Self-similar turbulent dynamo

The amplification of magnetic fields in a highly conducting fluid is studied numerically. During growth, the magnetic field is spatially intermittent: it does not uniformly fill the volume, but is concentrated in long thin folded structures. Contrary to a commonly held view, intermittency of the folded field does not increase indefinitely throughout the growth stage if diffusion is present. Instead, as we show, the probability-density function (PDF) of the field strength becomes self-similar. The normalized moments increase with magnetic Prandtl number in a powerlike fashion. We argue that the self-similarity is to be expected with a finite flow scale and system size. In the nonlinear saturated state, intermittency is reduced and the PDF is exponential. Parallels are noted with self-similar behavior recently observed for passive-scalar mixing and for map dynamos.Comment: revtex, 4 pages, 5 figures; minor changes to match published versio

Simulations of small-scale turbulent dynamo

We report an extensive numerical study of the small-scale turbulent dynamo at large magnetic Prandtl numbers Pm. A Pm scan is given for the model case of low-Reynolds-number turbulence. We concentrate on three topics: magnetic-energy spectra and saturation levels, the structure of the field lines, and the field-strength distribution. The main results are (1) the folded structure (direction reversals at the resistive scale, field lines curved at the scale of the flow) persists from the kinematic to the nonlinear regime; (2) the field distribution is self-similar and appears to be lognormal during the kinematic regime and exponential in the saturated state; and (3) the bulk of the magnetic energy is at the resistive scale in the kinematic regime and remains there after saturation, although the spectrum becomes much shallower. We propose an analytical model of saturation based on the idea of partial two-dimensionalization of the velocity gradients with respect to the local direction of the magnetic folds. The model-predicted spectra are in excellent agreement with numerical results. Comparisons with large-Re, moderate-Pm runs are carried out to confirm the relevance of these results. New features at large Re are elongation of the folds in the nonlinear regime from the viscous scale to the box scale and the presence of an intermediate nonlinear stage of slower-than-exponential magnetic-energy growth accompanied by an increase of the resistive scale and partial suppression of the kinetic-energy spectrum in the inertial range. Numerical results for the saturated state do not support scale-by-scale equipartition between magnetic and kinetic energies, with a definite excess of magnetic energy at small scales. A physical picture of the saturated state is proposed.Comment: aastex using emulateapj; 32 pages, final published version; a pdf file (4Mb) of the paper containing better-quality versions of figs. 5, 8, 12, 15, 17 is available from http://www.damtp.cam.ac.uk/user/as629 or by email upon request

The Murchison Widefield Array: The Square Kilometre Array Precursor at Low Radio Frequencies

The Murchison Widefield Array (MWA) is one of three Square Kilometre Array Precursor telescopes and is located at the Murchison Radio-astronomy Observatory in the Murchison Shire of the mid-west of Western Australia, a location chosen for its extremely low levels of radio frequency interference. The MWA operates at low radio frequencies, 80–300 MHz, with a processed bandwidth of 30.72 MHz for both linear polarisations, and consists of 128 aperture arrays (known as tiles) distributed over a ~3-km diameter area. Novel hybrid hardware/software correlation and a real-time imaging and calibration systems comprise the MWA signal processing backend. In this paper, the as-built MWA is described both at a system and sub-system level, the expected performance of the array is presented, and the science goals of the instrument are summarised

A Comparison of Methods to Harmonize Cortical Thickness Measurements Across Scanners and Sites

Results of neuroimaging datasets aggregated from multiple sites may be biased by site-specific profiles in participants’ demographic and clinical characteristics, as well as MRI acquisition protocols and scanning platforms. We compared the impact of four different harmonization methods on results obtained from analyses of cortical thickness data: (1) linear mixed-effects model (LME) that models site-specific random intercepts (LME INT), (2) LME that models both site-specific random intercepts and age-related random slopes (LME INT+SLP), (3) ComBat, and (4) ComBat with a generalized additive model (ComBat-GAM). Our test case for comparing harmonization methods was cortical thickness data aggregated from 29 sites, which included 1,340 cases with posttraumatic stress disorder (PTSD) (6.2–81.8 years old) and 2,057 trauma-exposed controls without PTSD (6.3–85.2 years old). We found that, compared to the other data harmonization methods, data processed with ComBat-GAM was more sensitive to the detection of significant case-control differences (Χ 2(3) = 63.704, p < 0.001) as well as case-control differences in age-related cortical thinning (Χ 2(3) = 12.082, p = 0.007). Both ComBat and ComBat-GAM outperformed LME methods in detecting sex differences (Χ 2(3) = 9.114, p = 0.028) in regional cortical thickness. ComBat-GAM also led to stronger estimates of age-related declines in cortical thickness (corrected p-values < 0.001), stronger estimates of case-related cortical thickness reduction (corrected p-values < 0.001), weaker estimates of age-related declines in cortical thickness in cases than controls (corrected p-values < 0.001), stronger estimates of cortical thickness reduction in females than males (corrected p-values < 0.001), and stronger estimates of cortical thickness reduction in females relative to males in cases than controls (corrected p-values < 0.001). Our results support the use of ComBat-GAM to minimize confounds and increase statistical power when harmonizing data with non-linear effects, and the use of either ComBat or ComBat-GAM for harmonizing data with linear effects

