229 research outputs found
Application of a MHD hybrid solar wind model with latitudinal dependences to Ulysses data at minimum
In a previous work, Ulysses data was analyzed to build a complete
axisymmetric MHD solution for the solar wind at minimum including rotation and
the initial flaring of the solar wind in the low corona. This model has some
problems in reproducing the values of magnetic field at 1 AU despite the
correct values of the velocity. Here, we intend to extend the previous analysis
to another type of solutions and to improve our modelling of the wind from the
solar surface to 1 AU. We compare the previous results to those obtained with a
fully helicoidal model and construct a hybrid model combining both previous
solutions, keeping the flexibility of the parent models in the appropriate
domain. From the solar surface to the Alfven, point, a three component solution
for velocity and magnetic field is used, reproducing the complex wind geometry
and the well-known flaring of the field lines observed in coronal holes. From
the Alfven radius to 1 AU and further, the hybrid model keeps the latitudinal
dependences as flexible as possible, in order to deal with the sharp variations
near the equator and we use the helicoidal solution, turning the poloidal
streamlines into radial ones. Despite the absence of the initial flaring, the
helicoidal model and the first hybrid solution suffer from the same low values
of the magnetic field at 1 AU. However, by adjusting the parameters with a
second hybrid solution, we are able to reproduce both the velocity and magnetic
profiles observed by Ulysses and a reasonable description of the low corona,
provided that a certain amount of energy deposit exists along the flow. The
present paper shows that analytical axisymmetric solutions can be constructed
to reproduce the solar structure and dynamics from 1 solar radius up to 1 AU.Comment: 12 pages, 16 figure
Shocks in relativistic transverse stratified jets, a new paradigm for radio-loud AGN
The transverse stratification of active galactic nuclei (AGN) jets is
suggested by observations and theoretical arguments, as a consequence of
intrinsic properties of the central engine (accretion disc + black hole) and
external medium. On the other hand, the one-component jet approaches are
heavily challenged by the various observed properties of plasmoids in radio
jets (knots), often associated with internal shocks. Given that such a
transverse stratification plays an important role on the jets acceleration,
stability, and interaction with the external medium, it should also induce
internal shocks with various strengths and configurations, able to describe the
observed knots behaviours. By establishing a relation between the transverse
stratification of the jets, the internal shock properties, and the multiple
observed AGN jet morphologies and behaviours, our aim is to provide a
consistent global scheme of the various AGN jet structures. Working on a large
sample of AGN radio jets monitored in very long baseline interferometry (VLBI)
by the MOJAVE collaboration, we determined the consistency of a systematic
association of the multiple knots with successive re-collimation shocks. We
then investigated the re-collimation shock formation and the influence of
different transverse stratified structures by parametrically exploring the two
relativistic outflow components with the specific relativistic hydrodynamic
(SRHD) code AMRVAC. We were able to link the different spectral classes of AGN
with specific stratified jet characteristics, in good accordance with their
VLBI radio properties and their accretion regimes.Comment: 16 pages, 12 figures, accepted for publication in A&
Counterrotation in magnetocentrifugally driven jets and other winds
Rotation measurement in jets from T Tauri stars is a rather difficult task.
Some jets seem to be rotating in a direction opposite to that of the underlying
disk, although it is not yet clear if this affects the totality or part of the
outflows. On the other hand, Ulysses data also suggest that the solar wind may
rotate in two opposite ways between the northern and southern hemispheres. We
show that this result is not as surprising as it may seem and that it emerges
naturally from the ideal MHD equations. Specifically, counterrotating jets
neither contradict the magnetocentrifugal driving of the flow nor prevent
extraction of angular momentum from the disk. The demonstration of this result
is shown by combining the ideal MHD equations for steady axisymmetric flows.
Provided that the jet is decelerated below some given threshold beyond the
Alfven surface, the flow will change its direction of rotation locally or
globally. Counterrotation is also possible for only some layers of the outflow
at specific altitudes along the jet axis. We conclude that the counterrotation
of winds or jets with respect to the source, star or disk, is not in
contradiction with the magnetocentrifugal driving paradigm. This phenomenon may
affect part of the outflow, either in one hemisphere, or only in some of the
outflow layers. From a time-dependent simulation, we illustrate this effect and
show that it may not be permanent.Comment: To appear in ApJ
Nonradial and nonpolytropic astrophysical outflows VIII. A GRMHD generalization for relativistic jets
Steady axisymmetric outflows originating at the hot coronal magnetosphere of
a Schwarzschild black hole and surrounding accretion disk are studied in the
framework of general relativistic magnetohydrodynamics (GRMHD). The assumption
of meridional self-similarity is adopted for the construction of
semi-analytical solutions of the GRMHD equations describing outflows close to
the polar axis. In addition, it is assumed that relativistic effects related to
the rotation of the black hole and the plasma are negligible compared to the
gravitational and other energetic terms. The constructed model allows us to
extend previous MHD studies for coronal winds from young stars to spine jets
from Active Galactic Nuclei surrounded by disk-driven outflows. The outflows
are thermally driven and magnetically or thermally collimated. The collimation
depends critically on an energetic integral measuring the efficiency of the
magnetic rotator, similarly to the non relativistic case. It is also shown that
relativistic effects affect quantitatively the depth of the gravitational well
and the coronal temperature distribution in the launching region of the
outflow. Similarly to previous analytical and numerical studies, relativistic
effects tend to increase the efficiency of the thermal driving but reduce the
effect of magnetic self-collimation.Comment: 20 page, Accepted in A&A 10/10/200
Counter-rotation in relativistic magnetohydrodynamic jets
Young stellar object observations suggest that some jets rotate in the
opposite direction with respect to their disk. In a recent study, Sauty et al.
(2012) have shown that this does not contradict the magnetocentrifugal
mechanism that is believed to launch such outflows. Signatures of motions
transverse to the jet axis and in opposite directions have recently been
measured in M87 (Meyer et al. 2013). One possible interpretation of this motion
is the one of counter rotating knots. Here, we extend our previous analytical
derivation of counter-rotation to relativistic jets, demonstrating that
counter-rotation can indeed take place under rather general conditions. We show
that both the magnetic field and a non-negligible enthalpy are necessary at the
origin of counter-rotating outflows, and that the effect is associated with a
transfer of energy flux from the matter to the electromagnetic field. This can
be realized in three cases : if a decreasing enthalpy causes an increase of the
Poynting flux, if the flow decelerates, or, if strong gradients of the magnetic
field are present. An illustration of the involved mechanism is given by an
example of relativistic MHD jet simulation.Comment: Accepted for publication in ApJ
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