2,960 research outputs found
On the penetration of meridional circulation below the solar convection zone
Meridional flows with velocities of a few meters per second are observed in
the uppermost regions of the solar convection zone. The amplitude and pattern
of the flows deeper in the solar interior, in particular near the top of the
radiative region, are of crucial importance to a wide range of solar
magnetohydrodynamical processes. In this paper, we provide a systematic study
of the penetration of large-scale meridional flows from the convection zone
into the radiative zone. In particular, we study the effects of the assumed
boundary conditions applied at the convective-radiative interface on the deeper
flows. Using simplified analytical models in conjunction with more complete
numerical methods, we show that penetration of the convectively-driven
meridional flows into the deeper interior is not necessarily limited to a
shallow Ekman depth but can penetrate much deeper, depending on how the
convective-radiative interface flows are modeled.Comment: 13 pages, 5 figures. Subitted to Ap
Fast magnetic reconnection in the plasmoid-dominated regime
A conceptual model of resistive magnetic reconnection via a stochastic
plasmoid chain is proposed. The global reconnection rate is shown to be
independent of the Lundquist number. The distribution of fluxes in the
plasmoids is shown to be an inverse square law. It is argued that there is a
finite probability of emergence of abnormally large plasmoids, which can
disrupt the chain (and may be responsible for observable large abrupt events in
solar flares and sawtooth crashes). A criterion for the transition from
magnetohydrodynamic to collisionless regime is provided.Comment: 4 pages, 1 figur
Magnetic Reconnection with Radiative Cooling. I. Optically-Thin Regime
Magnetic reconnection, a fundamental plasma process associated with a rapid
dissipation of magnetic energy, is believed to power many disruptive phenomena
in laboratory plasma devices, the Earth magnetosphere, and the solar corona.
Traditional reconnection research, geared towards these rather tenuous
environments, has justifiably ignored the effects of radiation on the
reconnection process. However, in many reconnecting systems in high-energy
astrophysics (e.g., accretion-disk coronae, relativistic jets, magnetar flares)
and, potentially, in powerful laser plasma and z-pinch experiments, the energy
density is so high that radiation, in particular radiative cooling, may start
to play an important role. This observation motivates the development of a
theory of high-energy-density radiative magnetic reconnection. As a first step
towards this goal, we present in this paper a simple Sweet--Parker-like theory
of non-relativistic resistive-MHD reconnection with strong radiative cooling.
First, we show how, in the absence of a guide magnetic field, intense cooling
leads to a strong compression of the plasma in the reconnection layer,
resulting in a higher reconnection rate. The compression ratio and the layer
temperature are determined by the balance between ohmic heating and radiative
cooling. The lower temperature in the radiatively-cooled layer leads to a
higher Spitzer resistivity and hence to an extra enhancement of the
reconnection rate. We then apply our general theory to several specific
astrophysically important radiative processes (bremsstrahlung, cyclotron, and
inverse-Compton) in the optically thin regime, for both the zero- and
strong-guide-field cases. We derive specific expressions for key reconnection
parameters, including the reconnection rate. We also discuss the limitations
and conditions for applicability of our theory.Comment: 31 pages, 1 figur
Fast and slow two-fluid magnetic reconnection
We present a two-fluid magnetohydrodynamics (MHD) model of quasi-stationary,
two-dimensional magnetic reconnection in an incompressible plasma composed of
electrons and ions. We find two distinct regimes of slow and fast reconnection.
The presence of these two regimes can provide a possible explanation for the
initial slow build up and subsequent rapid release of magnetic energy
frequently observed in cosmic and laboratory plasmas.Comment: 16 pages, 2 figures, 1 tabl
X-point collapse and saturation in the nonlinear tearing mode reconnection
We study the nonlinear evolution of the resistive tearing mode in slab
geometry in two dimensions. We show that, in the strongly driven regime (large
Delta'), a collapse of the X-point occurs once the island width exceeds a
certain critical value ~1/Delta'. A current sheet is formed and the
reconnection is exponential in time with a growth rate ~eta^1/2, where eta is
the resistivity. If the aspect ratio of the current sheet is sufficiently
large, the sheet can itself become tearing-mode unstable, giving rise to
secondary islands, which then coalesce with the original island. The saturated
state depends on the value of Delta'. For small Delta', the saturation
amplitude is ~Delta' and quantitatively agrees with the theoretical prediction.
