199 research outputs found
An observationally-driven kinetic approach to coronal heating
Coronal heating through the explosive release of magnetic energy remains an
open problem in solar physics. Recent hydrodynamical models attempt an
investigation by placing swarms of 'nanoflares' at random sites and times in
modeled one-dimensional coronal loops. We investigate the problem in three
dimensions, using extrapolated coronal magnetic fields of observed solar active
regions. We apply a nonlinear force-free field extrapolation above an observed
photospheric magnetogram of NOAA active region (AR) 11158. We then determine
the locations, energy contents, and volumes of 'unstable' areas, namely areas
prone to releasing magnetic energy due to locally accumulated electric current
density. Statistical distributions of these volumes and their fractal dimension
are inferred, investigating also their dependence on spatial resolution.
Further adopting a simple resistivity model, we infer the properties of the
fractally distributed electric fields in these volumes. Next, we monitor the
evolution of 10^5 particles (electrons and ions) obeying an initial Maxwellian
distribution with a temperature of 10 eV, by following their trajectories and
energization when subjected to the resulting electric fields. For computational
convenience, the length element of the magnetic-field extrapolation is 1
arcsec, much coarser than the particles collisional mean free path in the low
corona. The presence of collisions traps the bulk of the plasma around the
unstable volumes, or current sheets (UCS), with only a tail of the distribution
gaining substantial energy. Assuming that the distance between UCS is similar
to the collisional mean free path we find that the low active-region corona is
heated to 100-200 eV, corresponding to temperatures exceeding 2 MK, within tens
of seconds for electrons and thousands of seconds for ions. Fractally
distributed, nanoflare-triggening fragmented UCS ...Comment: accepted by A&
Non-Supersymmetric Seiberg Duality, Orientifold QCD and Non-Critical Strings
We propose an electric-magnetic duality and conjecture an exact conformal
window for a class of non-supersymmetric U(N_c) gauge theories with fermions in
the (anti)symmetric representation of the gauge group and N_f additional scalar
and fermion flavors. The duality exchanges N_c with N_f -N_c \mp 4 leaving N_f
invariant, and has common features with Seiberg duality in N=1 SQCD with SU or
SO/Sp gauge group. At large N the duality holds due to planar equivalence with
N=1 SQCD. At finite N we embed these gauge theories in a setup with D-branes
and orientifolds in a non-supersymmetric, but tachyon-free, non-critical type
0B string theory and argue in favor of the duality in terms of boundary and
crosscap state monodromies as in analogous supersymmetric situations. One can
verify explicitly that the resulting duals have matching global anomalies.
Finally, we comment on the moduli space of these gauge theories and discuss
other potential non-supersymmetric examples that could exhibit similar
dualities.Comment: 45 pages, 1 figur
Magnetic Helicity Estimations in Models and Observations of the Solar Magnetic Field. Part III: Twist Number Method
We study the writhe, twist and magnetic helicity of different magnetic flux
ropes, based on models of the solar coronal magnetic field structure. These
include an analytical force-free Titov--D\'emoulin equilibrium solution, non
force-free magnetohydrodynamic simulations, and nonlinear force-free magnetic
field models. The geometrical boundary of the magnetic flux rope is determined
by the quasi-separatrix layer and the bottom surface, and the axis curve of the
flux rope is determined by its overall orientation. The twist is computed by
the Berger--Prior formula that is suitable for arbitrary geometry and both
force-free and non-force-free models. The magnetic helicity is estimated by the
twist multiplied by the square of the axial magnetic flux. We compare the
obtained values with those derived by a finite volume helicity estimation
method. We find that the magnetic helicity obtained with the twist method
agrees with the helicity carried by the purely current-carrying part of the
field within uncertainties for most test cases. It is also found that the
current-carrying part of the model field is relatively significant at the very
location of the magnetic flux rope. This qualitatively explains the agreement
between the magnetic helicity computed by the twist method and the helicity
contributed purely by the current-carrying magnetic field.Comment: To be published in Ap
Histological insights into the pathogenesis of postâRouxâenâY hyperinsulinaemic hypoglycaemia
