37 research outputs found
The sub-arcsecond hard X-ray structure of loop footpoints in a solar flare
The newly developed X-ray visibility forward fitting technique is applied to
Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) data of a limb
flare to investigate the energy and height dependence on sizes, shapes, and
position of hard X-ray chromospheric footpoint sources. This provides
information about the electron transport and chromospheric density structure.
The spatial distribution of two footpoint X-ray sources is analyzed using
PIXON, Maximum Entropy Method, CLEAN and visibility forward fit algorithms at
nonthermal energies from to keV. We report, for the first
time, the vertical extents and widths of hard X-ray chromospheric sources
measured as a function of energy for a limb event. Our observations suggest
that both the vertical and horizontal sizes of footpoints are decreasing with
energy. Higher energy emission originates progressively deeper in the
chromosphere consistent with downward flare accelerated streaming electrons.
The ellipticity of the footpoints grows with energy from at keV to at keV. The positions of X-ray emission are in
agreement with an exponential density profile of scale height ~km.
The characteristic size of the hard X-ray footpoint source along the limb is
decreasing with energy suggesting a converging magnetic field in the footpoint.
The vertical sizes of X-ray sources are inconsistent with simple collisional
transport in a single density scale height but can be explained using a
multi-threaded density structure in the chromosphere.Comment: 7 pages, 7 figures, submitted to Ap
The influence of albedo on the size of hard X-ray flare sources
Context: Hard X-rays from solar flares are an important diagnostic of
particle acceleration and transport in the solar atmosphere. Any observed X-ray
flux from on-disc sources is composed of direct emission plus Compton
backscattered photons (albedo). This affects both the observed spectra and
images as well as the physical quantities derived from them such as the spatial
and spectral distributions of accelerated electrons or characteristics of the
solar atmosphere. Aims: We propose a new indirect method to measure albedo and
to infer the directivity of X-rays in imaging using RHESSI data. Methods:
Visibility forward fitting is used to determine the size of a disc event
observed by RHESSI as a function of energy. This is compared to the sizes of
simulated sources from a Monte Carlo simulation code of photon transport in the
chromosphere for different degrees of downward directivity and true source
sizes to find limits on the true source size and the directivity. Results: The
observed full width half maximum of the source varies in size between 7.4
arcsec and 9.1 arcsec with the maximum between 30 and 40 keV. Such behaviour is
expected in the presence of albedo and is found in the simulations. A source
size smaller than 6 arcsec is improbable for modest directivities and the true
source size is likely to be around 7 arcsec for small directivities.
Conclusions: While it is difficult to image the albedo patch directly, the
effect of backscattered photons on the observed source size can be estimated.
The increase in source size caused by albedo has to be accounted for when
computing physical quantities that include the size as a parameter such as
flare energetics. At the same time, the study of the albedo signature provides
vital information about the directivity of X-rays and related electrons.Comment: 8 pages, 6 figures, A&A (accepted
Instrumental oscillations in RHESSI count rates during solar flares
Aims: We seek to illustrate the analysis problems posed by RHESSI spacecraft
motion by studying persistent instrumental oscillations found in the
lightcurves measured by RHESSI's X-ray detectors in the 6-12 keV and 12-25 keV
energy range during the decay phase of the flares of 2004 November 4 and 6.
Methods: The various motions of the RHESSI spacecraft which may contribute to
the manifestation of oscillations are studied. The response of each detector in
turn is also investigated. Results: We find that on 2004 November 6 the
observed oscillations correspond to the nutation period of the RHESSI
instrument. These oscillations are also of greatest amplitude for detector 5,
while in the lightcurves of many other detectors the oscillations are small or
undetectable. We also find that the variation in detector pointing is much
larger during this flare than the counterexample of 2004 November 4.
