519 research outputs found
Turbulence in nearly incompressible fluids: density spectrum, flows, correlations and implication to the interstellar medium
International audienceInterstellar scintillation and angular radio wave broadening measurements show that interstellar and solar wind (electron) density fluctuations exhibit a Kolmogorov-like k-5/3 power spectrum extending over many decades in wavenumber space. The ubiquity of the Kolmogorov-like interstellar medium (ISM) density spectrum led to an explanation based on coupling incompressible magnetohydrodynamic (MHD) fluctuations to density fluctuations through a "pseudosound" relation within the context of "nearly incompressible" (NI) hydrodynamics (HD) and MHD models. The NI theory provides a fundamentally different explanation for the observed ISM density spectrum in that the density fluctuations can be a consequence of passive scalar convection due to background incompressible fluctuations. The theory further predicts generation of long-scale structures and various correlations between the density, temperature and the (magneto) acoustic as well as convective pressure fluctuations in the compressible ISM fluids in different thermal regimes that are determined purely by the thermal fluctuation level. In this paper, we present the results of our two dimensional nonlinear fluid simulations, exploring various nonlinear aspects that lead to inertial range ISM turbulence within the context of a NI hydrodymanics model. In qualitative agreement with the NI predictions and the in-situ observations, we find that i) the density fluctuations exhibit a Kolmogorov-like spectrum via a passive convection in the field of the background incompressible fluctuations, ii) the compressible ISM fluctuations form long scale flows and structures, and iii) the density and the temperature fluctuations are anti-correlated
Quasilinear Drift Of Cosmic Rays In Weak Turbulent Electromagnetic Fields
A general quasilinear transport parameter for particle drift in arbitrary
turbulence geometry is presented. The new drift coefficient is solely
characterized by a nonresonant term and is evaluated for slab and
two-dimensional turbulence geometry. The calculations presented here
demonstrate that fluctuating electric fields are a key quantity for
understanding quasilinear particle drift in slab geometry. It is shown that
particle drift does not exist in unpolarized and purely magnetic slab
fluctuations. This is in stark contrast to previous models, which are
restricted to slab geometry and the field line random walk limit. The
evaluation of the general transport parameter for two-dimensional turbulence
geometry, presented here for the first time for dynamical magnetic turbulence,
results in a drift coefficient valid for a magnetic power spectrum and
turbulence decay rate varying arbitrarily in wavenumber. For a two-component,
slab/two-dimensional turbulence model, numerical calculations are presented.
The new quasilinear drift, induced by the magnetic perturbations, is compared
with a standard drift expression related to the curvature and gradient of an
unperturbed heliospheric background magnetic field. The considerations
presented here offer a solid ground and natural explanation for the hitherto
puzzling observation that drift models often describe observations much better
when drift effects are reduced.Comment: 23 pages, 6 figures, accepted for publication in Ap
Radial Velocity along the Voyager 1 Trajectory: The Effect of Solar Cycle
As Voyager 1 and Voyager 2 are approaching the heliopause (HP)âthe boundary between the solar wind (SW) and the local interstellar medium (LISM)âwe expect new, unknown features of the heliospheric interface to be revealed. A seeming puzzle reported recently by Krimigis et al. concerns the unusually low, even negative, radial velocity components derived from the energetic ion distribution. Steady-state plasma models of the inner heliosheath (IHS) show that the radial velocity should not be equal to zero even at the surface of the HP. Here we demonstrate that the velocity distributions observed by Voyager 1 are consistent with time-dependent simulations of the SW-LISM interaction. In this Letter, we analyze the results from a numerical model of the large-scale heliosphere that includes solar cycle effects. Our simulations show that prolonged periods of low to negative radial velocity can exist in the IHS at substantial distances from the HP. It is also shown that Voyager 1 was more likely to observe such regions than Voyager 2
Shock Geometry and Spectral Breaks in Large SEP Events
