352 research outputs found
A Comparative Study of the Depth of Maximum of Simulated Air Shower Longitudinal Profiles
A comparative study of simulated air shower longitudinal profiles is
presented. An appropriate thinning level for the calculations is first
determined empirically. High statistics results are then provided, over a wide
energy range, (10^14.0 to 10^20.5 eV), for proton & iron primaries, using four
combinations of the MOCCA & CORSIKA program frameworks, and the SIBYLL & QGSJET
high energy hadronic interaction models. These results are compared to existing
experimental data. The way in which the first interaction controls Xmax is
investigated, as is the distribution of Xmax.Comment: 13 pages, 5 figures. Accepted by Astroparticle Physics. (Revised
according to referee's comments.
Determining the race structure of Leptosphaeria maculans in western Canada
Non-Peer ReviewedEach year blackleg disease of canola (Brassica napus L.), caused by Leptosphaeria maculans (Desmaz.) Ces. & De Not., is responsible for significant yield loss of Brassica napus L., oilseed rape and canola worldwide. In western Canada, blackleg disease has been managed using a combination of four-year rotations and resistant canola varieties. To determine the current race structures present in western Canada, isolates were collected from eight locations across Alberta, Saskatchewan, and Manitoba in 2007 and 2008. These isolates were inoculated onto eight different canola varieties containing different resistant genes: âWestarâ, âQuintaâ, âGlacierâ, âMT29â, âSamouraiâ, âQuantumâ, âFalconâ and âDarmorâ. Using the gene-for-gene theory, race structure was determined based on variety resistance to L. maculans. A scale of 0 (no disease symptoms) to 9 (severely diseased) (Newman 1980) was implemented to assess plant resistance, with a score of 5 and above suggesting susceptibility. Preliminary results have identified 18 different races, nine of which contain the majority of isolates. These results have also shown that the frequency of L. maculans avirulent alleles greatly varies between the different sites in western Canada
Intensity and velocity oscillations in a flaring active region
Chromospheric oscillations can give us insight into the physical environment in the solar atmosphere, both in quiet Sun and flaring conditions. Many authors have reported increases in the prevalence of 3-minute oscillations which are thought to be excited by events which impact the chromosphere such as flares. In this study, we utilized the CaâII 8542âĂ
line to study the oscillatory behaviour of the chromosphere in an active region which underwent two B-class flares. We analysed oscillations in both intensity and velocity, and found different behaviours in both. Intensity oscillations were most prevalent over the umbrae of sunspots and magnetic pores in the active region, and the extent of the area which contained significant oscillations was found to decrease when comparing times after the flares to before. By measuring the evolution of the magnetic field, we found that this could be because the areas surrounding the umbrae were becoming more âpenumbralâ with an increase to the magnetic field inclination. Velocity oscillations were found across the active region both before and after the flares but were observed clearly in areas which were brightened by the second flare. By comparing to EUV imaging, it was seen that strong chromospheric velocity oscillations with 3â4-minute periods occurred at the same time and location as a flare loop cooling 30âmin after the second flare peak. This could be evidence of disturbances in the loop exciting a response from the chromosphere at its acoustic cut-off frequency
A Measurement of the Cosmic Ray Spectrum and Composition at the Knee
The energy spectrum and primary composition of cosmic rays with energy
between and 3\times10^{16}\unit{eV} have been studied using
the CASA-BLANCA detector. CASA measured the charged particle distribution of
air showers, while BLANCA measured the lateral distribution of Cherenkov light.
The data are interpreted using the predictions of the CORSIKA air shower
simulation coupled with four different hadronic interaction codes.
The differential flux of cosmic rays measured by BLANCA exhibits a knee in
the range of 2--3 PeV with a width of approximately 0.5 decades in primary
energy. The power law indices of the differential flux below and above the knee
are and .