Background ÎČâcell hyperplasia has been implicated in the aetiology of post RouxâenâY gastric bypass hyperinsulinaemic hypoglycaemia, but the pathogenesis of this condition is still unclear. Case report We report a case of a 52âyearâold man with postâRouxâenâY gastric bypass hyperinsulinaemic hypoglycaemia who underwent distal pancreatectomy to alleviate his symptoms. Pancreatic histopathology showed chronic pancreatitis with a corresponding loss of exocrine tissue and islet retention. Amyloid deposition was found in pancreatic islets. These features are more typically associated with Type 2 diabetes. Discussion This case highlights the potential multifactorial pathogenesis of symptomatic hypoglycaemia after RouxâenâY gastric bypass. What's new? We present a case of a patient with post RouxâenâY hypoglycaemia and pancreatic histology typically associated with diabetes. We show that hyperinsulinaemic hypoglycaemia has a multifactorial aetiology.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/109602/1/dme12571.pd
Additivity of relative magnetic helicity in finite volumes
CONTEXT: Relative magnetic helicity is conserved by magneto-hydrodynamic evolution even in the presence of moderate resistivity. For that reason, it is often invoked as the most relevant constraint on the dynamical evolution of plasmas in complex systems, such as solar and stellar dynamos, photospheric flux emergence, solar eruptions, and relaxation processes in laboratory plasmas. However, such studies often indirectly imply that relative magnetic helicity in a given spatial domain can be algebraically split into the helicity contributions of the composing subvolumes, in other words that it is an additive quantity. A limited number of very specific applications have shown that this is not the case. AIMS: Progress in understanding the nonadditivity of relative magnetic helicity requires removal of restrictive assumptions in favor of a general formalism that can be used in both theoretical investigations and numerical applications. METHODS: We derive the analytical gauge-invariant expression for the partition of relative magnetic helicity between contiguous finite volumes, without any assumptions on either the shape of the volumes and interface, or the employed gauge. RESULTS: We prove the nonadditivity of relative magnetic helicity in finite volumes in the most general, gauge-invariant formalism, and verify this numerically. We adopt more restrictive assumptions to derive known specific approximations, which yields a unified view of the additivity issue. As an example, the case of a flux rope embedded in a potential field shows that the nonadditivity term in the partition equation is, in general, non-negligible. CONCLUSIONS: The nonadditivity of relative magnetic helicity can potentially be a serious impediment to the application of relative helicity conservation as a constraint on the complex dynamics of magnetized plasmas. The relative helicity partition formula can be applied to numerical simulations to precisely quantify the effect of nonadditivity on global helicity budgets of complex physical processes
Magnetic Helicity Evolution and Eruptive Activity in NOAA Active Region 11158
Coronal mass ejections are among the Sunâs most energetic activity events yet the physical mechanisms that lead to their occurrence are not yet fully understood. They can drive major space weather impacts at Earth, so knowing why and when these ejections will occur is required for accurate space weather forecasts. In this study we use a 4 day time series of a quantity known as the helicity ratio, âŁH
J
âŁ/âŁH
V
⣠(helicity of the current-carrying part of the active region field to the total relative magnetic helicity within the volume), which has been computed from nonlinear force-free field extrapolations of NOAA active region 11158. We compare the evolution of âŁH
J
âŁ/âŁH
V
⣠with the activity produced in the corona of the active region and show this ratio can be used to indicate when the active region is prone to eruption. This occurs when âŁH
J
âŁ/âŁH
V
⣠exceeds a value of 0.1, as suggested by previous studies. We find the helicity ratio variations to be more pronounced during times of strong flux emergence, collision and reconnection between fields of different bipoles, shearing motions, and reconfiguration of the corona through failed and successful eruptions. When flux emergence, collision, and shearing motions have lessened, the changes in helicity ratio are somewhat subtle despite the occurrence of significant eruptive activity during this time
In which shell-type SNRs should we look for gamma-rays and neutrinos from p-p collisions?