Solar energetic particle (SEP) events are traditionally classified as "impulsive" or "gradual." It is now widely accepted that in gradual SEP events, particles are accelerated at coronal mass ejection-driven (CME-driven) shocks. In many of these large SEP events, particle spectra exhibit double power law or exponential rollover features, with the break energy or rollover energy ordered as (Q/A)^α, with Q being the ion charge in e and A the ion mass in units of proton mass m_p . This Q/A dependence of the spectral breaks provides an opportunity to study the underlying acceleration mechanism. In this paper, we examine how the Q/A dependence may depend on shock geometry. Using the nonlinear guiding center theory, we show that α ~ 1/5 for a quasi-perpendicular shock. Such a weak Q/A dependence is in contrast to the quasi-parallel shock case where α can reach 2. This difference in α reflects the difference of the underlying parallel and perpendicular diffusion coefficients Îș_(||) and Îș â„. We also examine the Q/A dependence of the break energy for the most general oblique shock case. Our analysis offers a possible way to remotely examine the geometry of a CME-driven shock when it is close to the Sun, where the acceleration of particle to high energies occurs
MHDâdriven kinetic dissipation in the solar wind and corona
Mechanisms for the deposition of heat in the lower coronal plasma are discussed, emphasizing recent attempts to reconcile the fluid and kinetic perspectives. Structures at magnetohydrodynamic (MHD) scales may drive a nonlinear cascade, preferentially exciting high perpendicular wavenumber fluctuations. Relevant dissipative kinetic processes must be identified that can absorb the associated energy flux. The relationship between the MHD cascade and direct cyclotron absorption, including cyclotron sweep, is discussed. We conclude that for coronal and solar wind parameters the perpendicular cascade cannot be neglected and may be more rapid than cyclotron sweep. Solar wind observational evidence suggests the relevance of the ion inertial scale, which is associated with current sheet thickness during reconnection. We conclude that a significant fraction of dissipation in the corona and solar wind likely proceeds through a perpendicular cascade and small-scale reconnection, coupled to kinetic processes that act at oblique wavevectors
Simulations of MHD Turbulence in a Strongly Magnetized Medium
We analyze 3D numerical simulations of driven incompressible
magnetohydrodynamic (MHD) turbulence in a periodic box threaded by a moderately
strong external magnetic field. We sum over nonlinear interactions within
Fourier wavebands and find that the time scale for the energy cascade is
consistent with the Goldreich-Sridhar model of strong MHD turbulence. Using
higher order longitudinal structure functions we show that the turbulent
motions in the plane perpendicular to the local mean magnetic field are similar
to ordinary hydrodynamic turbulence while motions parallel to the field are
consistent with a scaling correction which arises from the eddy anisotropy. We
present the structure tensor describing velocity statistics of Alfvenic and
pseudo-Alfvenic turbulence. Finally, we confirm that an imbalance of energy
moving up and down magnetic field lines leads to a slow decay of turbulent
motions and speculate that this imbalance is common in the interstellar medium
where injection of energy is intermittent both in time and space.Comment: ApJ accepted, 29 pages, 10 figures; some revisions, new figure
An introductory guide to fluid models with anisotropic temperatures Part 1 -- CGL description and collisionless fluid hierarchy
We present a detailed guide to advanced collisionless fluid models that
incorporate kinetic effects into the fluid framework, and that are much closer
to the collisionless kinetic description than traditional magnetohydrodynamics.
Such fluid models are directly applicable to modeling turbulent evolution of a
vast array of astrophysical plasmas, such as the solar corona and the solar
wind, the interstellar medium, as well as accretion disks and galaxy clusters.
The text can be viewed as a detailed guide to Landau fluid models and it is
divided into two parts. Part 1 is dedicated to fluid models that are obtained
by closing the fluid hierarchy with simple (non Landau fluid) closures. Part 2
is dedicated to Landau fluid closures. Here in Part 1, we discuss the CGL fluid
model in great detail, together with fluid models that contain dispersive
effects introduced by the Hall term and by the finite Larmor radius (FLR)
corrections to the pressure tensor. We consider dispersive effects introduced
by the non-gyrotropic heat flux vectors. We investigate the parallel and
oblique firehose instability, and show that the non-gyrotropic heat flux
strongly influences the maximum growth rate of these instabilities.
Furthermore, we discuss fluid models that contain evolution equations for the
gyrotropic heat flux fluctuations and that are closed at the 4th-moment level
by prescribing a specific form for the distribution function. For the
bi-Maxwellian distribution, such a closure is known as the "normal" closure. We
also discuss a fluid closure for the bi-kappa distribution. Finally, by
considering one-dimensional Maxwellian fluid closures at higher-order moments,
we show that such fluid models are always unstable. The last possible non
Landau fluid closure is therefore the "normal" closure, and beyond the
4th-order moment, Landau fluid closures are required.Comment: Improved version, accepted to JPP Lecture Notes. Some parts were
shortened and some parts were expanded. The text now contains Conclusion
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