304,419 research outputs found
Direct measurement of the electron density of extended femtosecond laser pulse-induced filaments
We present direct time- and space- resolved measurements of the electron
density of femtosecond laser pulse-induced plasma filaments. The dominant
nonlinearity responsible for extended atmospheric filaments is shown to be
field-induced rotation of air molecules.Comment: 12 pages, 5 figure
A model for the formation of the active region corona driven by magnetic flux emergence
We present the first model that couples the formation of the corona of a
solar active region to a model of the emergence of a sunspot pair. This allows
us to study when, where, and why active region loops form, and how they evolve.
We use a 3D radiation MHD simulation of the emergence of an active region
through the upper convection zone and the photosphere as a lower boundary for a
3D MHD coronal model. The latter accounts for the braiding of the magnetic
fieldlines, which induces currents in the corona heating up the plasma. We
synthesize the coronal emission for a direct comparison to observations.
Starting with a basically field-free atmosphere we follow the filling of the
corona with magnetic field and plasma. Numerous individually identifiable hot
coronal loops form, and reach temperatures well above 1 MK with densities
comparable to observations. The footpoints of these loops are found where small
patches of magnetic flux concentrations move into the sunspots. The loop
formation is triggered by an increase of upwards-directed Poynting flux at
their footpoints in the photosphere. In the synthesized EUV emission these
loops develop within a few minutes. The first EUV loop appears as a thin tube,
then rises and expands significantly in the horizontal direction. Later, the
spatially inhomogeneous heat input leads to a fragmented system of multiple
loops or strands in a growing envelope.Comment: 13 pages, 10 figures, accepted to publication in A&
Magnetic Jam in the Corona of the Sun
The outer solar atmosphere, the corona, contains plasma at temperatures of
more than a million K, more than 100 times hotter that solar surface. How this
gas is heated is a fundamental question tightly interwoven with the structure
of the magnetic field in the upper atmosphere. Conducting numerical experiments
based on magnetohydrodynamics we account for both the evolving
three-dimensional structure of the atmosphere and the complex interaction of
magnetic field and plasma. Together this defines the formation and evolution of
coronal loops, the basic building block prominently seen in X-rays and extreme
ultraviolet (EUV) images. The structures seen as coronal loops in the EUV can
evolve quite differently from the magnetic field. While the magnetic field
continuously expands as new magnetic flux emerges through the solar surface,
the plasma gets heated on successively emerging fieldlines creating an EUV loop
that remains roughly at the same place. For each snapshot the EUV images
outline the magnetic field, but in contrast to the traditional view, the
temporal evolution of the magnetic field and the EUV loops can be different.
Through this we show that the thermal and the magnetic evolution in the outer
atmosphere of a cool star has to be treated together, and cannot be simply
separated as done mostly so far.Comment: Final version published online on 27 April 2015, Nature Physics 12
pages and 8 figure
Hooge's Constant of Carbon Nanotube Field Effect Transistors
The 1/f noise in individual semiconducting carbon nanotubes (s-CNT) in a
field effect transistor configuration has been measured in ultra-high vacuum
and following exposure to air. The amplitude of the normalized current spectral
noise density is independent of source-drain current, indicating the noise is
due to mobility rather than number fluctuations. Hooge's constant for s-CNT is
found to be 9.3 plus minus 0.4x10^-3. The magnitude of the 1/f noise is
substantially degreased by exposing the devices to air
Non-exponential kinetic behavior of confined water
We present the results of molecular dynamics simulations of SPC/E water
confined in a realistic model of a silica pore. The single-particle dynamics
have been studied at ambient temperature for different hydration levels. The
confinement near the hydrophilic surface makes the dynamic behaviour of the
liquid strongly dependent on the hydration level. Upon decrease of the number
of water molecules in the pore we observe the onset of a slow dynamics due to
the ``cage effect''. The conventional picture of a stochastic single-particle
diffusion process thus looses its validity
Multi-wavelength variability properties of Fermi blazar S5 0716+714
S5 0716+714 is a typical BL Lacertae object. In this paper we present the
analysis and results of long term simultaneous observations in the radio,
near-infrared, optical, X-ray and -ray bands, together with our own
photometric observations for this source. The light curves show that the
variability amplitudes in -ray and optical bands are larger than those
in the hard X-ray and radio bands and that the spectral energy distribution
(SED) peaks move to shorter wavelengths when the source becomes brighter, which
are similar to other blazars, i.e., more variable at wavelengths shorter than
the SED peak frequencies. Analysis shows that the characteristic variability
timescales in the 14.5 GHz, the optical, the X-ray, and the -ray bands
are comparable to each other. The variations of the hard X-ray and 14.5 GHz
emissions are correlated with zero-lag, so are the V band and -ray
variations, which are consistent with the leptonic models. Coincidences of
-ray and optical flares with a dramatic change of the optical
polarization are detected. Hadronic models do not have the same nature
explanation for these observations as the leptonic models. A strong optical
flare correlating a -ray flare whose peak flux is lower than the
average flux is detected. Leptonic model can explain this variability
phenomenon through simultaneous SED modeling. Different leptonic models are
distinguished by average SED modeling. The synchrotron plus synchrotron
self-Compton (SSC) model is ruled out due to the extreme input parameters.
Scattering of external seed photons, such as the hot dust or broad line region
emission, and the SSC process are probably both needed to explain the
-ray emission of S5 0716+714.Comment: 43 pages, 13 figures, 3 tables, to be appeared in Ap
Reciprocatory magnetic reconnection in a coronal bright point
Coronal bright points (CBPs) are small-scale and long-duration brightenings
in the lower solar corona. They are often explained in terms of magnetic
reconnection. We aim to study the sub-structures of a CBP and clarify the
relationship among the brightenings of different patches inside the CBP. The
event was observed by the X-ray Telescope (XRT) aboard the Hinode spacecraft on
2009 August 2223. The CBP showed repetitive brightenings (or CBP flashes).
During each of the two successive CBP flashes, i.e., weak and strong flashes
which are separated by 2 hr, the XRT images revealed that the CBP was
composed of two chambers, i.e., patches A and B. During the weak flash, patch A
brightened first, and patch B brightened 2 min later. During the
transition, the right leg of a large-scale coronal loop drifted from the right
side of the CBP to the left side. During the strong flash, patch B brightened
first, and patch A brightened 2 min later. During the transition, the
right leg of the large-scale coronal loop drifted from the left side of the CBP
to the right side. In each flash, the rapid change of the connectivity of the
large-scale coronal loop is strongly suggestive of the interchange
reconnection. For the first time we found reciprocatory reconnection in the
CBP, i.e., reconnected loops in the outflow region of the first reconnection
process serve as the inflow of the second reconnection process.Comment: 13 pages, 8 figure
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