29 research outputs found
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 Turbulent Convection and Magnetic Dynamo Action in the Solar Envelope
The operation of the solar global dynamo appears to involve many dynamical
elements. Self-consistent MHD simulations which realistically incorporate all
of these processes are not yet computationally feasible, though some elements
can now be studied with reasonable fidelity. Here we consider the manner in
which turbulent compressible convection within the bulk of the solar convection
zone can generate large-scale magnetic fields through dynamo action. We
accomplish this through a series of three-dimensional numerical simulations of
MHD convection within rotating spherical shells using our ASH code on massively
parallel supercomputers. Since differential rotation is a key ingredient in all
dynamo models, we also examine here the nature of the rotation profiles that
can be sustained within the deep convection zone as strong magnetic fields are
built and maintained. We find that the convection is able to maintain a
solar-like angular velocity profile despite the influence of Maxwell stresses
which tend to oppose Reynolds stresses and thus reduce the latitudinal angular
velocity contrast throughout the convection zone. The dynamo-generated magnetic
fields exhibit a complex structure and evolution, with radial fields
concentrated in downflow lanes and toroidal fields organized into twisted
ribbons which are extended in longitude and which achieve field strengths of up
to 5000 G. The flows and fields exhibit substantial kinetic and magnetic
helicity although systematic hemispherical patterns are only apparent in the
former. Fluctuating fields dominate the magnetic energy and account for most of
the back-reaction on the flow via Lorentz forces. Mean fields are relatively
weak and do not exhibit systematic latitudinal propagation or periodic polarity
reversals as in the sun. This may be attributed to the absence of a tachocline.Comment: 55 pages (ApJ refereeing format), 15 figures (low res), published by
ApJ on October 2004 (abstract slightly reduced in order to fit in 24 lines
limit) see also Browning, Miesch, Brun & Toomre 2006, ApJL, 648, 157
(astro-ph/0609153) for the effect of a tachocline in organizing the mean
field
Simulations of core convection in rotating A-type stars: Differential rotation and overshooting
We present the results of 3--D simulations of core convection within A-type
stars of 2 solar masses, at a range of rotation rates. We consider the inner
30% by radius of such stars, thereby encompassing the convective core and some
of the surrounding radiative envelope. We utilize our anelastic spherical
harmonic (ASH) code, which solves the compressible Navier-Stokes equations in
the anelastic approximation, to examine highly nonlinear flows that can span
multiple scale heights. The cores of these stars are found to rotate
differentially, with central cylindrical regions of strikingly slow rotation
achieved in our simulations of stars whose convective Rossby number (R_{oc}) is
less than unity. Such differential rotation results from the redistribution of
angular momentum by the nonlinear convection that strongly senses the overall
rotation of the star. Penetrative convective motions extend into the overlying
radiative zone, yielding a prolate shape (aligned with the rotation axis) to
the central region in which nearly adiabatic stratification is achieved. This
is further surrounded by a region of overshooting motions, the extent of which
is greater at the equator than at the poles, yielding an overall spherical
shape to the domain experiencing at least some convective mixing. We assess the
overshooting achieved as the stability of the radiative exterior is varied, and
the weak circulations that result in that exterior. The convective plumes serve
to excite gravity waves in the radiative envelope, ranging from localized
ripples of many scales to some remarkable global resonances.Comment: 48 pages, 16 figures, some color. Accepted to Astrophys. J. Color
figures compressed with appreciable loss of quality; a PDF of the paper with
better figures is available at
http://lcd-www.colorado.edu/~brownim/core_convectsep24.pd
Turbulent Convection Under the Influence of Rotation: Sustaining a Strong Differential Rotation
The intense turbulence present in the solar convection zone is a major
challenge to both theory and simulation as one tries to understand the origins
of the striking differential rotation profile with radius and latitude that has
been revealed by helioseismology. The differential rotation must be an
essential element in the operation of the solar magnetic dynamo and its cycles
of activity, yet there are many aspects of the interplay between convection,
rotation and magnetic fields that are still unclear. We have here carried out a
series of 3--D numerical simulations of turbulent convection within deep
spherical shells using our anelastic spherical harmonic (ASH) code on massively
