27,658 research outputs found
Spatially hybrid computations for streamer discharges with generic features of pulled fronts: I. Planar fronts
Streamers are the first stage of sparks and lightning; they grow due to a
strongly enhanced electric field at their tips; this field is created by a thin
curved space charge layer. These multiple scales are already challenging when
the electrons are approximated by densities. However, electron density
fluctuations in the leading edge of the front and non-thermal stretched tails
of the electron energy distribution (as a cause of X-ray emissions) require a
particle model to follow the electron motion. As super-particle methods create
wrong statistics and numerical artifacts, modeling the individual electron
dynamics in streamers is limited to early stages where the total electron
number still is limited.
The method of choice is a hybrid computation in space where individual
electrons are followed in the region of high electric field and low density
while the bulk of the electrons is approximated by densities (or fluids). We
here develop the hybrid coupling for planar fronts. First, to obtain a
consistent flux at the interface between particle and fluid model in the hybrid
computation, the widely used classical fluid model is replaced by an extended
fluid model. Then the coupling algorithm and the numerical implementation of
the spatially hybrid model are presented in detail, in particular, the position
of the model interface and the construction of the buffer region. The method
carries generic features of pulled fronts that can be applied to similar
problems like large deviations in the leading edge of population fronts etc.Comment: 33 pages, 15 figures and 2 table
On the formation of current sheets in response to the compression or expansion of a potential magnetic field
The compression or expansion of a magnetic field that is initially potential
is considered. It was recently suggested by Janse & Low [2009, ApJ, 690, 1089]
that, following the volumetric deformation, the relevant lowest energy state
for the magnetic field is another potential magnetic field that in general
contains tangential discontinuities (current sheets). Here we examine this
scenario directly using a numerical relaxation method that exactly preserves
the topology of the magnetic field. It is found that of the magnetic fields
discussed by Janse & Low, only those containing magnetic null points develop
current singularities during an ideal relaxation, while the magnetic fields
without null points relax toward smooth force-free equilibria with finite
non-zero current.Comment: Accepted for publication in Ap
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