Conclusions: Sufficiently large nutation motions of the RHESSI spacecraft lead
to clearly observable oscillations in count rates, posing a significant hazard
for data analysis. This issue is particularly problematic for detector 5 due to
its design characteristics. Dynamic correction of the RHESSI counts, accounting
for the livetime, data gaps, and the transmission of the bi-grid collimator of
each detector, is required to overcome this issue. These corrections should be
applied to all future oscillation studies.Comment: 8 pages, 10 figure
Temporal Correlation of Hard X-rays and Meter/Decimeter Radio Structures in Solar Flares
We investigate the relative timing between hard X-ray (HXR) peaks and
structures in metric and decimetric radio emissions of solar flares using data
from the RHESSI and Phoenix-2 instruments. The radio events under consideration
are predominantly classified as type III bursts, decimetric pulsations and
patches. The RHESSI data are demodulated using special techniques appropriate
for a Phoenix-2 temporal resolution of 0.1s. The absolute timing accuracy of
the two instruments is found to be about 170 ms, and much better on the
average. It is found that type III radio groups often coincide with enhanced
HXR emission, but only a relatively small fraction ( 20%) of the groups
show close correlation on time scales 1s. If structures correlate, the HXRs
precede the type III emissions in a majority of cases, and by 0.690.19 s
on the average. Reversed drift type III bursts are also delayed, but
high-frequency and harmonic emission is retarded less. The decimetric
pulsations and patches (DCIM) have a larger scatter of delays, but do not have
a statistically significant sign or an average different from zero. The time
delay does not show a center-to-limb variation excluding simple propagation
effects. The delay by scattering near the source region is suggested to be the
most efficient process on the average for delaying type III radio emission
X-Ray Polarization of Solar Flares Measured with Rhessi
The degree of linear polarization in solar flares has not yet been precisely
determined despite multiple attempts to measure it with different missions. The
high energy range in particular has very rarely been explored, due to its
greater instrumental difficulties. We approached the subject using the Reuven
Ramaty High Energy Spectroscopic Imager (RHESSI) satellite to study 6 X-class
and 1 M-class flares in the energy range between 100 keV and 350 keV. Using
RHESSI as a polarimeter requires the application of strict cuts to the event
list in order to extract those photons that are Compton scattered between two
detectors. Our measurements show polarization values between 2% and 54%, with
errors ranging from 10% to 26% in 1 sigma level. In view of the large
uncertainties in both the magnitude and direction of the polarization vector,
the results can only reject source models with extreme properties.Comment: 26 pages, 11 figures, accepted for publication by Solar Physic
Women's empowerment, production choices, and crop diversity in Burkina Faso, India, Malawi, and Tanzania: a secondary analysis of cross-sectional data
BACKGROUND: Bolstering farm-level crop diversity is one strategy to strengthen food system resilience and achieve global food security. Women who live in rural areas play an essential role in food production; therefore, we aimed to assess the associations between women's empowerment and crop diversity. METHODS: In this secondary analysis of cross-sectional data, we used data from four cluster-randomised controlled trials done in Burkina Faso, India, Malawi, and Tanzania. We assessed women's empowerment using indicators from the Women's Empowerment in Agriculture Index. Farm-level crop diversity measures were the number of food crops grown, number of food groups grown, and if nutrient-dense crops were grown. We used a two-stage modelling approach. First, we analysed covariate-adjusted country-specific associations between women's empowerment and crop diversity indicators using multivariable generalised linear models. Second, we pooled country-specific associations using random-effects models. FINDINGS: The final analytic sample included 1735 women from Burkina Faso, 4450 women from India, 547 women from Malawi, and 574 women from Tanzania. Across all countries, compared with households in which women provided input into fewer productive decisions, households of women with greater input into productive decisions produced more food crops (mean difference 0Β·36 [95% CI 0Β·16-0Β·55]), a higher number of food groups (mean difference 0Β·16 [0Β·06-0Β·25]), and more nutrient-dense crops (percentage point difference 3 [95% CI 3-4]). Across all countries, each additional community group a woman actively participated in was associated with cultivating a higher number of food crops (mean difference 0Β·20 [0Β·04-0Β·35]) and a higher number of food groups (mean difference 0Β·11 [0Β·03-0Β·18]), but not more nutrient-dense crops. In pooled associations from Burkina Faso and India, asset ownership was associated with cultivating a higher number of food crops (mean difference 0Β·08 [0Β·04-0Β·12]) and a higher number of food groups (mean difference 0Β·05 [0Β·04-0Β·07]), but not more nutrient-dense crops. INTERPRETATION: Greater women's empowerment was associated with higher farm-level crop diversity among low-income agricultural households, suggesting that it could help enhance efforts to strengthen food system resilience. FUNDING: Bill & Melinda Gates Foundation
