557 research outputs found
Meter-scale spark X-ray spectrumstatistics
X-ray emission by sparks implies bremsstrahlung from a population of
energetic electrons, but the details of this process remain a mystery. We
present detailed statistical analysis of X-ray spectra detected by multiple
detectors during sparks produced by 1 MV negative high-voltage pulses with 1
s risetime. With over 900 shots, we statistically analyze the signals,
assuming that the distribution of spark X-ray fluence behaves as a power law
and that the energy spectrum of X-rays detectable after traversing 2 m of
air and a thin aluminum shield is exponential. We then determine the parameters
of those distributions by fitting cumulative distribution functions to the
observations. The fit results match the observations very well if the mean of
the exponential X-ray energy distribution is 86 7 keV and the spark X-ray
fluence power law distribution has index -1.29 0.04 and spans at least 3
orders of magnitude in fluence
A conjecture on the origin of dark energy
The physical origin of holographic dark energy (HDE) is investigated. The
main existing explanations, namely the UV/IR connection argument of Cohen et
al, Thomas' bulk holography argument, and Ng's spacetime foam argument, are
shown to be not satisfactory. A new explanation of the HDE model is then
proposed based on the ideas of Thomas and Ng. It is suggested that the dark
energy might originate from the quantum fluctuations of spacetime limited by
the event horizon of the universe. Several potential problems of the
explanation are also discussed.Comment: 11 pages, no figure
Properties of Flares-Generated Seismic Waves on the Sun
The solar seismic waves excited by solar flares (``sunquakes'') are observed
as circular expanding waves on the Sun's surface. The first sunquake was
observed for a flare of July 9, 1996, from the Solar and Heliospheric
Observatory (SOHO) space mission. However, when the new solar cycle started in
1997, the observations of solar flares from SOHO did not show the seismic
waves, similar to the 1996 event, even for large X-class flares during the
solar maximum in 2000-2002. The first evidence of the seismic flare signal in
this solar cycle was obtained for the 2003 ``Halloween'' events, through
acoustic ``egression power'' by Donea and Lindsey. After these several other
strong sunquakes have been observed. Here, I present a detailed analysis of the
basic properties of the helioseismic waves generated by three solar flares in
2003-2005. For two of these flares, X17 flare of October 28, 2003, and X1.2
flare of January 15, 2005, the helioseismology observations are compared with
simultaneous observations of flare X-ray fluxes measured from the RHESSI
satellite. These observations show a close association between the flare
seismic waves and the hard X-ray source, indicating that high-energy electrons
accelerated during the flare impulsive phase produced strong compression waves
in the photosphere, causing the sunquake. The results also reveal new physical
properties such as strong anisotropy of the seismic waves, the amplitude of
which varies significantly with the direction of propagation. The waves travel
through surrounding sunspot regions to large distances, up to 120 Mm, without
significant decay. These observations open new perspectives for helioseismic
diagnostics of flaring active regions on the Sun and for understanding the
mechanisms of the energy release and transport in solar flares.Comment: 12 pages, 4 figures, submitted to Ap
Investigating the maturation of microstructure and radial orientation in the preterm human cortex with diffusion MRI
Preterm birth disrupts and alters the complex developmental processes in the cerebral cortex. This disruption may be a contributing factor to widespread delay and cognitive difficulties in the preterm population. Diffusion-weighted magnetic resonance imaging (DW MRI) is a noninvasive imaging technique that makes inferences about cellular structures, at scales smaller than the imaging resolution. One established finding is that DW MRI shows a transient radial alignment in the preterm cortex. In this study, we quantify this maturational process with the âradiality indexâ, a parameter that measures directional coherence, which we expect to change rapidly in the perinatal period. To measure this index, we used structural T2-weighted MRI to segment the cortex and generate cortical meshes. We obtained normal vectors for each face of the mesh and compared them to the principal diffusion direction, calculated by both the DTI and DIAMOND models, to generate the radiality index. The subjects included in this study were 89 infants born at fewer than 34 weeks completed gestation, each imaged at up to four timepoints between 27 and 42 weeks gestational age. In this manuscript, we quantify the longitudinal trajectory of radiality, fractional anisotropy and mean diffusivity from the DTI and DIAMOND models. For the radiality index and fractional anisotropy, the DIAMOND model offers improved sensitivity over the DTI model. The radiality index has a consistent progression across time, with the rate of change depending on the cortical lobe. The occipital lobe changes most rapidly, and the frontal and temporal least: this is commensurate with known developmental anatomy. Analysing the radiality index offers information complementary to other diffusion parameters
SUSY Stops at a Bump
We discuss collider signatures of the "natural supersymmetry" scenario with
baryon-number violating R-parity violation. We argue that this is one of the
few remaining viable incarnations of weak scale supersymmetry consistent with
full electroweak naturalness. We show that this intriguing and challenging
scenario contains distinctive LHC signals, resonances of hard jets in
conjunction with relatively soft leptons and missing energy, which are easily
overlooked by existing LHC searches. We propose novel strategies for
distinguishing these signals above background, and estimate their potential
reach at the 8 TeV LHC. We show that other multi-lepton signals of this
scenario can be seen by currently existing searches with increased statistics,
but these opportunities are more spectrum-dependent.Comment: 23 pages, 7 figures, 3 tables. V2: spectrum discussion corrected,
