444 research outputs found
Resistive Magnetohydrodynamic Simulations of Relativistic Magnetic Reconnection
Resistive relativistic magnetohydrodynamic (RRMHD) simulations are applied to
investigate the system evolution of relativistic magnetic reconnection. A
time-split Harten--Lan--van Leer method is employed. Under a localized
resistivity, the system exhibits a fast reconnection jet with an Alfv\'{e}nic
Lorentz factor inside a narrow Petschek-type exhaust. Various shock structures
are resolved in and around the plasmoid such as the post-plasmoid vertical
shocks and the "diamond-chain" structure due to multiple shock reflections.
Under a uniform resistivity, Sweet--Parker-type reconnection slowly evolves.
Under a current-dependent resistivity, plasmoids are repeatedly formed in an
elongated current sheet. It is concluded that the resistivity model is of
critical importance for RRMHD modeling of relativistic magnetic reconnection.Comment: published in ApJ
Scaling Law of Relativistic Sweet-Parker Type Magnetic Reconnection
Relativistic Sweet-Parker type magnetic reconnection is investigated by
relativistic resistive magnetohydrodynamic (RRMHD) simulations. As an initial
setting, we assume anti-parallel magnetic fields and a spatially uniform
resistivity. A perturbation imposed on the magnetic fields triggers magnetic
reconnection around a current sheet, and the plasma inflows into the
reconnection region. The inflows are then heated due to ohmic dissipation in
the diffusion region, and finally become relativistically hot outflows. The
outflows are not accelerated to ultra-relativistic speeds (i.e., Lorentz factor
~ 1), even when the magnetic energy dominates the thermal and rest mass
energies in the inflow region. Most of the magnetic energy in the inflow region
is converted into the thermal energy of the outflow during the reconnection
process. The energy conversion from magnetic to thermal energy in the diffusion
region results in an increase in the plasma inertia. This prevents the outflows
from being accelerated to ultra-relativistic speeds. We find that the
reconnection rate R obeys the scaling relation R S^{-0.5}, where S is the
Lundquist number. This feature is the same as that of non-relativistic
reconnection. Our results are consistent with the theoretical predictions of
Lyubarsky (2005) for Sweet-Parker type magnetic reconnection.Comment: accepted for publication in ApJL, 6 pages, 4 figure
Beaming and rapid variability of high-energy radiation from relativistic pair plasma reconnection
We report on the first study of the angular distribution of energetic
particles and radiation generated in relativistic collisionless
electron-positron pair plasma reconnection, using two-dimensional
particle-in-cell simulations. We discover a strong anisotropy of the particles
accelerated by reconnection and the associated strong beaming of their
radiation. The focusing of particles and radiation increases with their energy;
in this sense, this "kinetic beaming" effect differs fundamentally from the
relativistic Doppler beaming usually invoked in high-energy astrophysics, in
which all photons are focused and boosted achromatically. We also present, for
the first time, the modeling of the synchrotron emission as seen by an external
observer during the reconnection process. The expected lightcurves comprise
several bright symmetric sub-flares emitted by the energetic beam of particles
sweeping across the line of sight intermittently, and exhibit super-fast time
variability as short as about one tenth of the system light-crossing time. The
concentration of the energetic particles into compact regions inside magnetic
islands and particle anisotropy explain the rapid variability. This radiative
signature of reconnection can account for the brightness and variability of the
gamma-ray flares in the Crab Nebula and in blazars.Comment: 14 pages, 5 figures, Accepted for publication in The Astrophysical
Journal Letter
Reconnection-Powered Linear Accelerator and Gamma-Ray Flares in the Crab Nebula
The recent discovery of day-long gamma-ray flares in the Crab Nebula,
presumed to be synchrotron emission by PeV (10^{15} eV) electrons in milligauss
magnetic fields, presents a strong challenge to particle acceleration models.
The observed photon energies exceed the upper limit (~100 MeV) obtained by
balancing the acceleration rate and synchrotron radiation losses under standard
conditions where the electric field is smaller than the magnetic field. We
argue that a linear electric accelerator, operating at magnetic reconnection
sites, is able to circumvent this difficulty. Sufficiently energetic electrons
have gyroradii so large that their motion is insensitive to small-scale
turbulent structures in the reconnection layer and is controlled only by
large-scale fields. We show that such particles are guided into the
reconnection layer by the reversing magnetic field as they are accelerated by
the reconnection electric field. As these electrons become confined within the
current sheet, they experience a decreasing perpendicular magnetic field that
may drop below the accelerating electric field. This enables them to reach
higher energies before suffering radiation losses and hence to emit synchrotron
radiation in excess of the 100 MeV limit, providing a natural resolution to the
Crab gamma-ray flare paradox.Comment: 14 pages including 4 figure
Relativistic magnetic reconnection at X-type neutral points
Relativistic effects in the oscillatory damping of magnetic disturbances near
two-dimensional X-points are investigated. By taking into account displacement
current, we study new features of extremely magnetized systems, in which the
Alfv\'en velocity is almost the speed of light. The frequencies of the
least-damped mode are calculated using linearized relativistic MHD equations
for wide ranges of the Lundquist number S and the magnetization parameter
. These timescales approach constant values in the large resistive
limit: the oscillation time becomes a few times the light crossing time,
irrespective of , and the decay time is proportional to and
therefore is longer for a highly magnetized system.Comment: 6 pages, 4 figure
The role of the Weibel instability at the reconnection jet front in relativistic pair plasma reconnection
The role of the Weibel instability is investigated for the first time in the
context of the large-scale magnetic reconnection problem. A late-time evolution
of magnetic reconnection in relativistic pair plasmas is demonstrated by
particle-in-cell (PIC) simulations. In the outflow regions, powerful
reconnection jet piles up the magnetic fields and then a tangential
discontinuity appears there. Further downstream, it is found that the
two-dimensional extension of the relativistic Weibel instability generates
electro-magnetic fields, which are comparable to the anti-parallel or piled-up
fields. In a microscopic viewpoint, the instability allows plasma's multiple
interactions with the discontinuity. In a macroscopic viewpoint, the
instability leads to rapid expansion of the current sheet and then the
reconnection jet front further propagates into the downstream. Possible
application to the three-dimensional case is briefly discussed.Comment: 25 pages, 9 figures; References and typos are fixe
- …