We present our data both as a mean depth of shower maximum and as a mean
nuclear mass. A multi-component fit using four elemental species shows the same
composition trends given by the mean quantities, and also indicates that QGSJET
and VENUS are the preferred hadronic interaction models. We find that an
initially mixed composition turns lighter between 1 and 3 PeV, and then becomes
heavier with increasing energy above 3 PeV.Comment: 25 pages, 10 figures. Submitted to Astroparticle Physic
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
Text Line Segmentation of Historical Documents: a Survey
There is a huge amount of historical documents in libraries and in various
National Archives that have not been exploited electronically. Although
automatic reading of complete pages remains, in most cases, a long-term
objective, tasks such as word spotting, text/image alignment, authentication
and extraction of specific fields are in use today. For all these tasks, a
major step is document segmentation into text lines. Because of the low quality
and the complexity of these documents (background noise, artifacts due to
aging, interfering lines),automatic text line segmentation remains an open
research field. The objective of this paper is to present a survey of existing
methods, developed during the last decade, and dedicated to documents of
historical interest.Comment: 25 pages, submitted version, To appear in International Journal on
Document Analysis and Recognition, On line version available at
http://www.springerlink.com/content/k2813176280456k3
Single-electron transport driven by surface acoustic waves: moving quantum dots versus short barriers
We have investigated the response of the acoustoelectric current driven by a
surface-acoustic wave through a quantum point contact in the closed-channel
regime. Under proper conditions, the current develops plateaus at integer
multiples of ef when the frequency f of the surface-acoustic wave or the gate
voltage Vg of the point contact is varied. A pronounced 1.1 MHz beat period of
the current indicates that the interference of the surface-acoustic wave with
reflected waves matters. This is supported by the results obtained after a
second independent beam of surface-acoustic wave was added, traveling in
opposite direction. We have found that two sub-intervals can be distinguished
within the 1.1 MHz modulation period, where two different sets of plateaus
dominate the acoustoelectric-current versus gate-voltage characteristics. In
some cases, both types of quantized steps appeared simultaneously, though at
different current values, as if they were superposed on each other. Their
presence could result from two independent quantization mechanisms for the
acoustoelectric current. We point out that short potential barriers determining
the properties of our nominally long constrictions could lead to an additional
quantization mechanism, independent from those described in the standard model
of 'moving quantum dots'.Comment: 25 pages, 12 figures, to be published in a special issue of J. Low
Temp. Phys. in honour of Prof. F. Pobel
The Composition of Cosmic Rays at the Knee
The observation of a small change in spectral slope, or 'knee' in the fluxes
of cosmic rays near energies 10^15 eV has caused much speculation since its
discovery over 40 years ago. The origin of this feature remains unknown. A
small workshop to review some modern experimental measurements of this region
was held at the Adler Planetarium in Chicago, USA in June 2000. This paper
summarizes the results presented at this workshop and the discussion of their
interpretation in the context of hadronic models of atmospheric airshowers.Comment: 36 pages, 10 figure
The Flare-energy Distributions Generated by Kink-unstable Ensembles of Zero-net-current Coronal Loops
It has been proposed that the million degree temperature of the corona is due
to the combined effect of barely-detectable energy releases, so called
nanoflares, that occur throughout the solar atmosphere. Alas, the nanoflare
density and brightness implied by this hypothesis means that conclusive
verification is beyond present observational abilities. Nevertheless, we
investigate the plausibility of the nanoflare hypothesis by constructing a
magnetohydrodynamic (MHD) model that can derive the energy of a nanoflare from
the nature of an ideal kink instability. The set of energy-releasing
instabilities is captured by an instability threshold for linear kink modes.
Each point on the threshold is associated with a unique energy release and so
we can predict a distribution of nanoflare energies. When the linear
instability threshold is crossed, the instability enters a nonlinear phase as
it is driven by current sheet reconnection. As the ensuing flare erupts and
declines, the field transitions to a lower energy state, which is modelled by
relaxation theory, i.e., helicity is conserved and the ratio of current to
field becomes invariant within the loop. We apply the model so that all the
loops within an ensemble achieve instability followed by energy-releasing
relaxation. The result is a nanoflare energy distribution. Furthermore, we
produce different distributions by varying the loop aspect ratio, the nature of
the path to instability taken by each loop and also the level of radial
expansion that may accompany loop relaxation. The heating rate obtained is just
sufficient for coronal heating. In addition, we also show that kink instability
cannot be associated with a critical magnetic twist value for every point along
the instability threshold
Modest agreement between magnetic resonance and pathological tumor regression after neoadjuvant therapy for rectal cancer in the real world.
Magnetic resonance imaging (MRI) is routinely used for preoperative tumor staging and to assess response to therapy in rectal cancer patients. The aim of our study was to evaluate the accuracy of MRI based restaging after neoadjuvant chemoradiotherapy (CRT) in predicting pathologic response. This multicenter cohort study included adult patients with histologically confirmed locally advanced rectal adenocarcinoma treated with neoadjuvant CRT followed by curative intent elective surgery between January 2014 and December 2019 at four academic high-volume institutions. Magnetic resonance tumor regression grade (mrTRG) and pathologic tumor regression grade (pTRG) were reviewed and compared for all the patients. The agreement between radiologist and pathologist was assessed with the weighted k test. Risk factors for poor agreement were investigated using logistic regression. A total of 309 patients were included. Modest agreement was found between mrTRG and pTRG when regression was classified according to standard five-tier systems (k = 0.386). When only two categories were considered for each regression system, (pTRG 0-3 vs pTRG 4; mrTRG 2-5 vs mrTRG 1) an accuracy of 78% (95% confidence interval [CI] 0.73-0.83) was found between radiologic and pathologic assessment with a k value of 0.185. The logistic regression model revealed that "T3 greater than 5 mm extent" was the only variable significantly impacting on disagreement (OR 0.33, 95% CI 0.15-0.68, P = .0034). Modest agreement exists between mrTRG and pTRG. The chances of appropriate assessment of the regression grade after neoadjuvant CRT appear to be higher in case of a T3 tumor with at least 5 mm extension in the mesorectal fat at the pretreatment MRI
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