parallel supercomputers. These studies of the global dynamics of the solar
convection zone concentrate on how the differential rotation and meridional
circulation are established. We have analyzed the transport of angular momentum
in establishing such differential rotation, and clarified the roles played by
Reynolds stresses and the meridional circulation in this process. We have found
that the Reynolds stresses are crucial in transporting angular momentum toward
the equator. The effects of baroclinicity (thermal wind) have been found to
have a modest role in the resulting mean zonal flows. The simulations have
produced differential rotation profiles within the bulk of the convection zone
that make reasonable contact with ones inferred from helioseismic inversions,
namely possessing a fast equator, an angular velocity difference of about 30%
from equator to pole, and some constancy along radial lines at mid-latitudes.Comment: 25 pages, 14 very low resolution figures, shortened abstract,
published by ApJ. High resolution/complete version can be found at
http://lcd-www.colorado.edu/sabrun/index_cv.html then Scientific Publication
Strong Dynamo Action in Rapidly Rotating Suns
Stellar dynamos are driven by complex couplings between rotation and
turbulent convection, which drive global-scale flows and build and rebuild
stellar magnetic fields. When stars like our sun are young, they rotate much
more rapidly than the current solar rate. Observations generally indicate that
more rapid rotation is correlated with stronger magnetic activity and perhaps
more effective dynamo action. Here we examine the effects of more rapid
rotation on dynamo action in a star like our sun. We find that vigorous dynamo
action is realized, with magnetic field generated throughout the bulk of the
convection zone. These simulations do not possess a penetrative tachocline of
shear where global-scale fields are thought to be organized in our sun, but
despite this we find strikingly ordered fields, much like sea-snakes of
toroidal field, which are organized on global scales. We believe this to be a
novel finding.Comment: 8 pages, 4 figs. Published in conference proceedings "Unsolved
Problems in Stellar Physics", held July 2-6 2007 Cambridge, Englan
A chemical survey of exoplanets with ARIEL
Thousands of exoplanets have now been discovered with a huge range of masses, sizes and orbits: from rocky Earth-like planets to large gas giants grazing the surface of their host star. However, the essential nature of these exoplanets remains largely mysterious: there is no known, discernible pattern linking the presence, size, or orbital parameters of a planet to the nature of its parent star. We have little idea whether the chemistry of a planet is linked to its formation environment, or whether the type of host star drives the physics and chemistry of the planetâs birth, and evolution. ARIEL was conceived to observe a large number (~1000) of transiting planets for statistical understanding, including gas giants, Neptunes, super-Earths and Earth-size planets around a range of host star types using transit spectroscopy in the 1.25â7.8 ÎŒm spectral range and multiple narrow-band photometry in the optical. ARIEL will focus on warm and hot planets to take advantage of their well-mixed atmospheres which should show minimal condensation and sequestration of high-Z materials compared to their colder Solar System siblings. Said warm and hot atmospheres are expected to be more representative of the planetary bulk composition. Observations of these warm/hot exoplanets, and in particular of their elemental composition (especially C, O, N, S, Si), will allow the understanding of the early stages of planetary and atmospheric formation during the nebular phase and the following few million years. ARIEL will thus provide a representative picture of the chemical nature of the exoplanets and relate this directly to the type and chemical environment of the host star. ARIEL is designed as a dedicated survey mission for combined-light spectroscopy, capable of observing a large and well-defined planet sample within its 4-year mission lifetime. Transit, eclipse and phase-curve spectroscopy methods, whereby the signal from the star and planet are differentiated using knowledge of the planetary ephemerides, allow us to measure atmospheric signals from the planet at levels of 10â100 part per million (ppm) relative to the star and, given the bright nature of targets, also allows more sophisticated techniques, such as eclipse mapping, to give a deeper insight into the nature of the atmosphere. These types of observations require a stable payload and satellite platform with broad, instantaneous wavelength coverage to detect many molecular species, probe the thermal structure, identify clouds and monitor the stellar activity. The wavelength range proposed covers all the expected major atmospheric gases from e.g. H2O, CO2, CH4 NH3, HCN, H2S through to the more exotic metallic compounds, such as TiO, VO, and condensed species. Simulations of ARIEL performance in conducting exoplanet surveys have been performed â using conservative estimates of mission performance and a full model of all significant noise sources in the measurement â using a list of potential ARIEL targets that incorporates the latest available exoplanet statistics. The conclusion at the end of the Phase A study, is that ARIEL â in line with the stated mission objectives â will be able to observe about 1000 exoplanets depending on the details of the adopted survey strategy, thus confirming the feasibility of the main science objectives.Peer reviewedFinal Published versio
