296 research outputs found
Thermalisation of self-interacting solar flare fast electrons
Most theoretical descriptions of the production of solar flare bremsstrahlung
radiation assume the collision of dilute accelerated particles with a cold,
dense target plasma, neglecting interactions of the fast particles with each
other. This is inadequate for situations where collisions with this background
plasma are not completely dominant, as may be the case in, for example,
low-density coronal sources. We aim to formulate a model of a self-interacting,
entirely fast electron population in the absence of a dense background plasma,
to investigate its implications for observed bremsstrahlung spectra and the
flare energy budget. We derive approximate expressions for the time-dependent
distribution function of the fast electrons using a Fokker-Planck approach. We
use these expressions to generate synthetic bremsstrahlung X-ray spectra as
would be seen from a corresponding coronal source. We find that our model
qualitatively reproduces the observed behaviour of some flares. As the flare
progresses, the model's initial power-law spectrum is joined by a lower energy,
thermal component. The power-law component diminishes, and the growing thermal
component proceeds to dominate the total emission over timescales consistent
with flare observations. The power-law exhibits progressive spectral hardening,
as is seen in some flare coronal sources. We also find that our model requires
a factor of 7 - 10 fewer accelerated electrons than the cold, thick target
model to generate an equivalent hard X-ray flux. This model forms the basis of
a treatment of self-interactions among flare fast electrons, a process which
affords a more efficient means to produce bremsstrahlung photons and so may
reduce the efficiency requirements placed on the particle acceleration
mechanism. It also provides a useful description of the thermalisation of fast
electrons in coronal sources.Comment: 9 pages, 7 figures, accepted for Astronomy & Astrophysics; this
version clarifies arguments around Eqs. (11) and (20
Impact of Unexpected Events, Shocking News and Rumours on Foreign Exchange Market Dynamics
We analyze the dynamical response of the world's financial community to
various types of unexpected events, including the 9/11 terrorist attacks as
they unfolded on a minute-by-minute basis. We find that there are various
'species' of news, characterized by how quickly the news get absorbed, how much
meaning and importance is assigned to it by the community, and what subsequent
actions are then taken. For example, the response to the unfolding events of
9/11 shows a gradual collective understanding of what was happening, rather
than an immediate realization. For news items which are not simple economic
statements, and hence whose implications are not immediately obvious, we
uncover periods of collective discovery during which collective opinions seem
to oscillate in a remarkably synchronized way. In the case of a rumour, our
findings also provide a concrete example of contagion in inter-connected
communities. Practical applications of this work include the possibility of
producing selective newsfeeds for specific communities, based on their likely
impact
Certification in molecular pathology in the united states: An update from the association for molecular pathology training and education committee
The past 25 years have witnessed the field of molecular pathology evolving from an imprecisely defined discipline to a firmly established medical subspecialty that plays an essential role in patient care. During this time, the training, certification, and licensure requirements for directing and performing testing in a molecular pathology or molecular diagnostics laboratory have become better defined. The purpose of this document is to describe the various board certifications available to individuals seeking certification in molecular diagnostics at the level of laboratory director, supervisor, or technologist. Several national organizations offer certification in molecular pathology or molecular diagnostics for doctoral-level clinical scientists to function as the director of a molecular diagnostics laboratory. Furthermore, 12 states and Puerto Rico require licensing of medical technologists, including those working in molecular diagnostic laboratories. The information provided here updates a 2002 document by the Training and Education Committee of the Association for Molecular Pathology and has been expanded to include certification and licensing requirements for laboratory technologists
Regularized energy-dependent solar flare hard x-ray spectral index
The deduction from solar flare X-ray photon spectroscopic data of the energy
dependent model-independent spectral index is considered as an inverse problem.
Using the well developed regularization approach we analyze the energy
dependency of spectral index for a high resolution energy spectrum provided by
Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The regularization
technique produces much smoother derivatives while avoiding additional errors
typical of finite differences. It is shown that observations imply a spectral
index varying significantly with energy, in a way that also varies with time as
the flare progresses. The implications of these findings are discussed in the
solar flare context.Comment: 13 pages; 5 figures, Solar Physics in pres
Compton backscattered and primary X-rays from solar flares: angle dependent Green's function correction for photospheric albedo
The observed hard X-ray (HXR) flux spectrum from solar flares
is a combination of primary bremsstrahlung photons with a
spectrally modified component from photospheric Compton backscatter of downward
primary emission. The latter can be significant, distorting or hiding the true
features of the primary spectrum which are key diagnostics for acceleration and
propagation of high energy electrons and of their energy budget. For the first
time in solar physics, we use a Green's function approach to the backscatter
spectral deconvolution problem, constructing a Green's matrix including
photoelectric absorption. This approach allows spectrum-independent extraction
of the primary spectrum for several HXR flares observed by the {\it Ramaty High
Energy Solar Spectroscopic Imager} (RHESSI). We show that the observed and
primary spectra differ very substantially for flares with hard spectra close to
the disk centre. We show in particular that the energy dependent photon
spectral index is very different
for and for and that inferred mean source
electron spectra differ greatly. Even for a forward fitting of a
parametric to the data, a clear low-energy cutoff required to fit
essentially disappears when the fit is to - i.e.
