5,738 research outputs found
Dynamos in Stellar Convection Zones: of Wreaths and Cycles
We live near a magnetic star whose cycles of activity are driven by dynamo
action beneath the surface. In the solar convection zone, rotation couples with
plasma motions to build highly organized magnetic fields that erupt at the
surface and undergo relatively regular cycles of polarity reversal. Despite our
proximity to the Sun, the nature of its dynamo remains elusive, but
observations of other solar-type stars show that surface magnetism is a nearly
ubiquitous feature. In recent time, numerical simulations of convection and
dynamo action have taken tremendous strides forward. Global-scale organization
and cyclic magnetism are being achieved by several groups in distinctly
different solar and stellar simulations. Here I will talk about advances on the
numerical front including wreath-building dynamos which may occupy stellar
convection zones. I will discuss the interplay between the new simulations,
various classes of mean-field models, and current and upcoming solar and
stellar observations.Comment: 10 pages, 5 figures, a4 format; proceedings for SOHO 24/GONG 2010
conference: "A new era of seismology of the Sun and solar-like stars,"
Aix-en-Provence, France, June 27-July 4, 2010 (JPCS
Dissecting Monomer-Dimer Equilibrium of an RNase P Protein Provides Insight Into the Synergistic Flexibility of 5’ Leader Pre-tRNA Recognition
Ribonuclease P (RNase P) is a universal RNA-protein endonuclease that catalyzes 5’ precursor-tRNA (ptRNA) processing. The RNase P RNA plays the catalytic role in ptRNA processing; however, the RNase P protein is required for catalysis in vivo and interacts with the 5’ leader sequence. A single P RNA and a P protein form the functional RNase P holoenzyme yet dimeric forms of bacterial RNase P can interact with non-tRNA substrates and influence bacterial cell growth. Oligomeric forms of the P protein can also occur in vitro and occlude the 5’ leader ptRNA binding interface, presenting a challenge in accurately defining the substrate recognition properties. To overcome this, concentration and temperature dependent NMR studies were performed on a thermostable RNase P protein from Thermatoga maritima. NMR relaxation (R1, R2), heteronuclear NOE, and diffusion ordered spectroscopy (DOSY) experiments were analyzed, identifying a monomeric species through the determination of the diffusion coefficients (D) and rotational correlation times (τc). Experimental diffusion coefficients and τc values for the predominant monomer (2.17 ± 0.36 * 10−10 m2/s, τc = 5.3 ns) or dimer (1.87 ± 0.40* 10−10 m2/s, τc = 9.7 ns) protein assemblies at 45°C correlate well with calculated diffusion coefficients derived from the crystallographic P protein structure (PDB 1NZ0). The identification of a monomeric P protein conformer from relaxation data and chemical shift information enabled us to gain novel insight into the structure of the P protein, highlighting a lack of structural convergence of the N-terminus (residues 1–14) in solution. We propose that the N-terminus of the bacterial P protein is partially disordered and adopts a stable conformation in the presence of RNA. In addition, we have determined the location of the 5’ leader RNA in solution and measured the affinity of the 5’ leader RNA–P protein interaction. We show that the monomer P protein interacts with RNA at the 5’ leader binding cleft that was previously identified using X-ray crystallography. Data support a model where N-terminal protein flexibility is stabilized by holoenzyme formation and helps to accommodate the 5’ leader region of ptRNA. Taken together, local structural changes of the P protein and the 5’ leader RNA provide a means to obtain optimal substrate alignment and activation of the RNase P holoenzyme
Magnetic Wreaths and Cycles in Convective Dynamos
Solar-type stars exhibit a rich variety of magnetic activity. Seeking to
explore the convective origins of this activity, we have carried out a series
of global 3D magnetohydrodynamic (MHD) simulations with the anelastic spherical
harmonic (ASH) code. Here we report on the dynamo mechanisms achieved as the
effects of artificial diffusion are systematically decreased. The simulations
are carried out at a nominal rotation rate of three times the solar value
(3), but similar dynamics may also apply to the Sun. Our previous
simulations demonstrated that convective dynamos can build persistent toroidal
flux structures (magnetic wreaths) in the midst of a turbulent convection zone
and that high rotation rates promote the cyclic reversal of these wreaths. Here
we demonstrate that magnetic cycles can also be achieved by reducing the
diffusion, thus increasing the Reynolds and magnetic Reynolds numbers. In these
more turbulent models, diffusive processes no longer play a significant role in
the key dynamical balances that establish and maintain the differential
rotation and magnetic wreaths. Magnetic reversals are attributed to an
imbalance in the poloidal magnetic induction by convective motions that is
stabilized at higher diffusion levels. Additionally, the enhanced levels of
turbulence lead to greater intermittency in the toroidal magnetic wreaths,
promoting the generation of buoyant magnetic loops that rise from the deep
interior to the upper regions of our simulated domain. The implications of such
turbulence-induced magnetic buoyancy for solar and stellar flux emergence are
also discussed.Comment: 21 pages, 16 figures, accepted for publication in Ap
Global-scale wreath-building dynamos in stellar convection zones
When stars like our Sun are young they rotate rapidly and are very
magnetically active. We explore dynamo action in rapidly rotating suns with the
3-D MHD anelastic spherical harmonic (ASH) code. The magnetic fields built in
these dynamos are organized on global-scales into wreath-like structures that
span the convection zone. Wreath-building dynamos can undergo quasi-cyclic
reversals of polarity and such behavior is common in the parameter space we
have been able to explore. These dynamos do not appear to require tachoclines
to achieve their spatial or temporal organization. Wreath-building dynamos are
present to some degree at all rotation rates, but are most evident in the more
rapidly rotating simulations.Comment: 8 pages, 4 figures. To appear in IAU 271: "Astrophysical Dynamics:
from Stars to Galaxies
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