The Mass-dependence of Angular Momentum Evolution in Sun-like Stars
International audienceTo better understand the observed distributions of the rotation rate and magnetic activity of Sun-like and low-mass stars, we derive a physically motivated scaling for the dependence of the stellar wind torque on the Rossby number. The torque also contains an empirically derived scaling with stellar mass (and radius), which provides new insight into the mass-dependence of stellar magnetic and wind properties. We demonstrate that this new formulation explains why the lowest mass stars are observed to maintain rapid rotation for much longer than solar-mass stars, and simultaneously why older populations exhibit a sequence of slowly rotating stars, in which the low-mass stars rotate more slowly than solar-mass stars. The model also reproduces some previously unexplained features in the period-mass diagram for the Kepler field, notably: the particular shape of the "upper envelope" of the distribution, suggesting that ~95% of Kepler field stars with measured rotation periods are younger than ~4 Gyr; and the shape of the "lower envelope," corresponding to the location where stars transition between magnetically saturated and unsaturated regimes
Erratum: âThe Mass-dependence of Angular Momentum Evolution in Sun-like Starsâ (<A href="http://doi.org/10.1088/2041-8205/799/2/l23">2015, ApJL, 799, L23</A>)
This is the final version. Available from the American Astronomical Society via the DOI in this recordThe article to which this is the erratum is in ORE at http://hdl.handle.net/10871/16813In the original Letter, the moments of inertia of all stars used in the model were too large by an exact factor of 3/2, due to a conceptual error. This affects the value of the moment of inertia of the Sun that is listed in Table 1, and we include here the full, corrected table. The only change in the table is to the value of Ie. Because the error was a constant multiplicative factor on all moments of inertia, the only change this makes is in the value for the normalization for the torque. Specifically, all results presented in the original Letter are unchanged, except that Equation (8) should read T0 = 6.3 Ă 1030 erg (Râ/Râ)3.1 (Mâ/Mâ)0.5, which corresponds to the requirement that Tâ = 6.3 Ă 1030 erg. (Table Presented)
Stirring the Base of the Solar Wind: On Heat Transfer and Vortex Formation
Current models of the solar wind must approximate (or ignore) the small-scale
dynamics within the solar atmosphere, however these are likely important in
shaping the emerging wave-turbulence spectrum and ultimately
heating/accelerating the coronal plasma. The Bifrost code produces realistic
simulations of the solar atmosphere that facilitate the analysis of spatial and
temporal scales which are currently at, or beyond, the limit of modern solar
telescopes. For this study, the Bifrost simulation is configured to represent
the solar atmosphere in a coronal hole region, from which the fast solar wind
emerges. The simulation extends from the upper-convection zone (2.5 Mm below
the photosphere) to the low-corona (14.5 Mm above the photosphere), with a
horizontal extent of 24 Mm x 24 Mm. The twisting of the coronal magnetic field
by photospheric flows, efficiently injects energy into the low-corona. Poynting
fluxes of up to kWm are commonly observed inside twisted magnetic
structures with diameters in the low-corona of 1 - 5 Mm. Torsional Alfv\'en
waves are favourably transmitted along these structures, and will subsequently
escape into the solar wind. However, reflections of these waves from the upper
boundary condition make it difficult to unambiguously quantify the emerging
Alfv\'en wave-energy flux. This study represents a first step in quantifying
the conditions at the base of the solar wind using Bifrost simulations. It is
shown that the coronal magnetic field is readily braided and twisted by
photospheric flows. Temperature and density contrasts form between regions with
active stirring motions and those without. Stronger whirlpool-like flows in the
convection, concurrent with magnetic concentrations, launch torsional Alfv\'en
waves up through the magnetic funnel network, which are expected to enhance the
turbulent generation of magnetic switchbacks in the solar wind.Comment: Accepted to A&A. 22 Pages + Appendix. 20 Figures + 5 Appendix Figure