575 research outputs found
The Role of the Magnetorotational Instability in the Sun
We calculate growth rates for nonaxisymmetric instabilities including the
magnetorotational instability (MRI) throughout the Sun. We first derive a
dispersion relation for nonaxisymmetric instability including the effects of
shear, convective buoyancy, and three diffusivities (thermal conductivity,
resistivity, and viscosity). We then use a solar model evolved with the stellar
evolution code MESA and angular velocity profiles determined by Global
Oscillations Network Group (GONG) helioseismology to determine the unstable
modes present at each location in the Sun and the associated growth rates. The
overall instability has unstable modes throughout the convection zone and also
slightly below it at middle and high latitudes. It contains three classes of
modes: large-scale hydrodynamic convective modes, large-scale hydrodynamic
shear modes, and small-scale magnetohydrodynamic (MHD) shear modes, which may
be properly called MRI modes. While large-scale convective modes are the most
rapidly growing modes in most of the convective zone, MRI modes are important
in both stably stratified and convectively unstable locations near the
tachocline at colatitudes theta less than 53 degrees. Nonaxisymmetric MRI modes
grow faster than the corresponding axisymmetric modes; for some poloidal
magnetic fields, the nonaxisymmetric MRI growth rates are similar to the
angular rotation frequency Omega, while axisymmetric modes are stabilized. We
briefly discuss the saturation of the field produced by MRI modes, finding that
the implied field at the base of the convective zone in the Sun is comparable
to that derived based on dynamos active in the tachocline and that the
saturation of field resulting from the MRI may be of importance even in the
upper convection zone.Comment: 20 pages, 11 figure
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Magnetic instabilities and resulting energy conversion in astrophysics
textBecause the universe is primarily composed of plasma, the interaction of plasmas and magnetic fields is of great importance for astrophysics. In this dissertation, we investigate three magnetic instabilities and examine their possible effects on astrophysical objects. First, we model solar coronal structures as Double Beltrami states, which are the lowest energy equilibria of Hall magnetohydrodynamics. We find that these states can undergo a catastrophe with characteristics similar to those of a solar eruption, such as a flare or coronal mass ejection. We then investigate magnetic reconnection and particle acceleration in moderately magnetized relativistic pair plasmas with three-dimensional particle-in-cell simulations of a kinetic-scale current sheet. We find that in three dimensions the tearing instability produces a network of interconnected and interacting magnetic flux ropes. In its nonlinear evolution, the current sheet evolves toward a three-dimensional, disordered state in which the resulting flux rope segments contain magnetic substructure on kinetic scales and sites of temporally and spatially intermittent dissipation. We find that reconnection produces significant particle acceleration, primarily due to the electric field in the X-line regions between flux ropes; the resulting particle energy spectrum can extend to high Lorentz factors. We find that the highest energy particles are moderately beamed within.Astronom
The Role of the Magnetorotational Instability in Massive Stars
The magnetorotational instability (MRI) is key physics in accretion disks and
is widely considered to play some role in massive-star core collapse. Models of
rotating massive stars naturally develop very strong shear at composition
boundaries, a necessary condition for MRI instability, and the MRI is subject
to triply-diffusive destabilizing effects in radiative regions. We have used
the MESA stellar evolution code to compute magnetic effects due to the
Spruit-Taylor mechanism and the MRI, separately and together, in a sample of
massive star models. We find that the MRI can be active in the later stages of
massive star evolution, leading to mixing effects that are not captured in
models that neglect the MRI. The MRI and related magneto-rotational effects can
move models of given ZAMS mass across "boundaries" from degenerate CO cores to
degenerate O/Ne/Mg cores and from degenerate O/Ne/Mg cores to iron cores, thus
affecting the final evolution and the physics of core collapse. The MRI acting
alone can slow the rotation of the inner core in general agreement with the
observed "initial" rotation rates of pulsars. The MRI analysis suggests that
localized fields ~10^{12} G may exist at the boundary of the iron core. With
both the ST and MRI mechanisms active in the 20 solar mass model, we find that
the helium shell mixes entirely out into the envelope. Enhanced mixing could
yield a population of yellow or even blue supergiant supernova progenitors that
would not be standard SN IIP.Comment: 32 pages, 19 figures, accepted for publication in the Astrophysical
Journa
Right-Handed New Physics Remains Strangely Beautiful
Current data on CP violation in B_d -> eta' K_S and B_d -> phi K_S, taken
literally, suggest new physics contributions in b -> s transitions. Despite a
claim to the contrary, we point out that right-handed operators with a single
weak phase can account for both deviations thanks to the two-fold ambiguity in
the extraction of the weak phase from the corresponding CP-asymmetry. This
observation is welcome since large mixing in the right-handed sector is favored
by many GUT models and frameworks which address the flavor puzzle. There are
also interesting correlations with the B_s system which provide a way to test
this scenario in the near future.Comment: 7 pages, 9 figures; published version: added 1 reference and 1
clarificatio
Stress, Physical Activity, and Resilience Resources: Tests of Direct and Moderation Effects in Young Adults
Stress is an important consideration for understanding why individuals take part in limited or no physical activity (PA). The effects of stress on PA do not hold for everyone, so examinations of possible moderators that protect individuals from the harmful effects of stress are required. Aligned with a resilience framework, individual resources (e.g., hope and self-efficacy) may buffer the maladaptive effects of stress, such that people who have access to these resources in greater quantity may be more "resilient" to the deleterious effects of stress on PA. This study was designed to test this expectation. In total, 140 Australian undergraduate students (70.7% female, Mage = 21.68 ± 4.88) completed a multisection survey and provided a sample for hair cortisol concentration analysis using immunoassays. Main effects demonstrated primarily small and nonsignificant associations between perceived stress and hair cortisol concentration with different intensities of PA. Similar findings were observed between individual-level resilience resources and PA intensities, with the exception of hope (i.e., positive association with vigorous PA and negative association with sitting), self-efficacy (i.e., positive association with vigorous PA), and resilience (i.e., positive association with walking). Although certain individual-level resilience resources were perceived as beneficial for PA and sedentary time, the moderating role of resilience resources was not supported by the findings. The direct and moderating effects between stress, PA, and resilience resources require further testing using longitudinal designs in which stressful periods occur naturally (e.g., exams for students) or are experimentally manipulated
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