when albedo correction is included. The self-consistent correction for
backscattered photons is thus shown to be crucial in determining the energy
spectra of flare accelerated electrons, and hence their total number and
energy.Comment: 8 pages, 8 figures, Accepted to Astronomy and Astrophysic
Human candidate gene polymorphisms and risk of severe malaria in children in Kilifi, Kenya: a case-control association study
Background: Human genetic factors are important determinants of malaria risk. We investigated associations between multiple candidate polymorphismsâmany related to the structure or function of red blood cellsâand risk for severe Plasmodium falciparum malaria and its specific phenotypes, including cerebral malaria, severe malaria anaemia, and respiratory distress. Methods: We did a case-control study in Kilifi County, Kenya. We recruited as cases children presenting with severe malaria to the high-dependency ward of Kilifi County Hospital. We included as controls infants born in the local community between Aug 1, 2006, and Sept 30, 2010, who were part of a genetics study. We tested for associations between a range of candidate malaria-protective genes and risk for severe malaria and its specific phenotypes. We used a permutation approach to account for multiple comparisons between polymorphisms and severe malaria. We judged p values less than 0·005 significant for the primary analysis of the association between candidate genes and severe malaria. Findings: Between June 11, 1995, and June 12, 2008, 2244 children with severe malaria were recruited to the study, and 3949 infants were included as controls. Overall, 263 (12%) of 2244 children with severe malaria died in hospital, including 196 (16%) of 1233 with cerebral malaria. We investigated 121 polymorphisms in 70 candidate severe malaria-associated genes. We found significant associations between risk for severe malaria overall and polymorphisms in 15 genes or locations, of which most were related to red blood cells: ABO, ATP2B4, ARL14, CD40LG, FREM3, INPP4B, G6PD, HBA (both HBA1 and HBA2), HBB, IL10, LPHN2 (also known as ADGRL2), LOC727982, RPS6KL1, CAND1, and GNAS. Combined, these genetic associations accounted for 5·2% of the variance in risk for developing severe malaria among individuals in the general population. We confirmed established associations between severe malaria and sickle-cell trait (odds ratio [OR] 0·15, 95% CI 0·11â0·20; p=2·61 Ă 10â58), blood group O (0·74, 0·66â0·82; p=6·26 Ă 10â8), and âα3·7-thalassaemia (0·83, 0·76â0·90; p=2·06 Ă 10â6). We also found strong associations between overall risk of severe malaria and polymorphisms in both ATP2B4 (OR 0·76, 95% CI 0·63â0·92; p=0·001) and FREM3 (0·64, 0·53â0·79; p=3·18 Ă 10â14). The association with FREM3 could be accounted for by linkage disequilibrium with a complex structural mutation within the glycophorin gene region (comprising GYPA, GYPB, and GYPE) that encodes for the rare Dantu blood group antigen. Heterozygosity for Dantu was associated with risk for severe malaria (OR 0·57, 95% CI 0·49â0·68; p=3·22 Ă 10â11), as was homozygosity (0·26, 0·11â0·62; p=0·002). Interpretation: Both ATP2B4 and the Dantu blood group antigen are associated with the structure and function of red blood cells. ATP2B4 codes for plasma membrane calcium-transporting ATPase 4 (the major calcium pump on red blood cells) and the glycophorins are ligands for parasites to invade red blood cells. Future work should aim at uncovering the mechanisms by which these polymorphisms can result in severe malaria protection and investigate the implications of these associations for wider health. Funding: Wellcome Trust, UK Medical Research Council, European Union, and Foundation for the National Institutes of Health as part of the Bill & Melinda Gates Grand Challenges in Global Health Initiative
Modelling the impact of intermittent preventive treatment for malaria on selection pressure for drug resistance
BACKGROUND: Intermittent preventive treatment (IPT) is a promising intervention for malaria control, although there are concerns about its impact on drug resistance. METHODS: The key model inputs are age-specific values for a) baseline anti-malarial dosing rate, b) parasite prevalence, and c) proportion of those treated with anti-malarials (outside IPT) who are infected. These are used to estimate the immediate effect of IPT on the genetic coefficient of selection (s). The scenarios modelled were year round IPT to infants in rural southern Tanzania, and three doses at monthly intervals of seasonal IPT in Senegal. RESULTS: In the simulated Tanzanian setting, the model suggests a high selection pressure for drug resistance, but that IPTi would only increase this by a small amount (4.4%). The percent change in s is larger if parasites are more concentrated in infants, or if baseline drug dosing is less common or less specific. If children aged up to five years are included in the Tanzanian scenario then the predicted increase in s rises to 31%. The Senegalese seasonal IPT scenario, in children up to five years, results in a predicted increase in s of 16%. CONCLUSION: There is a risk that the useful life of drugs will be shortened if IPT is implemented over a wide childhood age range. On the other hand, IPT delivered only to infants is unlikely to appreciably shorten the useful life of the drug used
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