Remodeling of the Cortical Structural Connectome in Posttraumatic Stress Disorder:Results from the ENIGMA-PGC PTSD Consortium

BACKGROUND: Posttraumatic stress disorder (PTSD) is accompanied by disrupted cortical neuroanatomy. We investigated alteration in covariance of structural networks associated with PTSD in regions that demonstrate the case-control differences in cortical thickness (CT) and surface area (SA). METHODS: Neuroimaging and clinical data were aggregated from 29 research sites in >1,300 PTSD cases and >2,000 trauma-exposed controls (age 6.2-85.2 years) by the ENIGMA-PGC PTSD working group. Cortical regions in the network were rank-ordered by effect size of PTSD-related cortical differences in CT and SA. The top-n (n = 2 to 148) regions with the largest effect size for PTSD > non-PTSD formed hypertrophic networks, the largest effect size for PTSD < non-PTSD formed atrophic networks, and the smallest effect size of between-group differences formed stable networks. The mean structural covariance (SC) of a given n-region network was the average of all positive pairwise correlations and was compared to the mean SC of 5,000 randomly generated n-region networks. RESULTS: Patients with PTSD, relative to non-PTSD controls, exhibited lower mean SC in CT-based and SA-based atrophic networks. Comorbid depression, sex and age modulated covariance differences of PTSD-related structural networks. CONCLUSIONS: Covariance of structural networks based on CT and cortical SA are affected by PTSD and further modulated by comorbid depression, sex, and age. The structural covariance networks that are perturbed in PTSD comport with converging evidence from resting state functional connectivity networks and networks impacted by inflammatory processes, and stress hormones in PTSD

Searches for long-lived charged particles in pp collisions at √s =7 and 8 TeV

Results of searches for heavy stable charged particles produced in pp collisions at √s =7 and 8 TeV are presented corresponding to an integrated luminosity of 5.0 fb⁻¹ and 18.8 fb⁻¹, respectively. Data collected with the CMS detector are used to study the momentum, energy deposition, and time-of-flight of signal candidates. Leptons with an electric charge between e/3 and 8e, as well as bound states that can undergo charge exchange with the detector material, are studied. Analysis results are presented for various combinations of signatures in the inner tracker only, inner tracker and muon detector, and muon detector only. Detector signatures utilized are long time-of-flight to the outer muon system and anomalously high (or low) energy deposition in the inner tracker. The data are consistent with the expected background, and upper limits are set on the production cross section of long-lived gluinos, scalar top quarks, and scalar τ leptons, as well as pair produced long-lived leptons. Corresponding lower mass limits, ranging up to 1322 GeV/c² for gluinos, are the most stringent to date

Initial Report of Second‐Line FOLFIRI in Combination with Ramucirumab in Advanced Gastroesophageal Adenocarcinomas: A Multi‐Institutional Retrospective Analysis

BackgroundThe randomized phase III RAINBOW trial established paclitaxel (pac) plus ramucirumab (ram) as a global standard for second-line (2L) therapy in advanced gastric and gastroesophageal junction adenocarcinoma, together gastroesophageal adenocarcinoma (GEA). Patients (pts) receiving first-line (1L) FOLFOX often develop neuropathy that renders continued neurotoxic agents in the 2L setting unappealing and other regimens more desirable. As such, FOLFIRI-ram has become an option for patients with 2L GEA. FOLFIRI-ramucirumab (ram) has demonstrated safety and activity in 2L colorectal cancer, but efficacy/safety data in GEA are lacking.Subjects, materials, and methodsPatients with GEA treated with 2L FOLFIRI-ram between August 2014 and April 2018 were identified. Clinicopathologic data including oxaliplatin neurotoxicity rates/grades (G), 2L treatment response, progression-free survival (PFS), overall survival (OS), safety, and molecular features were abstracted from three U.S. academic institutions. Kaplan-Meier survival analysis was used to generate PFS/OS; the likelihood ratio test was used to determine statistical significance.ResultsWe identified 29 pts who received 2L FOLFIRI-ram. All pts received 1L platinum + fluoropyrimidine, and 23 of 29 (79%) had post-1L neuropathy; 12 (41%) had G1, and 11 (38%) had G2. Patients were evenly split between esophagus/gastroesophageal junction (12; 41%) and gastric cancer (17; 59%). Among evaluable pts (26/29), the overall response rate was 23% (all partial response) with a disease control rate of 79%. Median PFS was 6.0 months and median OS was 13.4 months among all evaluable pts. Six- and 12-month OS were 90% (n = 18/20) and 41% (n = 7/17). There were no new safety signals.ConclusionWe provide the first data suggesting FOLFIRI-ram is a safe, non-neurotoxic regimen comparing favorably with the combination of pac + ram used in the seminal RAINBOW trial.Implications for practiceResults of this study provide initial support for the safety and efficacy of second-line (2L) FOLFIRI-ramucirumab (ram) after progression on first-line platinum/fluoropyrimidine in patients with gastroesophageal adenocarcinoma (GEA). The overall response, progression-free survival, overall survival, and toxicity profile compare favorably with paclitaxel (pac) + ram and highlight the importance of the ongoing phase II RAMIRIS trial examining FOLFIRI-ram versus pac + ram in 2L GEA (NCT03081143). FOLFIRI-ram may warrant consideration for inclusion as an alternate regimen in consensus guidelines for GEA