If Delta' is large enough for the X-point collapse to have occured, the
saturation amplitude increases noticeably and becomes independent of Delta'.Comment: revtex4, 4 pages, 18 figure
Steady Hall Magnetohydrodynamics Near a X-type Magnetic Neutral Line
Hall magnetohydrodynamics (MHD) properties near a two-dimensional (2D) X-type
magnetic neutral line in the steady state are considered via heuristic and
rigorous developments. Upon considering the steady-state as the asymptotic
limit of the corresponding \textit{time-dependent} problem and using a rigorous
development, Hall effects are shown to be able to sustain the hyperbolicity of
the magnetic field (and hence a more open X-point configuration) near the
neutral line in the steady state. The heuristic development misses this subtle
connection of the steady state with the corresponding \textit{time-dependent}
problem and predicts only an elongated current-sheet configuration (as in
resistive MHD). However, the heuristic development turns out to be useful in
providing insight into the lack of dependence of the reconnection rate on the
mechanism breaking the frozen-in condition of the magnetic field lines. The
latter result can be understood in terms of the ability of the ions and
electrons to transport equal amounts of magnetic flux per unit time out of the
reconnection region.Comment: 1-10 page
Formation of Plasmoid Chains in Magnetic Reconnection
A detailed numerical study of magnetic reconnection in resistive MHD for very
large, previously inaccessible, Lundquist numbers () is
reported. Large-aspect-ratio Sweet-Parker current sheets are shown to be
unstable to super-Alfv\'enically fast formation of plasmoid (magnetic-island)
chains. The plasmoid number scales as and the instability growth rate
in the linear stage as , in agreement with the theory by Loureiro et
al. [Phys. Plasmas {\bf 14}, 100703 (2007)]. In the nonlinear regime, plasmoids
continue to grow faster than they are ejected and completely disrupt the
reconnection layer. These results suggest that high-Lundquist-number
reconnection is inherently time-dependent and hence call for a substantial
revision of the standard Sweet-Parker quasi-stationary picture for .Comment: submitted to Phys. Rev. Let
A Model for Patchy Reconnection in Three Dimensions
We show, theoretically and via MHD simulations, how a short burst of
reconnection localized in three dimensions on a one-dimensional current sheet
creates a pair of reconnected flux tubes. We focus on the post-reconnection
evolution of these flux tubes, studying their velocities and shapes. We find
that slow-mode shocks propagate along these reconnected flux tubes, releasing
magnetic energy as in steady-state Petschek reconnection. The geometry of these
three-dimensional shocks, however, differs dramatically from the classical
two-dimensional geometry. They propagate along the flux tube legs in four
isolated fronts, whereas in the two-dimensional Petschek model, they form a
continuous, stationary pair of V-shaped fronts.
We find that the cross sections of these reconnected flux tubes appear as
teardrop shaped bundles of flux propagating away from the reconnection site.
Based on this, we argue that the descending coronal voids seen by Yohkoh SXT,
LASCO, and TRACE are reconnected flux tubes descending from a flare site in the
high corona, for example after a coronal mass ejection. In this model, these
flux tubes would then settle into equilibrium in the low corona, forming an
arcade of post-flare coronal loops.Comment: 27 pages plus 16 figure
Can Extra Mixing in RGB and AGB Stars Be Attributed to Magnetic Mechanisms?
It is known that there must be some weak form of transport (called cool
bottom processing, or CBP) acting in low mass RGB and AGB stars, adding nuclei,
newly produced near the hydrogen-burning shell, to the convective envelope. We
assume that this extra-mixing originates in a stellar dynamo operated by the
differential rotation below the envelope, maintaining toroidal magnetic fields
near the hydrogen-burning shell. We use a phenomenological approach to the
buoyancy of magnetic flux tubes, assuming that they induce matter circulation
as needed by CBP models. This establishes requirements on the fields necessary
to transport material from zones where some nuclear burning takes place,
through the radiative layer, and into the convective envelope. Magnetic field
strengths are determined by the transport rates needed by CBP for the model
stellar structure of a star of initially 1.5 solar mass, in both the AGB and
RGB phases. The field required for the AGB star in the processing zone is B_0 ~
5x10^6 G; at the base of the convective envelope this yields an intensity B_E <
10^4 G (approximately). For the RGB case, B_0 ~ 5x10^4 to 4x10^5 G, and the
corresponding B_E are ~ 450 to 3500 G. These results are consistent with
existing observations on AGB stars. They also hint at the basis for high field
sources in some planetary nebulae and the very large fields found in some white
dwarfs. It is concluded that transport by magnetic buoyancy should be
considered as a possible mechanism for extra mixing through the radiative zone,
as is required by both stellar observations and the extensive isotopic data on
circumstellar condensates found in meteorites.Comment: 26 pages, 4 figures, accepted by Astrophysical Journa
The SAMI Galaxy Survey: Gas Streaming and Dynamical M/L in Rotationally Supported Systems
Line-of-sight velocities of gas and stars can constrain dark matter (DM)
within rotationally supported galaxies if they trace circular orbits
extensively. Photometric asymmetries may signify non-circular motions,
requiring spectra with dense spatial coverage. Our integral-field spectroscopy
of 178 galaxies spanned the mass range of the SAMI Galaxy Survey. We derived
circular speed curves (CSCs) of gas and stars from non-parametric Diskfit fits
out to . For 12/14 with measured H I profiles, ionized gas and H I
maximum velocities agreed. We fitted mass-follows-light models to 163 galaxies
by approximating the radial starlight profile as nested, very flattened mass
homeoids viewed as a S\'ersic form. Fitting broad-band SEDs to SDSS images gave
median stellar mass/light 1.7 assuming a Kroupa IMF vs. 2.6 dynamically.
Two-thirds of the dynamical mass/light measures were consistent with
star+remnant IMFs. One-fifth required upscaled starlight to fit, hence
comparable mass of unobserved baryons and/or DM distributed similarly across
the SAMI aperture that came to dominate motions as the starlight CSC declined
rapidly. The rest had mass distributed differently from starlight. Subtracting
fits of S\'ersic profiles to 13 VIKING Z-band images revealed residual weak
bars. Near the bar PA, we assessed m = 2 streaming velocities, and found
deviations usually <30 km/s from the CSC; three showed no deviation. Thus,
asymmetries rarely influenced our CSCs despite co-located shock-indicating,
emission-line flux ratios in more than 2/3.Comment: 21 pages, 15 figures. Accepted to MNRA
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