most of the changes are in Sec. 2. Benchmarks, analysis and conclusions
unchanged. References adde
Relationship between eruptions of active-region filaments and associated flares and CMEs
To better understand the dynamical process of active-region filament
eruptions and associated flares and CMEs, we carried out a statistical study of
120 events observed by BBSO, TRACE, and t(SOHO/EIT) from 1998 to 2007 and
combined filament observations with the NOAA's flare reports, MDI magnetograms,
and LASCO data, to investigate the relationship between active-region filament
eruptions and other solar activities. We found that 115 out of 120 filament
eruptions are associated with flares. 56 out of 105 filament eruptions are
found to be associated with CMEs except for 15 events without corresponding
LASCO data. We note the limitation of coronagraphs duo to geometry or
sensitivity, leading to many smaller CMEs that are Earth-directed or well out
of the plane of sky not being detected by near-Earth spacecraft. Excluding
those without corresponding LASCO data, the CME association rate of
active-region filament eruptions clearly increases with X-ray flare class from
about 32% for C-class flares to 100% for X-class flares. The eruptions of
active-region filaments associated with Halo CMEs are often accompanied by
large flares. About 92% events associated with X-class flare are associated
with Halo CMEs. Such a result is due to that the Earth-directed CMEs detected
as Halo CMEs are often the larger CMEs and many of the smaller ones are not
detected because of the geometry and low intensity. The average speed of the
associated CMEs of filament eruptions increases with X-ray flare size from
563.7 km/s for C-class flares to 1506.6 km/s for X-class flares. Moreover, the
magnetic emergence and cancellation play an important role in triggering
filament eruptions. These findings may be instructive to not only in respect to
the modeling of active-region filament eruptions but also in predicting flares
and CMEs.Comment: 19 Pages, 7 figures, Accepted for publication in MNRA
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
p-type Bi2Se3 for topological insulator and low temperature thermoelectric applications
The growth and elementary properties of p-type Bi2Se3 single crystals are
reported. Based on a hypothesis about the defect chemistry of Bi2Se3, the
p-type behavior has been induced through low level substitutions (1 percent or
less) of Ca for Bi. Scanning tunneling microscopy is employed to image the
defects and establish their charge. Tunneling and angle resolved photoemission
spectra show that the Fermi level has been lowered into the valence band by
about 400 meV in Bi1.98Ca0.02Se3 relative to the n-type material. p-type single
crystals with ab plane Seebeck coefficients of +180 microVK-1 at room
temperature are reported. These crystals show a giant anomalous peak in the
Seebeck coefficient at low temperatures, reaching +120 microVK-1 at 7 K, giving
them a high thermoelectric power factor at low temperatures. In addition to its
interesting thermoelectric properties, p-type Bi2Se3 is of substantial interest
for studies of technologies and phenomena proposed for topological insulators.Comment: v2: STM characterization of n and p type Bi2Se3 is adde
Reconstructing the properties of dark energy from recent observations
We explore the properties of dark energy from recent observational data,
including the Gold Sne Ia, the baryonic acoustic oscillation peak from SDSS,
the CMB shift parameter from WMAP3, the X-ray gas mass fraction in cluster and
the Hubble parameter versus redshift. The model with curvature
and two parameterized dark energy models are studied. For the
model, we find that the flat universe is consistent with observations at the
confidence level and a closed universe is slightly favored by these
data. For two parameterized dark energy models, with the prior given on the
present matter density, , with ,
and , our result seems to suggest that the
trend of dependence for an evolving dark energy from a
combination of the observational data sets is model-dependent.Comment: 16 pages, 15 figures, To appear in JCA
The Horizontal Component of Photospheric Plasma Flows During the Emergence of Active Regions on the Sun
The dynamics of horizontal plasma flows during the first hours of the
emergence of active region magnetic flux in the solar photosphere have been
analyzed using SOHO/MDI data. Four active regions emerging near the solar limb
have been considered. It has been found that extended regions of Doppler
velocities with different signs are formed in the first hours of the magnetic
flux emergence in the horizontal velocity field. The flows observed are
directly connected with the emerging magnetic flux; they form at the beginning
of the emergence of active regions and are present for a few hours. The Doppler
velocities of flows observed increase gradually and reach their peak values
4-12 hours after the start of the magnetic flux emergence. The peak values of
the mean (inside the +/-500 m/s isolines) and maximum Doppler velocities are
800-970 m/s and 1410-1700 m/s, respectively. The Doppler velocities observed
substantially exceed the separation velocities of the photospheric magnetic
flux outer boundaries. The asymmetry was detected between velocity structures
of leading and following polarities. Doppler velocity structures located in a
region of leading magnetic polarity are more powerful and exist longer than
those in regions of following polarity. The Doppler velocity asymmetry between
the velocity structures of opposite sign reaches its peak values soon after the
emergence begins and then gradually drops within 7-12 hours. The peak values of
asymmetry for the mean and maximal Doppler velocities reach 240-460 m/s and
710-940 m/s, respectively. An interpretation of the observable flow of
photospheric plasma is given.Comment: 20 pages, 10 figures, 3 tables. The results of article were presented
at the ESPM-13 (12-16 September 2011, Rhodes, Greece, Abstract Book p. 102,
P.4.12,
http://astro.academyofathens.gr/espm13/documents/ESPM13_abstract_programme_book.pdf
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