5,884 research outputs found
Non-Maxwellian electron distribution functions due to self-generated turbulence in collisionless guide-field reconnection
Non-Maxwellian electron velocity space distribution functions (EVDF) are
useful signatures of plasma conditions and non-local consequences of
collisionless magnetic reconnection. In the past, EVDFs were obtained mainly
for antiparallel reconnection and under the influence of weak guide-fields in
the direction perpendicular to the reconnection plane. EVDFs are, however, not
well known, yet, for oblique (or component-) reconnection in dependence on
stronger guide-magnetic fields and for the exhaust (outflow) region of
reconnection away from the diffusion region. In view of the multi-spacecraft
Magnetospheric Multiscale Mission (MMS), we derived the non-Maxwellian EVDFs of
collisionless magnetic reconnection in dependence on the guide-field strength
from small () to very strong () guide-fields, taking
into account the feedback of the self-generated turbulence. For this sake, we
carried out 2.5D fully-kinetic Particle-in-Cell simulations using the ACRONYM
code. We obtained anisotropic EVDFs and electron beams propagating along the
separatrices as well as in the exhaust region of reconnection. The beams are
anisotropic with a higher temperature in the direction perpendicular rather
than parallel to the local magnetic field. The beams propagate in the direction
opposite to the background electrons and cause instabilities. We also obtained
the guide-field dependence of the relative electron-beam drift speed, threshold
and properties of the resulting streaming instabilities including the strongly
non-linear saturation of the self-generated plasma turbulence. This turbulence
and its non-linear feedback cause non-adiabatic parallel electron acceleration
and EVDFs well beyond the limits of the quasi-linear approximation, producing
phase space holes and an isotropizing pitch-angle scattering.Comment: 21 pages, 8 figures. Revised to match with the version published in
Physics of Plasmas. An abridged version of the abstract is shown her
A theory of local and global processes which affect solar wind electrons. 2: Experimental support
The microscopic characteristics of the Coulomb cross section show that there are three natural subpopulations for plasma electrons: the subthermals with local kinetic energy E kT sub c; the transthermals with kT sub c E 7 kT sub c and the extrathermals E 7 kT sub c. Data from three experimental groups on three different spacecraft in the interplanetary medium over a radial range are presented to support the five interrelations projected between solar wind electron properties and changes in the interplanetary medium: (1) subthermals respond primarily to local changes (compression and rarefactions) in stream dynamics; (2) the extrathermal fraction of the ambient electron density should be anti-correlated with the asymptotic bulk speed; (3) the extrathermal "temperature" should be anti-correlated with the local wind speed at 1 AU; (4) the heat flux carried by electrons should be anti-correlated with the local bulk speed; and (5) the extrathermal differential 'temperature' should be nearly independent of radius within 1 AU
Formation of Relativistic Outflows in Shearing Black Hole Accretion Coronae
We examine the possibility that the relativistic jets observed in many active
galactic nuclei may be powered by the Fermi acceleration of protons in a
tenuous corona above a two-temperature accretion disk. In this picture the
acceleration arises as a consequence of the shearing motion of the magnetic
field in the corona, which is anchored in the underlying Keplerian disk. The
protons in the corona have a power-law distribution because the density there
is too low for proton-proton collisions to thermalize the energy supplied via
Fermi acceleration. The same shear acceleration mechanism also operates in the
disk itself, however, there the density is high enough for thermalization to
occur and consequently the disk protons have a Maxwellian distribution.
Particle acceleration in the corona leads to the development of a
pressure-driven wind that passes through a critical point and subsequently
transforms into a relativistic jet at large distances from the black hole. We
combine the critical conditions for the wind with the structure equations for
the disk and the corona to obtain a coupled disk/corona/wind model. Using the
coupled model we compute the asymptotic Lorentz factor of the
jet as a function of the cylindrical starting radius at the base of the
outflow, in the corona. Our results suggest that \Gamma_\infty \lapprox 10,
which is consistent with observations of superluminal motion in blazars. We
show that collisions between the jet and broad-line emission clouds can produce
high-energy radiation with a luminosity sufficient to power the -rays
observed from blazars.
Subject headings: radiation mechanisms: non-thermal, accretion, accretion
disks, acceleration of particles, gamma rays: theoryComment: 50 pages, 13 figures, accepted by ApJ, 199
Electron acceleration by cascading reconnection in the solar corona I Magnetic gradient and curvature effects
Aims: We investigate the electron acceleration in convective electric fields
of cascading magnetic reconnection in a flaring solar corona and show the
resulting hard X-ray (HXR) radiation spectra caused by Bremsstrahlung for the
coronal source. Methods: We perform test particle calculation of electron
motions in the framework of a guiding center approximation. The electromagnetic
fields and their derivatives along electron trajectories are obtained by
linearly interpolating the results of high-resolution adaptive mesh refinement
(AMR) MHD simulations of cascading magnetic reconnection. Hard X-ray (HXR)
spectra are calculated using an optically thin Bremsstrahlung model. Results:
Magnetic gradients and curvatures in cascading reconnection current sheet
accelerate electrons: trapped in magnetic islands, precipitating to the
chromosphere and ejected into the interplanetary space. The final location of
an electron is determined by its initial position, pitch angle and velocity.
