22,901 research outputs found
Backscatter of hard X-rays in the solar atmosphere
The solar photosphere backscatters a substantial fraction of the hard X rays from solar flares incident upon it. This reflection was studied using a Monte Carlo simulation which takes into account Compton scattering and photo-electric absorption. Both isotropic and anisotropic X ray sources are considered. The bremsstrahlung from an anisotropic distribution of electrons are evaluated. By taking the reflection into account, the inconsistency is removed between recent observational data regarding the center-to-limb variation of solar X ray emission and the predictions of models in which accelerated electrons are moving down toward the photosphere
Gamma-ray and microwave evidence for two phases of acceleration in solar flares
Relativistic electrons in large solar flares produce gamma ray continuum by bremsstrahlung and microwave emission by gyrosynchrotron radiation. Using observations of the 1972, August 4 flare, the electron spectrum and the physical properties of the common emitting region of these radiations were evaluated. Information was also obtained on energetic protons in this flare by using gamma ray lines. From the electron spectrum, the proton-to-electron ratio, and the time dependences of the microwave emission, the 2.2 MeV line and the gamma ray continuum, it was concluded that in large solar flares relativistic electrons and energetic nuclei are accelerated by a mechanism which is different from the mechanism which accelerates approximately less than 100 keV electrons in flares
Time-Series Analysis of Super-Kamiokande Measurements of the Solar Neutrino Flux
The Super-Kamiokande Consortium has recently released data suitable for
time-series analysis. The binning is highly regular: the power spectrum of the
acquisition times has a huge peak (power S > 120) at the frequency (in cycles
per year) 35.98 (period 10.15 days), where power measurements are such that the
probability of obtaining a peak of strength S or more by chance at a specified
frequency is exp(-S). This inevitably leads to severe aliasing of the power
spectrum. The strongest peak in the range 0 - 100 in a power spectrum formed by
a likelihood procedure is at 26.57 (period 13.75 days) with S = 11.26. For the
range 0 - 40, the second-strongest peak is at 9.42 (period 38.82 days) with S =
7.3. Since 26.57 + 9.42 = 35.99, we conclude that the weaker peak at 9.42 is an
alias of the stronger peak at 26.57. We note that 26.57 falls in the band 26.36
- 27.66, formed from twice the range of synodic rotation frequencies of an
equatorial section of the Sun for normalized radius larger than 0.1.
Oscillations at twice the rotation frequency, attributable to "m = 2"
structures, are not uncommon in solar data. We find from the shuffle test that
the probability of obtaining a peak of S = 11.26 or more by chance in this band
is 0.1 %. This new result therefore supports at the 99.9% confidence level
previous evidence, found in Homestake and GALLEX-GNO data, for rotational
modulation of the solar neutrino flux. The frequency 25.57 points to a source
of modulation at or near the tachocline.Comment: 15 pages, 8 figure
High-Dimensional Inference with the generalized Hopfield Model: Principal Component Analysis and Corrections
We consider the problem of inferring the interactions between a set of N
binary variables from the knowledge of their frequencies and pairwise
correlations. The inference framework is based on the Hopfield model, a special
case of the Ising model where the interaction matrix is defined through a set
of patterns in the variable space, and is of rank much smaller than N. We show
that Maximum Lik elihood inference is deeply related to Principal Component
Analysis when the amp litude of the pattern components, xi, is negligible
compared to N^1/2. Using techniques from statistical mechanics, we calculate
the corrections to the patterns to the first order in xi/N^1/2. We stress that
it is important to generalize the Hopfield model and include both attractive
and repulsive patterns, to correctly infer networks with sparse and strong
interactions. We present a simple geometrical criterion to decide how many
attractive and repulsive patterns should be considered as a function of the
sampling noise. We moreover discuss how many sampled configurations are
required for a good inference, as a function of the system size, N and of the
amplitude, xi. The inference approach is illustrated on synthetic and
biological data.Comment: Physical Review E: Statistical, Nonlinear, and Soft Matter Physics
(2011) to appea
Testing linear hypotheses in high-dimensional regressions
For a multivariate linear model, Wilk's likelihood ratio test (LRT)
constitutes one of the cornerstone tools. However, the computation of its
quantiles under the null or the alternative requires complex analytic
approximations and more importantly, these distributional approximations are
feasible only for moderate dimension of the dependent variable, say .
On the other hand, assuming that the data dimension as well as the number
of regression variables are fixed while the sample size grows, several
asymptotic approximations are proposed in the literature for Wilk's \bLa
including the widely used chi-square approximation. In this paper, we consider
necessary modifications to Wilk's test in a high-dimensional context,
specifically assuming a high data dimension and a large sample size .
Based on recent random matrix theory, the correction we propose to Wilk's test
is asymptotically Gaussian under the null and simulations demonstrate that the
corrected LRT has very satisfactory size and power, surely in the large and
large context, but also for moderately large data dimensions like or
. As a byproduct, we give a reason explaining why the standard chi-square
approximation fails for high-dimensional data. We also introduce a new
procedure for the classical multiple sample significance test in MANOVA which
is valid for high-dimensional data.Comment: Accepted 02/2012 for publication in "Statistics". 20 pages, 2 pages
and 2 table
QCD Matter Thermalization at RHIC and LHC
Employing the perturbative QCD inspired parton cascade, we investigate
kinetic and chemical equilibration of the partonic matter created in central
heavy ion collisions at RHIC and LHC energies. Two types of initial conditions
are chosen. One is generated by the model of wounded nucleons using the PYTHIA
event generator and Glauber geometry. Another is considered as a color glass
condensate. We show that kinetic equilibration is almost independent on the
chosen initial conditions, whereas there is a sensitive dependence for chemical
equilibration. The time scale of thermalization lies between 1 and 1.5 fm/c.
The final parton transverse energy obtained from BAMPS calculations is compared
with the RHIC data and is estimated for the LHC energy.Comment: 8 pages, 10 figures, plenary talk at International Conference on
Strangeness in Quark Matter 2008, Beijing, China, October 6-10, 200
Tuneable Dual-band Antenna for Sub 1 GHz Cellular Mobile Radio Applications
In this paper, a compact tuneable dual-band slot
antenna operating over the frequency range from 560 MHz to 1
GHz is presented and evaluated through a hardware-in-the-loop
test-bed. The co-existence of high order modulation schemes is
investigated in DTT and low LTE bands. EVM results show that
the proposed antenna can support two operating frequency
bands simultaneously where each band can be tuned
independently for carrier aggregation with negligible crosstalk
Concurrent, Tunable, Multi-band, Single Chain Radio Receivers for 5G RANs
A concurrent, tunable, tri-band, single chain radio receiver for 5G radio access networks is evaluated. The three concurrent bands are independently tunable over a frequency range from 600 MHz to 2.7 GHz. A hardware-in-the-loop test-bed provides a system level evaluation of the proposed receiver using direct RF digitization. The test-bed emulates a 5G heterogeneous network supporting three wideband, simultaneous connections. By measuring the receiver EVM, we demonstrate sufficient isolation between concurrent bands achieving 60 MHz of aggregated bandwidth as well as strong resilience to adjacent blockers
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