16,267 research outputs found
An Online Tutor for Astronomy: The GEAS Self-Review Library
We introduce an interactive online resource for use by students and college
instructors in introductory astronomy courses. The General Education Astronomy
Source (GEAS) online tutor guides students developing mastery of core
astronomical concepts and mathematical applications of general astronomy
material. It contains over 12,000 questions, with linked hints and solutions.
Students who master the material quickly can advance through the topics, while
under-prepared or hesitant students can focus on questions on a certain topic
for as long as needed, with minimal repetition. Students receive individual
accounts for study and course instructors are provided with overview tracking
information, by time and by topic, for entire cohorts of students. Diagnostic
tools support self-evaluation and close collaboration between instructor and
student, even for distance learners. An initial usage study shows clear trends
in performance which increase with study time, and indicates that distance
learners using these materials perform as well as or better than a comparison
cohort of on-campus astronomy students. We are actively seeking new
collaborators to use this resource in astronomy courses and other educational
venues.Comment: 15 pages, 9 figures; Vogt, N. P., and A. S. Muise. 2015. An online
tutor for general astronomy: The GEAS self-review library. Cogent Education,
2 (1
Evolution of virulence in opportunistic pathogens: generalism, plasticity, and control
Standard virulence evolution theory assumes that virulence factors are maintained because they aid parasitic exploitation, increasing growth within and/or transmission between hosts. An increasing number of studies now demonstrate that many opportunistic pathogens (OPs) do not conform to these assumptions, with virulence factors maintained instead because of advantages in non-parasitic contexts. Here we review virulence evolution theory in the context of OPs and highlight the importance of incorporating environments outside a focal virulence site. We illustrate that virulence selection is constrained by correlations between these external and focal settings and pinpoint drivers of key environmental correlations, with a focus on generalist strategies and phenotypic plasticity. We end with a summary of key theoretical and empirical challenges to be met for a fuller understanding of OPs
The Low-High-Low Trend of Type III Radio Burst Starting Frequencies and Solar Flare Hard X-rays
Using simultaneous X-ray and radio observations from solar flares, we
investigate the link between the type III radio burst starting frequency and
hard X-ray spectral index. For a proportion of events the relation derived
between the starting height (frequency) of type III radio bursts and the
electron beam velocity spectral index (deduced from X-rays) is used to infer
the spatial properties (height and size) of the electron beam acceleration
region. Both quantities can be related to the distance travelled before an
electron beam becomes unstable to Langmuir waves. To obtain a list of suitable
events we considered the RHESSI catalogue of X-ray flares and the Phoenix 2
catalogue of type III radio bursts. From the 200 events that showed both type
III and X-ray signatures, we selected 30 events which had simultaneous emission
in both wavelengths, good signal to noise in the X-ray domain and > 20 seconds
duration. We find that > 50 % of the selected events show a good correlation
between the starting frequencies of the groups of type III bursts and the hard
X-ray spectral indices. A low-high-low trend for the starting frequency of type
III bursts is frequently observed. Assuming a background electron density model
and the thick target approximation for X-ray observations, this leads to a
correlation between starting heights of the type III emission and the beam
electron spectral index. Using this correlation we infer the altitude and
vertical extents of the flare acceleration regions. We find heights from 183 Mm
down to 25 Mm while the sizes range from 13 Mm to 2 Mm. These values agree with
previous work that places an extended flare acceleration region high in the
corona. We analyse the assumptions required and explore possible extensions to
our assumed model. We discuss these results with respect to the acceleration
heights and sizes derived from X-ray observations alone.Comment: 15 pages, 8 figures, Accepted to Astronomy and Astrophysic
Low frequency radio observations of bi-directional electron beams in the solar corona
The radio signature of a shock travelling through the solar corona is known
as a type II solar radio burst. In rare cases these bursts can exhibit a fine
structure known as `herringbones', which are a direct indicator of particle
acceleration occurring at the shock front. However, few studies have been
performed on herringbones and the details of the underlying particle
acceleration processes are unknown. Here, we use an image processing technique
known as the Hough transform to statistically analyse the herringbone fine
structure in a radio burst at 20-90 MHz observed from the Rosse
Solar-Terrestrial Observatory on 2011 September 22. We identify 188 individual
bursts which are signatures of bi-directional electron beams continuously
accelerated to speeds of 0.16. This occurs at a shock
acceleration site initially at a constant altitude of 0.6 R in
the corona, followed by a shift to 0.5 R. The anti-sunward
beams travel a distance of 170 Mm (and possibly further) away
from the acceleration site, while those travelling toward the sun come to a
stop sooner, reaching a smaller distance of 112 Mm. We show that
the stopping distance for the sunward beams may depend on the total number
density and the velocity of the beam. Our study concludes that a detailed
statistical analysis of herringbone fine structure can provide information on
the physical properties of the corona which lead to these relatively rare radio
bursts
Transient microbiota exposures activate dormant Escherichia coli infection in the bladder and drive severe outcomes of recurrent disease
Pathogens often inhabit the body asymptomatically, emerging to cause disease in response to unknown triggers. In the bladder, latent intracellular Escherichia coli reservoirs are regarded as likely origins of recurrent urinary tract infection (rUTI), a problem affecting millions of women worldwide. However, clinically plausible triggers that activate these reservoirs are unknown. Clinical studies suggest that the composition of a woman's vaginal microbiota influences her susceptibility to rUTI, but the mechanisms behind these associations are unclear. Several lines of evidence suggest that the urinary tract is routinely exposed to vaginal bacteria, including Gardnerella vaginalis, a dominant member of the vaginal microbiota in some women. Using a mouse model, we show that bladder exposure to G. vaginalis triggers E. coli egress from latent bladder reservoirs and enhances the potential for life-threatening outcomes of the resulting E. coli rUTI. Transient G. vaginalis exposures were sufficient to cause bladder epithelial apoptosis and exfoliation and interleukin-1-receptor-mediated kidney injury, which persisted after G. vaginalis clearance from the urinary tract. These results support a broader view of UTI pathogenesis in which disease can be driven by short-lived but powerful urinary tract exposures to vaginal bacteria that are themselves not "uropathogenic" in the classic sense. This "covert pathogenesis" paradigm may apply to other latent infections, (e.g., tuberculosis), or for diseases currently defined as noninfectious because routine culture fails to detect microbes of recognized significance
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