2,832 research outputs found
Standard 1D solar atmosphere as initial condition for MHD simulations and switch-on effects
Many applications in Solar physics need a 1D atmospheric model as initial
condition or as reference for inversions of observational data. The VAL
atmospheric models are based on observations and are widely used since decades.
Complementary to that, the FAL models implement radiative hydrodynamics and
showed the shortcomings of the VAL models since almost equally long time. In
this work, we present a new 1D layered atmosphere that spans not only from the
photosphere to the transition region, but from the solar interior up to far in
the corona. We also discuss typical mistakes that are done when switching on
simulations based on such an initial condition and show how the initial
condition can be equilibrated so that a simulation can start smoothly. The 1D
atmosphere we present here served well as initial condition for HD and MHD
simulations and should also be considered as reference data for solving inverse
problems.Comment: 10 pages, 3 figures, published versio
Mechanisms of jet formation on the giant planets
The giant planet atmospheres exhibit alternating prograde (eastward) and
retrograde (westward) jets of different speeds and widths, with an equatorial
jet that is prograde on Jupiter and Saturn and retrograde on Uranus and
Neptune. The jets are variously thought to be driven by differential radiative
heating of the upper atmosphere or by intrinsic heat fluxes emanating from the
deep interior. But existing models cannot account for the different flow
configurations on the giant planets in an energetically consistent manner. Here
a three-dimensional general circulation model is used to show that the
different flow configurations can be reproduced by mechanisms universal across
the giant planets if differences in their radiative heating and intrinsic heat
fluxes are taken into account. Whether the equatorial jet is prograde or
retrograde depends on whether the deep intrinsic heat fluxes are strong enough
that convection penetrates into the upper troposphere and generates strong
equatorial Rossby waves there. Prograde equatorial jets result if convective
Rossby wave generation is strong and low-latitude angular momentum flux
divergence owing to baroclinic eddies generated off the equator is sufficiently
weak (Jupiter and Saturn). Retrograde equatorial jets result if either
convective Rossby wave generation is weak or absent (Uranus) or low-latitude
angular momentum flux divergence owing to baroclinic eddies is sufficiently
strong (Neptune). The different speeds and widths of the off-equatorial jets
depend, among other factors, on the differential radiative heating of the
atmosphere and the altitude of the jets, which are vertically sheared. The
simulations have closed energy and angular momentum balances that are
consistent with observations of the giant planets.Comment: 21 pages, 10 figure
Relaxed plasma equilibria and entropy-related plasma self-organization principles
The concept of plasma relaxation as a constrained energy minimization is reviewed. Recent work by the authors on generalizing this approach to partially relaxed threedimensional plasma systems in a way consistent with chaos theory is discussed, with a view to clarifying the thermodynamic aspects of the variational approach used. Other entropy-related approaches to finding long-time steady states of turbulent or chaotic plasma systems are also briefly reviewed
Effect of stellar wind induced magnetic fields on planetary obstacles of non-magnetized hot Jupiters
We investigate the interaction between the magnetized stellar wind plasma and
the partially ionized hydrodynamic hydrogen outflow from the escaping upper
atmosphere of non- or weakly magnetized hot Jupiters. We use the well-studied
hot Jupiter HD 209458b as an example for similar exoplanets, assuming a
negligible intrinsic magnetic moment. For this planet, the stellar wind plasma
interaction forms an obstacle in the planet's upper atmosphere, in which the
position of the magnetopause is determined by the condition of pressure balance
between the stellar wind and the expanded atmosphere, heated by the stellar
extreme ultraviolet (EUV) radiation. We show that the neutral atmospheric atoms
penetrate into the region dominated by the stellar wind, where they are ionized
by photo-ionization and charge exchange, and then mixed with the stellar wind
flow. Using a 3D magnetohydrodynamic (MHD) model, we show that an induced
magnetic field forms in front of the planetary obstacle, which appears to be
much stronger compared to those produced by the solar wind interaction with
Venus and Mars. Depending on the stellar wind parameters, because of the
induced magnetic field, the planetary obstacle can move up to ~0.5-1 planetary
radii closer to the planet. Finally, we discuss how estimations of the
intrinsic magnetic moment of hot Jupiters can be inferred by coupling
hydrodynamic upper planetary atmosphere and MHD stellar wind interaction models
together with UV observations. In particular, we find that HD 209458b should
likely have an intrinsic magnetic moment of 10-20% that of Jupiter.Comment: 8 pages, 6 figures, 2 tables, accepted to MNRA
The Dynamo Effects in Laboratory Plasmas
A concise review of observations of the dynamo effect in laboratory
plasmas is given. Unlike many astrophysical systems, the laboratory pinch
plasmas are driven magnetically. When the system is overdriven, the resultant
instabilities cause magnetic and flow fields to fluctuate, and their
correlation induces electromotive forces along the mean magnetic field. This
-effect drives mean parallel electric current, which, in turn, modifies
the initial background mean magnetic structure towards the stable regime. This
drive-and-relax cycle, or the so-called self-organization process, happens in
magnetized plasmas in a time scale much shorter than resistive diffusion time,
thus it is a fast and unquenched dynamo process. The observed -effect
redistributes magnetic helicity (a measure of twistedness and knottedness of
magnetic field lines) but conserves its total value. It can be shown that fast
and unquenched dynamos are natural consequences of a driven system where
fluctuations are statistically either not stationary in time or not homogeneous
in space, or both. Implications to astrophysical phenomena will be discussed.Comment: 21 pages, 15 figures, submitted to Magnetohydrodynamic
Magnetohydrodynamic activity inside a sphere
We present a computational method to solve the magnetohydrodynamic equations
in spherical geometry. The technique is fully nonlinear and wholly spectral,
and uses an expansion basis that is adapted to the geometry:
Chandrasekhar-Kendall vector eigenfunctions of the curl. The resulting lower
spatial resolution is somewhat offset by being able to build all the boundary
conditions into each of the orthogonal expansion functions and by the
disappearance of any difficulties caused by singularities at the center of the
sphere. The results reported here are for mechanically and magnetically
isolated spheres, although different boundary conditions could be studied by
adapting the same method. The intent is to be able to study the nonlinear
dynamical evolution of those aspects that are peculiar to the spherical
geometry at only moderate Reynolds numbers. The code is parallelized, and will
preserve to high accuracy the ideal magnetohydrodynamic (MHD) invariants of the
system (global energy, magnetic helicity, cross helicity). Examples of results
for selective decay and mechanically-driven dynamo simulations are discussed.
In the dynamo cases, spontaneous flips of the dipole orientation are observed.Comment: 15 pages, 19 figures. Improved figures, in press in Physics of Fluid
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