147 research outputs found
Limit on UHE Neutrino Flux from the Parkes Lunar Radio Cherenkov Experiment
The first search for ultra-high energy (UHE) neutrinos using a radio
telescope was conducted by Hankins, Ekers and O'Sullivan (1996). This was a
search for nanosecond duration radio Cherenkov pulses from electromagnetic
cascades initiated by ultra-high energy (UHE) neutrino interactions in the
lunar regolith, and was made using a broad-bandwidth receiver fitted to the
Parkes radio telescope, Australia. At the time, no simulations were available
to convert the null result into a neutrino flux limit. Since then, similar
experiments at Goldstone, USA, and Kalyazin, Russia, have also recorded null
results, and computer simulations have been used to model the experimental
sensitivities of these two experiments and put useful limits on the UHE
neutrino flux.
Proposed future experiments include the use of broad-bandwidth receivers,
making the sensitivity achieved by the Parkes experiment highly relevant to the
future prospects of this field. We have therefore calculated the effective
aperture for the Parkes experiment and found that when pointing at the lunar
limb, the effective aperture at all neutrino energies was superior to
single-antenna, narrow-bandwidth experiments, and that the detection threshold
was comparable to that of the double-antenna experiment at Goldstone. However,
because only a small fraction of the observing time was spent pointing the
limb, the Parkes experiment places only comparatively weak limits on the UHE
neutrino flux. Future efforts should use multiple telescopes and
broad-bandwidth receivers.Comment: 6 pages, 2 figures, accepted for publication in MNRA
LUNASKA experiments using the Australia Telescope Compact Array to search for ultra-high energy neutrinos and develop technology for the lunar Cherenkov technique
We describe the design, performance, sensitivity and results of our recent
experiments using the Australia Telescope Compact Array (ATCA) for lunar
Cherenkov observations with a very wide (600 MHz) bandwidth and nanosecond
timing, including a limit on an isotropic neutrino flux. We also make a first
estimate of the effects of small-scale surface roughness on the effective
experimental aperture, finding that contrary to expectations, such roughness
will act to increase the detectability of near-surface events over the neutrino
energy-range at which our experiment is most sensitive (though distortions to
the time-domain pulse profile may make identification more difficult). The aim
of our "Lunar UHE Neutrino Astrophysics using the Square Kilometer Array"
(LUNASKA) project is to develop the lunar Cherenkov technique of using
terrestrial radio telescope arrays for ultra-high energy (UHE) cosmic ray (CR)
and neutrino detection, and in particular to prepare for using the Square
Kilometer Array (SKA) and its path-finders such as the Australian SKA
Pathfinder (ASKAP) and the Low Frequency Array (LOFAR) for lunar Cherenkov
experiments.Comment: 27 pages, 18 figures, 4 tables
LUNASKA Experiment Observational Limits on UHE Neutrinos from Centaurus A and the Galactic Center
We present the first observational limits to the ultra-high energy (UHE)
neutrino flux from the Galactic Center, and from Centaurus A which is the
nearest active galactic nucleus (AGN). These results are based on our "Lunar
UHE Neutrino Astrophysics using the Square Kilometer Array" (LUNASKA) project
experiments at the Australia Telescope Compact Array (ATCA). We also derive
limits for the previous experiments and compare these limits with expectations
for acceleration and super-heavy dark matter models of the origin of UHE cosmic
rays.Comment: 5 pages, 4 figures, additional experimental details and references
given, limits now calculated also for NuMoon, uncertainties in calculation of
LUNASKA limits now discussed, figures updated. Accepted for publication in
MNRAS on 5 August 201
Tissue tolerance: an essential but elusive trait for salt-tolerant crops
For a plant to persist in saline soil, osmotic adjustment of all plant cells is essential. The more salt-tolerant species accumulate Na+ and Clâ to concentrations in leaves and roots that are similar to the external solution, thus allowing energy-efficient osmotic adjustment. Adverse effects of Na+ and Clâ on metabolism must be avoided, resulting in a situation known as âtissue toleranceâ. The strategy of sequestering Na+ and Clâ in vacuoles and keeping concentrations low in the cytoplasm is an important contributor to tissue tolerance. Although there are clear differences between species in the ability to accommodate these ions in their leaves, it remains unknown whether there is genetic variation in this ability within a species. This viewpoint considers the concept of tissue tolerance, and how to measure it. Four conclusions are drawn: (1) osmotic adjustment is inseparable from the trait of tissue tolerance; (2) energy-efficient osmotic adjustment should involve ions and only minimal organic solutes; (3) screening methods should focus on measuring tolerance, not injury; and (4) high-throughput protocols that avoid the need for control plants and multiple Na+ or Cl-
measurements should be developed. We present guidelines to identify useful genetic variation in tissue tolerance that can be harnessed for plant breeding of salt tolerance
