Influence of the Lower Hybrid Drift Instability on the onset of Magnetic Reconnection

Two-dimensional and three-dimensional kinetic simulation results reveal the importance of the Lower-Hybrid Drift Instability LHDI to the onset of magnetic reconnection. Both explicit and implicit kinetic simulations show that the LHDI heats electrons anisotropically and increases the peak current density. Linear theory predicts these modifications can increase the growth rate of the tearing instability by almost two orders of magnitude and shift the fastest growing modes to significantly shorter wavelengths. These predictions are confirmed by nonlinear kinetic simulations in which the growth and coalescence of small scale magnetic islands leads to a rapid onset of large scale reconnection

Dependence of radio halo properties on star formation activity and galaxy mass

We investigate the relation between the existence and size of radio halos, which are believed to be created by star formation (SF) related energy input into the interstellar medium, and other galaxy properties, most importantly star formation activity and galaxy mass. Based on radio continuum and H-alpha observations of a sample of seven late-type spiral galaxies we find a direct, linear correlation of the radial extent of gaseous halos on the size of the actively star-forming parts of the galaxy disks. Data of a larger sample of 22 galaxies indicate that the threshold energy input rate into the disk ISM per unit surface area for the creation of a gaseous halo depends on the mass surface density of the galaxy, in the sense that a higher threshold must be surpassed for galaxies with a higher surface density. Because of the good prediction of the existence of a radio halo from these two parameters, we conclude that they are important, albeit not the only contributors. The compactness of the SF-related energy input is also found to be a relevant factor. Galaxies with relatively compact SF distributions are more likely to have gaseous halos than others with more widespread SF activity. These results quantify the so-called "break-out" condition for matter to escape from galaxy disks, as used in all current models of the interstellar medium and first defined by Norman and Ikeuchi (1989).Comment: accepted for publication in Astronomy & Astrophysic

Magnetogenesis from Cosmic String Loops

Large-scale coherent magnetic fields are observed in galaxies and clusters, but their ultimate origin remains a mystery. We reconsider the prospects for primordial magnetogenesis by a cosmic string network. We show that the magnetic flux produced by long strings has been overestimated in the past, and give improved estimates. We also compute the fields created by the loop population, and find that it gives the dominant contribution to the total magnetic field strength on present-day galactic scales. We present numerical results obtained by evolving semi-analytic models of string networks (including both one-scale and velocity-dependent one-scale models) in a Lambda-CDM cosmology, including the forces and torques on loops from Hubble redshifting, dynamical friction, and gravitational wave emission. Our predictions include the magnetic field strength as a function of correlation length, as well as the volume covered by magnetic fields. We conclude that string networks could account for magnetic fields on galactic scales, but only if coupled with an efficient dynamo amplification mechanism.Comment: 10 figures; v3: small typos corrected to match published version. MagnetiCS, the code described in paper, is available at http://markcwyman.com/ and http://www.damtp.cam.ac.uk/user/dhw22/code/index.htm

Sheared Flow As A Stabilizing Mechanism In Astrophysical Jets

It has been hypothesized that the sustained narrowness observed in the asymptotic cylindrical region of bipolar outflows from Young Stellar Objects (YSO) indicates that these jets are magnetically collimated. The j cross B force observed in z-pinch plasmas is a possible explanation for these observations. However, z-pinch plasmas are subject to current driven instabilities (CDI). The interest in using z-pinches for controlled nuclear fusion has lead to an extensive theory of the stability of magnetically confined plasmas. Analytical, numerical, and experimental evidence from this field suggest that sheared flow in magnetized plasmas can reduce the growth rates of the sausage and kink instabilities. Here we propose the hypothesis that sheared helical flow can exert a similar stabilizing influence on CDI in YSO jets.Comment: 13 pages, 2 figure

Serotonin tranporter methylation and response to cognitive behaviour therapy in children with anxiety disorders

Anxiety disorders that are the most commonly occurring psychiatric disorders in childhood, are associated with a range of social and educational impairments and often continue into adulthood. Cognitive behaviour therapy (CBT) is an effective treatment option for the majority of cases, although up to 35-45% of children do not achieve remission. Recent research suggests that some genetic variants may be associated with a more beneficial response to psychological therapy. Epigenetic mechanisms such as DNA methylation work at the interface between genetic and environmental influences. Furthermore, epigenetic alterations at the serotonin transporter (SERT) promoter region have been associated with environmental influences such as stressful life experiences. In this study, we measured DNA methylation upstream of SERT in 116 children with an anxiety disorder, before and after receiving CBT. Change during treatment in percentage DNA methylation was significantly different in treatment responders vs nonresponders. This effect was driven by one CpG site in particular, at which responders increased in methylation, whereas nonresponders showed a decrease in DNA methylation. This is the first study to demonstrate differences in SERT methylation change in association with response to a purely psychological therapy. These findings confirm that biological changes occur alongside changes in symptomatology following a psychological therapy such as CBT

Dynamics and Radiation of Young Type-Ia Supernova Remnants: Important Physical Processes

We examine and analyze the physical processes that should be taken into account when modeling young type-Ia SNRs, with ages of several hundred years. It is shown, that energy losses in the metal-rich ejecta can be essential for remnants already at this stage of evolution. The influence of electron thermal conduction and the rate of the energy exchange between electrons and ions on the temperature distribution and the X-radiation from such remnants is studied. The data for Tycho SNR from the XMM-Newton X-ray telescope have been employed for the comparison of calculations with observations.Comment: 19 pages, 8 figure

The Ligand Binding Domain of GCNF Is Not Required for Repression of Pluripotency Genes in Mouse Fetal Ovarian Germ Cells

In mice, successful development and reproduction require that all cells, including germ cells, transition from a pluripotent to a differentiated state. This transition is associated with silencing of the pluripotency genes Oct4 and Nanog. Interestingly, these genes are repressed at different developmental timepoints in germ and somatic cells. Ovarian germ cells maintain their expression until about embryonic day (E) 14.5, whereas somatic cells silence them much earlier, at about E8.0. In both somatic cells and embryonic stem cells, silencing of Oct4 and Nanog requires the nuclear receptor GCNF. However, expression of the Gcnf gene has not been investigated in fetal ovarian germ cells, and whether it is required for silencing Oct4 and Nanog in that context is not known. Here we demonstrate that Gcnf is expressed in fetal ovarian germ cells, peaking at E14.5, when Oct4 and Nanog are silenced. However, conditional ablation of the ligand-binding domain of Gcnf using a ubiquitous, tamoxifen-inducible Cre indicates that Gcnf is not required for the down-regulation of pluripotency genes in fetal ovarian germ cells, nor is it required for initiation of meiosis and oogenesis. These results suggest that the silencing of Oct4 and Nanog in germ cells occurs via a different mechanism from that operating in somatic cells during gastrulation.Howard Hughes Medical InstituteNational Institutes of Health (U.S.) (2R01HG00257-20)National Human Genome Research Institute (U.S.) (2R01HG00257-20