We present a simple analytic model for the various contributions to the
non-thermal emission from shell type SNRs, and show that this model's results
reproduce well the results of previous detailed calculations. We show that the
\geq 1 TeV gamma ray emission from the shell type SNRs RX J1713.7-3946 and RX
J0852.0-4622 is dominated by inverse-Compton scattering of CMB photons (and
possibly infra-red ambient photons) by accelerated electrons. Pion decay (due
to proton-proton collisions) is shown to account for only a small fraction,
\lesssim10^-2, of the observed flux, as assuming a larger fractional
contribution would imply nonthermal radio and X-ray synchrotron emission and
thermal X-ray Bremsstrahlung emission that far exceed the observed radio and
X-ray fluxes. Models where pion decay dominates the \geq 1 TeV flux avoid the
implied excessive synchrotron emission (but not the implied excessive thermal
X-ray Bremsstrahlung emission) by assuming an extremely low efficiency of
electron acceleration, K_ep \lesssim 10^-4 (K_ep is the ratio of the number of
accelerated electrons and the number of accelerated protons at a given energy).
We argue that observations of SNRs in nearby galaxies imply a lower limit of
K_ep \gtrsim 10^-3, and thus rule out K_ep values \lesssim 10^-4 (assuming that
SNRs share a common typical value of K_ep). It is suggested that SNRs with
strong thermal X-ray emission, rather than strong non-thermal X-ray emission,
are more suitable candidates for searches of gamma rays and neutrinos resulting
from proton-proton collisions. In particular, it is shown that the neutrino
flux from the SNRs above is probably too low to be detected by current and
planned neutrino observatories (Abridged).Comment: 13 pages, 1 figure, accepted for publication in JCAP, minor revision
Universal Third Trimester Ultrasonic Screening Using Fetal Macrosomia in the Prediction of Adverse Perinatal Outcome, a Systematic Review and Meta-analysis of Diagnostic Test Accuracy.
Background: The effectiveness of screening for macrosomia is not well established. One of the critical elements of an effective screening program is the diagnostic accuracy of a test at predicting the condition. The objective of this study is to investigate the diagnostic effectiveness of universal ultrasonic fetal biometry in predicting the delivery of a macrosomic infant, shoulder dystocia, and associated neonatal morbidity in low- and mixed-risk populations. Methods and findings: We conducted a predefined literature search in Medline, Excerpta Medica database (EMBASE), the Cochrane library and ClinicalTrials.gov from inception to May 2020. No language restrictions were applied. We included studies where the ultrasound was performed as part of universal screening and those that included low- and mixed-risk pregnancies and excluded studies confined to high risk pregnancies. We used the estimated fetal weight (EFW) (multiple formulas and thresholds) and the abdominal circumference (AC) to define suspected large for gestational age (LGA). Adverse perinatal outcomes included macrosomia (multiple thresholds), shoulder dystocia, and other markers of neonatal morbidity. The risk of bias was assessed using the Quality Assessment of Diagnostic Accuracy Studies (QUADAS-2) tool. Meta-analysis was carried out using the hierarchical summary receiver operating characteristic (ROC) and the bivariate logit-normal (Reitsma) models. We identified 41 studies that met our inclusion criteria involving 112,034 patients in total. These included 11 prospective cohort studies (N = 9986), one randomized controlled trial (RCT) (N = 367), and 29 retrospective cohort studies (N = 101,681). The quality of the studies was variable, and only three studies blinded the ultrasound findings to the clinicians. Both EFW >4,000 g (or 90th centile for the gestational age) and AC >36 cm (or 90th centile) had >50% sensitivity for predicting macrosomia (birthweight above 4,000 g or 90th centile) at birth with positive likelihood ratios (LRs) of 8.74 (95% confidence interval [CI] 6.84â11.17) and 7.56 (95% CI 5.85â9.77), respectively. There was significant heterogeneity at predicting macrosomia, which could reflect the different study designs, the characteristics of the included populations, and differences in the formulas used. An EFW >4,000 g (or 90th centile) had 22% sensitivity at predicting shoulder dystocia with a positive likelihood ratio of 2.12 (95% CI 1.34â3.35). There was insufficient data to analyze other markers of neonatal morbidity. Conclusions: In this study, we found that suspected LGA is strongly predictive of the risk of delivering a large infant in low- and mixed-risk populations. However, it is only weakly (albeit statistically significantly) predictive of the risk of shoulder dystocia. There was insufficient data to analyze other markers of neonatal morbidity
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