Peatland Initiation, Carbon Accumulation, and 2 ka Depth in the James Bay Lowland and Adjacent Regions
Copyright © 2014 University of Colorado at Boulder, Institute of Arctic and Alpine ResearchPeatlands surrounding Hudson and James Bays form the second largest peatland complex in the world and contain major stores of soil carbon (C). This study utilized a transect of eight ombrotrophic peat cores from remote regions of central and northern Ontario to quantify the magnitude and rate of C accumulation since peatland initiation and for the past 2000 calendar years before present (2 ka). These new data were supplemented by 17 millennially resolved chronologies from a literature review covering the Boreal Shield, Hudson Plains, and Taiga Shield bordering Hudson and James Bays. Peatlands initiated in central and northern Ontario by 7.8 ka following deglaciation and isostatic emergence of northern areas to above sea level. Total C accumulated since inception averaged 109.7 ± (std. dev.) 36.2 kg C mâ2. Approximately 40% of total soil C has accumulated since 2 ka at an average apparent rate of 20.2 ± 6.9 g C mâ2 yrâ1. The 2 ka depths correlate significantly and positively with modern gridded climate estimates for mean annual precipitation, mean annual air temperature, growing degree-days > 0 °C, and photosynthetically active radiation integrated over days > 0 °C. There are significantly shallower depths in permafrost peatlands. Vertical peat accumulation was likely constrained by temperature, growing season length, and photosynthetically active radiation over the last 2 ka in the Hudson Bay Lowlands and surrounding regions.US National Science Foundatio
Measurement of the cosmic ray spectrum above eV using inclined events detected with the Pierre Auger Observatory
A measurement of the cosmic-ray spectrum for energies exceeding
eV is presented, which is based on the analysis of showers
with zenith angles greater than detected with the Pierre Auger
Observatory between 1 January 2004 and 31 December 2013. The measured spectrum
confirms a flux suppression at the highest energies. Above
eV, the "ankle", the flux can be described by a power law with
index followed by
a smooth suppression region. For the energy () at which the
spectral flux has fallen to one-half of its extrapolated value in the absence
of suppression, we find
eV.Comment: Replaced with published version. Added journal reference and DO
Energy Estimation of Cosmic Rays with the Engineering Radio Array of the Pierre Auger Observatory
The Auger Engineering Radio Array (AERA) is part of the Pierre Auger
Observatory and is used to detect the radio emission of cosmic-ray air showers.
These observations are compared to the data of the surface detector stations of
the Observatory, which provide well-calibrated information on the cosmic-ray
energies and arrival directions. The response of the radio stations in the 30
to 80 MHz regime has been thoroughly calibrated to enable the reconstruction of
the incoming electric field. For the latter, the energy deposit per area is
determined from the radio pulses at each observer position and is interpolated
using a two-dimensional function that takes into account signal asymmetries due
to interference between the geomagnetic and charge-excess emission components.
The spatial integral over the signal distribution gives a direct measurement of
the energy transferred from the primary cosmic ray into radio emission in the
AERA frequency range. We measure 15.8 MeV of radiation energy for a 1 EeV air
shower arriving perpendicularly to the geomagnetic field. This radiation energy
-- corrected for geometrical effects -- is used as a cosmic-ray energy
estimator. Performing an absolute energy calibration against the
surface-detector information, we observe that this radio-energy estimator
scales quadratically with the cosmic-ray energy as expected for coherent
emission. We find an energy resolution of the radio reconstruction of 22% for
the data set and 17% for a high-quality subset containing only events with at
least five radio stations with signal.Comment: Replaced with published version. Added journal reference and DO
Measurement of the Radiation Energy in the Radio Signal of Extensive Air Showers as a Universal Estimator of Cosmic-Ray Energy
We measure the energy emitted by extensive air showers in the form of radio
emission in the frequency range from 30 to 80 MHz. Exploiting the accurate
energy scale of the Pierre Auger Observatory, we obtain a radiation energy of
15.8 \pm 0.7 (stat) \pm 6.7 (sys) MeV for cosmic rays with an energy of 1 EeV
arriving perpendicularly to a geomagnetic field of 0.24 G, scaling
quadratically with the cosmic-ray energy. A comparison with predictions from
state-of-the-art first-principle calculations shows agreement with our
measurement. The radiation energy provides direct access to the calorimetric
energy in the electromagnetic cascade of extensive air showers. Comparison with
our result thus allows the direct calibration of any cosmic-ray radio detector
against the well-established energy scale of the Pierre Auger Observatory.Comment: Replaced with published version. Added journal reference and DOI.
Supplemental material in the ancillary file
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