262 research outputs found
Shell-models of RMHD turbulence and the heating of solar coronal loops
A simplified non-linear numerical model for the development of incompressible
magnetohydrodynamics (MHD) in the presence of a strong magnetic field B0 and
stratification, nicknamed Shell-Atm, is presented. In planes orthogonal to the
mean field, the non-linear incompressible dynamics is replaced by 2D
shell-models for the complex variables u and b, allowing one to reach large
Reynolds numbers while at the same time carrying out sufficiently long time
integrations to obtain a good statistics at moderate computational cost. The
shell-models of different planes are coupled by Alfven waves propagating along
B0. The model may be applied to open or closed magnetic field configurations
where the axial field dominates and the plasma pressure is low; here we apply
it to the specific case of a magnetic loop of the solar corona heated via
turbulence driven by photospheric motions, and we use statistics for its
analysis. The Alfven waves interact non-linearly and form turbulent spectra in
the directions perpendicular and, via propagation, also parallel to the mean
field. A heating function is obtained, and is shown to be intermittent; the
average heating is consistent with values required for sustaining a hot corona,
and is proportional to the aspect ratio of the loop to the power -1.5;
characteristic properties of heating events are distributed as power-laws.
Cross-correlations show a delay of dissipation compared to energy content.Comment: 12 pages, 16 figures, accepted for publication in Ap
Ion kinetic energy conservation and magnetic field strength constancy in multi-fluid solar wind Alfv\'enic turbulence
We investigate properties of the plasma fluid motion in the large amplitude
low frequency fluctuations of highly Alfv\'enic fast solar wind. We show that
protons locally conserve total kinetic energy when observed from an effective
frame of reference comoving with the fluctuations. For typical properties of
the fast wind, this frame can be reasonably identified by alpha particles,
which, owing to their drift with respect to protons at about the Alfv\'en speed
along the magnetic field, do not partake in the fluid low frequency
fluctuations. Using their velocity to transform proton velocity into the frame
of Alfv\'enic turbulence, we demonstrate that the resulting plasma motion is
characterized by a constant absolute value of the velocity, zero electric
fields, and aligned velocity and magnetic field vectors as expected for
unidirectional Alfv\'enic fluctuations in equilibrium. We propose that this
constraint, via the correlation between velocity and magnetic field in
Alfv\'enic turbulence, is at the origin of the observed constancy of the
magnetic field: while the constant velocity corresponding to constant energy
can be only observed in the frame of the fluctuations, the correspondingly
constant total magnetic field, invariant for Galilean transformations, remains
the observational signature, in the spacecraft frame, of the constant total
energy in the Alfv\'en turbulence frame.Comment: 6 pages, 6 figures, Accepted for publication in The Astrophysical
Journa
Magnetic Effects Change Our View of the Heliosheath
There is currently a controversy as to whether Voyager 1 has already crossed
the Termination Shock, the first boundary of the Heliosphere. The region
between the Termination Shock and the Heliopause, the Helisheath, is one of the
most unknown regions theoretically. In the Heliosheath magnetic effects are
crucial, as the solar magnetic field is compressed at the Termination Shock by
the slowing flow. Recently, our simulations showed that the Heliosheath
presents remarkable dynamics, with turbulent flows and the presence of a jet
flow at the current sheet that is unstable due to magnetohydrodynamic
instabilities \cite{opher,opher1}. In this paper we review these recent
results, and present an additional simulation with constant neutral atom
background. In this case the jet is still present but with reduced intensity.
Further study, e.g., including neutrals and the tilt of the solar rotation from
the magnetic axis, is required before we can definitively address how the
Heliosheath behaves. Already we can say that this region presents remarkable
dynamics, with turbulent flows, indicating that the Heliosheath might be very
different from what we previously thought.Comment: 6 pages, 5 figures, to appear in IGPP 3rd Annual International
Astrophysics Conference, "PHYSICS OF THE OUTER HELIOSPHERE
Three-dimensional magnetic reconnection simulations using the Eulerian Conservative High Order (ECHO) code
Magnetic reconnection and shear driven instabilities are pervasive phenomena in the heliosphere and in astrophysical plasmas in general. Magnetic reconnection and
Kelvin-Helmholtz-like instabilities require the use of high-order numerical approximations to study their linear and non-linear evolution. At the same time, in compressible MHD the dynamical activity following reconnection processes leads to formation of discontinuous modes which should be treated by shock-capturing numerical schemes. For this purpose we have designed an Eulerian Conservative High Order (ECHO) code in which, i) explicit diffusivity is taken into account, ii) high-order numerical approximations of flux derivatives are included and iii) shock-capturing algorithms are employed in managing flux discontinuities.
This code has been applied successfully in studying the linear and non-linear 3D evolution of the tearing instability and in following the 3D evolution of a current
sheet embedded in a sheared flow
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
Transverse oscillations in solar coronal loops induced by propagating Alfvenic pulses
The propagation and the evolution of Alfvenic pulses in the solar coronal
arcades is investigated by means of MHD numerical simulations. Significant
transverse oscillations in coronal loops, triggered by nearby flare events, are
often measured in EUV lines and are generally interpreted as standing kink
modes. However, the damping times of these oscillations are typically very
short (from one to a few periods) and the physical mechanism responsible for
the decay is still a matter of debate. Moreover, the majority of the observed
cases actually appears to be better modeled by propagating, rather than
standing, modes. Here we perform 2.5-D compressible MHD simulations of
impulsively generated Alfven waves propagating in a potential magnetic arcade
(assumed as a simplified 2-D loop model), taking into account the
stratification of the solar atmosphere with height from the photosphere to the
corona. The results show a strong spreading of the initially localized pulses
along the loop, due to the variations in the Alfven velocity with height, and
correspondingly an efficient damping of the amplitude of the oscillations. We
believe that simple explanations based on the effects of wave propagation in
highly inhomogeneous media may apply to the majority of the reported cases, and
that variations of the background density and Alfven speed along the loop
should be considered as key ingredients in future models.Comment: Accepted for publication in A&A on 26 October 2004; 10 pages, 8
figure
A Three-dimensional Model of the Solar Wind Incorporating Solar Magnetogram Observations
We present a new compressible MHD model for simulating the three-dimensional structure of the solar wind under steady state conditions. The initial potential magnetic field is reconstructed throughout the computational volume using the source surface method, in which the necessary boundary conditions for the field are provided by solar magnetogram data. The solar wind in our simulations is powered by the energy interchange between the plasma and large-scale MHD turbulence, assuming that the additional energy is stored in the "turbulent" internal degrees of freedom. In order to reproduce the observed bimodal structure of the solar wind, the thermodynamic quantities for the initial state are varied with the heliographic latitude and longitude depending on the strength of the radial magnetic field
Gossamer roadmap technology reference study for a solar polar mission
A technology reference study for a solar polar mission is presented. The study uses novel analytical methods to quantify the mission design space including the required sail performance to achieve a given solar polar observation angle within a given timeframe and thus to derive mass allocations for the remaining spacecraft sub-systems, that is excluding the solar sail sub-system. A parametric, bottom-up, system mass budget analysis is then used to establish the required sail technology to deliver a range of science payloads, and to establish where such payloads can be delivered to within a given timeframe. It is found that a solar polar mission requires a solar sail of side-length 100 – 125 m to deliver a ‘sufficient value’ minimum science payload, and that a 2. 5μm sail film substrate is typically required, however the design is much less sensitive to the boom specific mass
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