46,595 research outputs found
Single pulse modeling and the bi-drifting subpulses of radio pulsar B1839-04
We study the bi-drifting pulsar B1839-04, where the observed subpulse drift
direction in the two leading pulse components is opposite from that in the two
trailing components. Such diametrically opposed apparent motions challenge our
understanding of an underlying structure. We find that for the geometry spanned
by the observer and the pulsar magnetic and rotation axes, the observed
bi-drifting in B1839-04 can be reproduced assuming a non-dipolar configuration
of the surface magnetic field. Acceptable solutions are found to either have
relatively weak or strong
surface magnetic fields. Our single pulse modeling shows that a global electric
potential variation at the polar cap that leads to a solid-body-like rotation
of spark forming regions is favorable in reproducing the observed drift
characteristics. This variation of the potential additionally ensures that the
variability is identical in all pulse components resulting in the observed
phase locking of subpulses. Thorough and more general studies of pulsar
geometry show that a low ratio of impact factor to opening angle increases the likelihood of bi-drifting to be observed. We thus conclude
that bi-drifting is visible when our line of sight crosses close to the
magnetic pole.Comment: 15 pages, 14 figures, accepted for publication in Ap
Generalised models for torsional spine and fan magnetic reconnection
Three-dimensional null points are present in abundance in the solar corona,
and the same is likely to be true in other astrophysical environments. Recent
studies suggest that reconnection at such 3D nulls may play an important role
in the coronal dynamics. In this paper the properties of the torsional spine
and torsional fan modes of magnetic reconnection at 3D nulls are investigated.
New analytical models are developed, which for the first time include a current
layer that is spatially localised around the null, extending along either the
spine or the fan of the null. These are complemented with numerical
simulations. The principal aim is to investigate the effect of varying the
degree of asymmetry of the null point magnetic field on the resulting
reconnection process - where previous studies always considered a non-generic
radially symmetric null. The geometry of the current layers within which
torsional spine and torsional fan reconnection occur is found to be strongly
dependent on the symmetry of the magnetic field. Torsional spine reconnection
still occurs in a narrow tube around the spine, but with elliptical
cross-section when the fan eigenvalues are different, and with the short axis
of the ellipse being along the strong field direction. The spatiotemporal peak
current, and the peak reconnection rate attained, are found not to depend
strongly on the degree of asymmetry. For torsional fan reconnection, the
reconnection occurs in a planar disk in the fan surface, which is again
elliptical when the symmetry of the magnetic field is broken. The short axis of
the ellipse is along the weak field direction, with the current being peaked in
these weak field regions. The peak current and peak reconnection rate in this
case are clearly dependent on the asymmetry, with the peak current increasing
but the reconnection rate decreasing as the degree of asymmetry is increased
Magnetic field amplification and electron acceleration to near-energy equipartition with ions by a mildly relativistic quasi-parallel plasma protoshock
The prompt emissions of gamma-ray bursts are seeded by radiating
ultrarelativistic electrons. Internal shocks propagating through a jet launched
by a stellar implosion, are expected to amplify the magnetic field & accelerate
electrons. We explore the effects of density asymmetry & a quasi-parallel
magnetic field on the collision of plasma clouds. A 2D relativistic PIC
simulation models the collision of two plasma clouds, in the presence of a
quasi-parallel magnetic field. The cloud density ratio is 10. The densities of
ions & electrons & the temperature of 131 keV are equal in each cloud. The mass
ratio is 250. The peak Lorentz factor of the electrons is determined, along
with the orientation & strength of the magnetic field at the cloud collision
boundary. The magnetic field component orthogonal to the initial plasma flow
direction is amplified to values that exceed those expected from shock
compression by over an order of magnitude. The forming shock is
quasi-perpendicular due to this amplification, caused by a current sheet which
develops in response to the differing deflection of the incoming upstream
electrons & ions. The electron deflection implies a charge separation of the
upstream electrons & ions; the resulting electric field drags the electrons
through the magnetic field, whereupon they acquire a relativistic mass
comparable to the ions. We demonstrate how a magnetic field structure
resembling the cross section of a flux tube grows in the current sheet of the
shock transition layer. Plasma filamentation develops, as well as signatures of
orthogonal magnetic field striping. Localized magnetic bubbles form. Energy
equipartition between the ion, electron & magnetic energy is obtained at the
shock transition layer. The electronic radiation can provide a seed photon
population that can be energized by secondary processes (e.g. inverse Compton).Comment: 12 pages, 15 Figures, accepted to A&
Extreme plasma states in laser-governed vacuum breakdown
Triggering vacuum breakdown at the upcoming laser facilities can provide
rapid electron-positron pair production for studies in laboratory astrophysics
and fundamental physics. However, the density of the emerging plasma should
seemingly stop rising at the relativistic critical density, when the plasma
becomes opaque. Here we identify the opportunity of breaking this limit using
optimal beam configuration of petawatt-class lasers. Tightly focused laser
fields allow plasma generation in a small focal volume much less than
, and creating extreme plasma states in terms of density and
produced currents. These states can be regarded as a new object of nonlinear
plasma physics. Using 3D QED-PIC simulations we demonstrate the possibility of
reaching densities of more than cm, which is an order of
magnitude higher than previously expected. Controlling the process via the
initial target parameters gives the opportunity to reach the discovered plasma
states at the upcoming laser facilities
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