3,182,297 research outputs found
Cosmic Infrared Background Fluctuations and Zodiacal Light
We have performed a specific observational test to measure the effect that
the zodiacal light can have on measurements of the spatial fluctuations of the
near-IR background. Previous estimates of possible fluctuations caused by
zodiacal light have often been extrapolated from observations of the thermal
emission at longer wavelengths and low angular resolution, or from IRAC
observations of high latitude fields where zodiacal light is faint and not
strongly varying with time. The new observations analyzed here target the
COSMOS field, at low ecliptic latitude where the zodiacal light intensity
varies by factors of over the range of solar elongations at which the
field can be observed. We find that the white noise component of the spatial
power spectrum of the background is correlated with the modeled zodiacal light
intensity. Roughly half of the measured white noise is correlated with the
zodiacal light, but a more detailed interpretation of the white noise is
hampered by systematic uncertainties that are evident in the zodiacal light
model. At large angular scales () where excess power above the
white noise is observed, we find no correlation of the power with the modeled
intensity of the zodiacal light. This test clearly indicates that the large
scale power in the infrared background is not being caused by the zodiacal
light.Comment: 17 pp. Accepted for publication in the Ap
G-band and Hard X-ray Emissions of the 2006 December 14 flare observed by Hinode/SOT and RHESSI
We report on G-band emission observed by the Solar Optical Telescope onboard
the Hinode satellite in association with the X1.5-class flare on 2006 December
14. The G-band enhancements originate from the footpoints of flaring coronal
magnetic loops, coinciding with non-thermal hard X-ray bremsstrahlung sources
observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager. At the
available 2 minute cadence, the G-band and hard X-ray intensities are
furthermore well correlated in time. Assuming that the G-band enhancements are
continuum emission from a blackbody, we derived the total radiative losses of
the white-light flare (white-light power). If the G-band enhancements
additionally have a contribution from lines, the derived values are
overestimates. We compare the white-light power with the power in hard X-ray
producing electrons using the thick target assumption. Independent of the
cutoff energy of the accelerated electron spectrum, the white-light power and
the power of accelerated electrons are roughly proportional. Using the observed
upper limit of ~30 keV for the cutoff energy, the hard X-ray producing
electrons provide at least a factor of 2 more power than needed to produce the
white-light emission. For electrons above 40 keV, the powers roughly match for
all four of the time intervals available during the impulsive phase. Hence, the
flare-accelerated electrons contain enough energy to produce the white-light
flare emissions. The observed correlation in time, space, and power strongly
suggests that electron acceleration and white-light production in solar flares
are closely related. However, the results also call attention to the
inconsistency in apparent source heights of the hard X-ray (chromosphere) and
white-light (upper photosphere) sources.Comment: 15 pages, 7 figures, accepted for publication in Ap
Path-Length-Resolved Dynamic Light Scattering: Modeling the Transition From Single to Diffusive Scattering
Dynamic light-scattering spectroscopy is used to study Brownian motion within highly scattering samples. The fluctuations of the light field that is backscattered by a suspension of polystyrene microspheres are measured as power spectra by use of low-coherence interferometry to obtain path-length resolution. The data are modeled as the sum of contributions to the detected light weighted by a Poisson probability for the number of events that each component has experienced. By analyzing the broadening of the power spectra as a function of the path length for various sizes of particles, we determine the contribution of multiple scattering to the detected signal as a function of scattering anisotropy
Nonperturbative Description of Deep Inelastic Structure Functions in Light-Front QCD
We explore the deep inelastic structure functions of hadrons
nonperturbatively in an inverse power expansion of the light-front energy of
the probe in the framework of light-front QCD. We arrive at the general
expressions for various structure functions as the Fourier transform of matrix
elements of different components of bilocal vector and axial vector currents on
the light-front in a straightforward manner. The complexities of the structure
functions are mainly carried by the multi-parton wave functions of the hadrons,
while, the bilocal currents have a dynamically dependent yet simple structure
on the light-front in this description. We also present a novel analysis of the
power corrections based on light-front power counting which resolves some
ambiguities of the conventional twist analysis in deep inelastic processes.
Further, the factorization theorem and the scale evolution of the structure
functions are presented in this formalism by using old-fashioned light-front
time-ordered perturbation theory with multi-parton wave functions.
Nonperturbative QCD dynamics underlying the structure functions can be explored
in the same framework. Once the nonperturbative multi-parton wave functions are
known from low-energy light-front QCD, a complete description of deep inelastic
structure functions can be realized.Comment: Revtex, 30 pages and no figur
Distributions of Upper PAPR and Lower PAPR of OFDM Signals in Visible Light Communications
Orthogonal frequency-division multiplexing (OFDM) in visible light
communications (VLC) inherits the disadvantage of high peak-to-average power
ratio (PAPR) from OFDM in radio frequency (RF) communications. The upper peak
power and lower peak power of real-valued VLC-OFDM signals are both limited by
the dynamic constraints of light emitting diodes (LEDs). The efficiency and
transmitted electrical power are directly related with the upper PAPR (UPAPR)
and lower PAPR (LPAPR) of VLC-OFDM. In this paper, we will derive the
complementary cumulative distribution function (CCDF) of UPAPR and LPAPR, and
investigate the joint distribution of UPAPR and LPAPR.Comment: acceptted by IEEE ICASSP 2014. arXiv admin note: text overlap with
arXiv:1304.019
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