These initial conditions also influence electron acceleration efficiency. Most
of electrons have enhanced perpendicular energy. Trapped electrons are
considered to cause the observed bright spots along coronal mass ejection
CME-trailing current sheets as well as the flare loop-top HXR emissions.Comment: submitted to A&
On the unconstrained expansion of a spherical plasma cloud turning collisionless : case of a cloud generated by a nanometer dust grain impact on an uncharged target in space
Nano and micro meter sized dust particles travelling through the heliosphere
at several hundreds of km/s have been repeatedly detected by interplanetary
spacecraft. When such fast moving dust particles hit a solid target in space,
an expanding plasma cloud is formed through the vaporisation and ionisation of
the dust particles itself and part of the target material at and near the
impact point. Immediately after the impact the small and dense cloud is
dominated by collisions and the expansion can be described by fluid equations.
However, once the cloud has reached micro-m dimensions, the plasma may turn
collisionless and a kinetic description is required to describe the subsequent
expansion. In this paper we explore the late and possibly collisionless
spherically symmetric unconstrained expansion of a single ionized ion-electron
plasma using N-body simulations. Given the strong uncertainties concerning the
early hydrodynamic expansion, we assume that at the time of the transition to
the collisionless regime the cloud density and temperature are spatially
uniform. We do also neglect the role of the ambient plasma. This is a
reasonable assumption as long as the cloud density is substantially higher than
the ambient plasma density. In the case of clouds generated by fast
interplanetary dust grains hitting a solid target some 10^7 electrons and ions
are liberated and the in vacuum approximation is acceptable up to meter order
cloud dimensions. ..
Baryon Loaded Relativistic Blastwaves in Supernovae
We provide a new analytic blastwave solution which generalizes the
Blandford-McKee solution to arbitrary ejecta masses and Lorentz factors. Until
recently relativistic supernovae have been discovered only through their
association with long duration Gamma Ray Bursts (GRB). The blastwaves of such
explosions are well described by the Blandford-McKee (in the ultra relativistic
regime) and Sedov-Taylor (in the non-relativistic regime) solutions during
their afterglows, as the ejecta mass is negligible in comparison to the swept
up mass. The recent discovery of the relativistic supernova SN 2009bb, without
a detected GRB, opens up the possibility of highly baryon loaded mildly
relativistic outflows which remains in nearly free expansion phase during the
radio afterglow. In this work, we consider a massive, relativistic shell,
launched by a Central Engine Driven EXplosion (CEDEX), decelerating
adiabatically due to its collision with the pre-explosion circumstellar wind
profile of the progenitor. We compute the synchrotron emission from
relativistic electrons in the shock amplified magnetic field. This models the
radio emission from the circumstellar interaction of a CEDEX. We show that this
model explains the observed radio evolution of the prototypical SN 2009bb and
demonstrate that SN 2009bb had a highly baryon loaded, mildly relativistic
outflow. We discuss the effect of baryon loading on the dynamics and
observational manifestations of a CEDEX. In particular, our predicted angular
size of SN 2009bb is consistent with VLBI upper limits on day 85, but is
presently resolvable on VLBI angular scales, since the relativistic ejecta is
still in the nearly free expansion phase.Comment: 13 pages, 6 figures, Accepted for publication in Ap
Shock Dissipation in Magnetically Dominated Impulsive Flows
We have revisited the issue of shock dissipation and emission and its
implications for the internal shock model of the prompt GRB emission and
studied it in the context of impulsive Poynting-dominated flows. Our results
show that unless the magnetization of GRB jets is extremely high, \sigma > 100
in the prompt emission zone, the magnetic model may still be compatible with
the observations. The main effect of reduced dissipation efficiency is merely
an increase in the size of the dissipation zone and even for highly magnetised
GRB jets this size may remain below the external shock radius, provided the
central engine can emit magnetic shells on the time scale well below the
typical observed variability scale of one second. Our analytical and numerical
results suggest that magnetic shells begin strongly interact with each other
well before they reach the coasting radius. As the result, the impulsive jet in
the dissipation zone is best described not as a collection of shells but as a
continuous highly magnetised flow with a high amplitude magnetosonic wave
component. How exactly the dissipated wave energy is distributed between the
radiation and the bulk kinetic energy of radial jets depends on the relative
rates of radiative and adiabatic cooling. In the fast radiative cooling regime,
the corresponding radiative efficiency can be as high as the wave contribution
to their energy budget, independently of the magnetization. Moreover, after
leaving the zone of prompt emission the jet may still remain
Poynting-dominated, leading to weaker emission from the reverse shock compared
to non-magnetic models.Comment: Submitted to MNRA
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