260 research outputs found
MHD modeling of the Heliosphere: a critical evaluation of different models
International audienceThe very complicated problem of the solar wind-interstellar medium interaction requires a very sophisticated numerical approach. To achieve such a goal first we need to understand very deeply the results obtained up-to-now. In this paper we gather the results from four different MHD models of the heliospheric interfaces. The comparison of the results invokes several questions. These questions should be addressed before we proceed with the next steps in the MHD modeling of the heliospheric interfaces. Is a jet created between the termination shock and the heliopause (Opher et al., 2003, 2004)? What is the physical meaning of the V-shape of termination shock and heliopause (Washimi and Tanaka, 2001, 2004)? Whether a similar looking result is either due to bending of the heliospheric current sheet as in the model by Pogorelov (2004), Pogorelov et al. (2004) or due to numerical "reconnection" as in the model by Ratkiewicz et al. (2004, 2005)? The purpose of this paper is to open a wider discussion to try to answer these questions
Effects of a Local Interstellar Magnetic Field on Voyager 1 and 2 Observations
We show that that an interstellar magnetic field can produce a north/south
asymmetry in solar wind termination shock. Using Voyager 1 and 2 measurements,
we suggest that the angle between the interstellar wind velocity and
magnetic field is . The distortion of the
shock is such that termination shock particles could stream outward along the
spiral interplanetary magnetic field connecting Voyager 1 to the shock when the
spacecraft was within of the shock. The shock distortion is larger
in the southern hemisphere, and Voyager 2 could be connected to the shock when
it is within of the shock, but with particles from the shock
streaming inward along the field. Tighter constraints on the interstellar
magnetic field should be possible when Voyager 2 crosses the shock in the next
several years.Comment: 12 pages, 5 figure
Long-time behaviour of discretizations of the simple pendulum equation
We compare the performance of several discretizations of the simple pendulum
equation in a series of numerical experiments. The stress is put on the
long-time behaviour. We choose for the comparison numerical schemes which
preserve the qualitative features of solutions (like periodicity). All these
schemes are either symplectic maps or integrable (preserving the energy
integral) maps, or both. We describe and explain systematic errors (produced by
any method) in numerical computations of the period and the amplitude of
oscillations. We propose a new numerical scheme which is a modification of the
discrete gradient method. This discretization preserves (almost exactly) the
period of small oscillations for any time step.Comment: 41 pages, including 18 figures and 4 table
Magnetic Effects at the Edge of the Solar System: MHD Instabilities, the de Laval nozzle Effect and an Extended Jet
To model the interaction between the solar wind and the interstellar wind,
magnetic fields must be included. Recently Opher et al. 2003 found that, by
including the solar magnetic field in a 3D high resolution simulation using the
University of Michigan BATS-R-US code, a jet-sheet structure forms beyond the
solar wind Termination Shock. Here we present an even higher resolution
three-dimensional case where the jet extends for beyond the Termination
Shock. We discuss the formation of the jet due to a de Laval nozzle effect and
it's su bsequent large period oscillation due to magnetohydrodynamic
instabilities. To verify the source of the instability, we also perform a
simplified two dimensional-geometry magnetohydrodynamic calculation of a plane
fluid jet embedded in a neutral sheet with the profiles taken from our 3D
simulation. We find remarkable agreement with the full three-dimensional
evolution. We compare both simulations and the temporal evolution of the jet
showing that the sinuous mode is the dominant mode that develops into a
velocity-shear-instability with a growth rate of . As a result, the outer edge of the heliosphere
presents remarkable dynamics, such as turbulent flows caused by the motion of
the jet. 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 this outer boundary behaves. Already, however, we can say that the magnetic
field effects are a major player in this region changing our previous notion of
how the solar system ends.Comment: 24 pages, 13 figures, accepted for publication in Astrophysical
Journal (2004
Comment on `conservative discretizations of the Kepler motion'
We show that the exact integrator for the classical Kepler motion, recently
found by Kozlov ({\it J. Phys. A: Math. Theor.\} {\bf 40} (2007) 4529-4539),
can be derived in a simple natural way (using well known exact discretization
of the harmonic oscillator). We also turn attention on important earlier
references, where the exact discretization of the 4-dimensional isotropic
harmonic oscillator has been applied to the perturbed Kepler problem.Comment: 6 page
Inverse-kinematics one-neutron pickup with fast rare-isotope beams
New measurements and reaction model calculations are reported for single
neutron pickup reactions onto a fast \nuc{22}{Mg} secondary beam at 84 MeV per
nucleon. Measurements were made on both carbon and beryllium targets, having
very different structures, allowing a first investigation of the likely nature
of the pickup reaction mechanism. The measurements involve thick reaction
targets and -ray spectroscopy of the projectile-like reaction residue
for final-state resolution, that permit experiments with low incident beam
rates compared to traditional low-energy transfer reactions. From measured
longitudinal momentum distributions we show that the \nuc{12}{C}
(\nuc{22}{Mg},\nuc{23}{Mg}+\gamma)X reaction largely proceeds as a direct
two-body reaction, the neutron transfer producing bound \nuc{11}{C} target
residues. The corresponding reaction on the \nuc{9}{Be} target seems to largely
leave the \nuc{8}{Be} residual nucleus unbound at excitation energies high in
the continuum. We discuss the possible use of such fast-beam one-neutron pickup
reactions to track single-particle strength in exotic nuclei, and also their
expected sensitivity to neutron high- (intruder) states which are often
direct indicators of shell evolution and the disappearance of magic numbers in
the exotic regime.Comment: 8 pages, 5 figure
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