parallel and perpendicular motions with
respect to the magnetic polarity inversion line (PIL). It is found that HXR
kernels and UV fronts exhibit similar apparent motion patterns and speeds. The
parallel motion dominates during the rise of the HXR emission, and the
perpendicular motion starts and dominates at the HXR peak, the apparent motion
speed being 10-40 km s-1. (3) We also find that UV emission is characterized by
a rapid rise correlated with HXRs, followed by a long decay on timescales of
15-30 minutes. The above analysis provides evidence that UV brightening is
primarily caused by beam heating, which also produces thick-target HXR
emission. The thermal origin of UV emission cannot be excluded, but would
produce weaker heating by one order of magnitude. The extended UV ribbons in
this event are most likely a result of sequential reconnection along the PIL,
which produces individual flux tubes (post-flare loops), subsequent non-thermal
energy release and heating in these flux tubes, and then the very long cooling
time of the transition region at the feet of these flux tubes.Comment: 8 figure
Vortex melting and decoupling transitions in YBaCuO single crystals
The vortex correlation along the c-axis in high quality single crystals of
YBaCuO has been investigated as a function of temperature T
in different magnetic fields, using the quasi-flux transformer configuration. A
simultaneous sharp drop associated with the vortex lattice melting is observed
in both the primary and secondary voltages(V and V). Just above
the melting temperature, the vortices form three-dimensional line liquid with
the correlation length along the c direction t, the sample
thickness. The temperature where a resistive peak in R develops
corresponds to the decoupling temperature T at which the vortices loose
their correlation along the c-direction and they dissolve into the two
dimensional pancake vortices. The H-T phase diagram for the
YBaCuO single crystal is obtained.Comment: 1 Text file, 3 eps figure
Single transverse-spin asymmetry in Drell-Yan lepton angular distribution
We calculate a single transverse-spin asymmetry for the Drell-Yan
lepton-pair's angular distribution in perturbative QCD. At leading order in the
strong coupling constant, the asymmetry is expressed in terms of a twist-3
quark-gluon correlation function T_F^{(V)}(x_1,x_2). In our calculation, the
same result was obtained in both light-cone and covariant gauge in QCD, while
keeping explicit electromagnetic current conservation for the virtual photon
that decays into the lepton pair. We also present a numerical estimate of the
asymmetry and compare the result to an existing other prediction.Comment: 15 pages, Revtex, 5 Postscript figures, uses aps.sty, epsfig.st
Cylindrical Invisibility Cloak with Simplified Material Parameters is Inherently Visible
It was proposed that perfect invisibility cloaks can be constructed for
hiding objects from electromagnetic illumination (Pendry et al., Science 312,
p. 1780). The cylindrical cloaks experimentally demonstrated (Schurig et al.,
Science 314, p. 997) and proposed (Cai et al., Nat. Photon. 1, p. 224) have
however simplified material parameters in order to facilitate easier
realization as well as to avoid infinities in optical constants. Here we show
that the cylindrical cloaks with simplified material parameters inherently
allow the zeroth-order cylindrical wave to pass through the cloak as if the
cloak is made of a homogeneous isotropic medium, and thus visible. To all
high-order cylindrical waves, our numerical simulation suggests that the
simplified cloak inherits some properties of the ideal cloak, but finite
scatterings exist.Comment: 10 pages, 3 figure
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