The Promoter of Rv0560c Is Induced by Salicylate and Structurally-Related Compounds in Mycobacterium tuberculosis
Mycobacterium tuberculosis, the causative agent of tuberculosis (TB), is a major global health threat. During infection, bacteria are believed to encounter adverse conditions such as iron depletion. Mycobacteria synthesize iron-sequestering mycobactins, which are essential for survival in the host, via the intermediate salicylate. Salicylate is a ubiquitous compound which is known to induce a mild antibiotic resistance phenotype. In M. tuberculosis salicylate highly induces the expression of Rv0560c, a putative methyltransferase. We identified and characterized the promoter and regulatory elements of Rv0560c. PRv0560c activity was highly inducible by salicylate in a dose-dependent manner. The induction kinetics of PRv0560c were slow, taking several days to reach maximal activity, which was sustained over several weeks. Promoter activity could also be induced by compounds structurally related to salicylate, such as aspirin or para-aminosalicylic acid, but not by benzoate, indicating that induction is specific to a structural motif. The β10 and β35 promoter elements were identified and residues involved in regulation of promoter activity were identified in close proximity to an inverted repeat spanning the β35 promoter element. We conclude that Rv0560c expression is controlled by a yet unknown repressor via a highly-inducible promoter
The dUTPase Enzyme Is Essential in Mycobacterium smegmatis
Thymidine biosynthesis is essential in all cells. Inhibitors of the enzymes involved in this pathway (e.g. methotrexate) are thus frequently used as cytostatics. Due to its pivotal role in mycobacterial thymidylate synthesis dUTPase, which hydrolyzes dUTP into the dTTP precursor dUMP, has been suggested as a target for new antitubercular agents. All mycobacterial genomes encode dUTPase with a mycobacteria-specific surface loop absent in the human dUTPase. Using Mycobacterium smegmatis as a fast growing model for Mycobacterium tuberculosis, we demonstrate that dUTPase knock-out results in lethality that can be reverted by complementation with wild-type dUTPase. Interestingly, a mutant dUTPase gene lacking the genus-specific loop was unable to complement the knock-out phenotype. We also show that deletion of the mycobacteria-specific loop has no major effect on dUTPase enzymatic properties in vitro and thus a yet to be identified loop-specific function seems to be essential within the bacterial cell context. In addition, here we demonstrated that Mycobacterium tuberculosis dUTPase is fully functional in Mycobacterium smegmatis as it rescues the lethal knock-out phenotype. Our results indicate the potential of dUTPase as a target for antitubercular drugs and identify a genus-specific surface loop on the enzyme as a selective target
Genome-wide association reveals host-specific genomic traits in Escherichia coli
Background: Escherichia coli is an opportunistic pathogen which colonizes various host species. However, to what extent genetic lineages of E. coli are adapted or restricted to specific hosts and the genomic determinants of such adaptation or restriction is poorly understood. Results: We randomly sampled E. coli isolates from four countries (Germany, UK, Spain, and Vietnam), obtained from five host species (human, pig, cattle, chicken, and wild boar) over 16 years, from both healthy and diseased hosts, to construct a collection of 1198 whole-genome sequenced E. coli isolates. We identified associations between specific E. coli lineages and the host from which they were isolated. A genome-wide association study (GWAS) identified several E. coli genes that were associated with human, cattle, or chicken hosts, whereas no genes associated with the pig host could be found. In silico characterization of nine contiguous genes (collectively designated as nan-9) associated with the human host indicated that these genes are involved in the metabolism of sialic acids (Sia). In contrast, the previously described sialic acid regulon known as sialoregulon (i.e. nanRATEK-yhcH, nanXY, and nanCMS) was not associated with any host species. In vitro growth experiments with a Ξnan-9 E. coli mutant strain, using the sialic acids 5-N-acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc) as sole carbon source, showed impaired growth behaviour compared to the wild-type. Conclusions: This study provides an extensive analysis of genetic determinants which may contribute to host specificity in E. coli. Our findings should inform risk analysis and epidemiological monitoring of (antimicrobial resistant) E. coli