697 research outputs found
Jet dynamics. Recollimation shocks and helical patterns
The dynamics and stability of extragalactic jets may be strongly influenced
by small (and probable) differences in pressure between the jet and the ambient
and within the jet itself. The former give rise to expansion and recollimation
of the jet. This occurs in the form of conical shocks, or Mach disks, if the
pressure difference is large enough. Pressure asymmetries within the jet may
trigger the development of helical patterns via coupling to kink current-driven
instability, or to helical Kelvin-Helmholtz instability, depending on the
physical conditions in the jet. I summarize here the evidence collected during
the last years on the presence of recollimation shocks and waves in jets. In
the jet of CTA 102 evidence has been found for (traveling)shock-(standing)shock
interaction in the core-region (0.1 mas from the core), using information from
the light-curve of the source combined with VLBI data. The conclusions derived
have been confirmed by numerical simulations combined with emission
calculations that have allowed to study the spectral evolution of the perturbed
jet. Helical structures can also be identified in radio-jets. The ridge-line of
emission of the jet of S5~0836+710 has been identified as a physical structure
corresponding to a wave developing in the jet flow. I review here the evidence
that has allowed to reach this conclusion, along with an associated caveat.
Current data do not allow to distinguish between magnetic or hydrodynamical
instabilities. I finally discuss the importance of these linear and non-linear
waves for jet evolution.Comment: 11 pages. Proceedings of the conference: The innermost regions of
relativistic jets and their magnetic fields. Corrected typos and added
reference
Numerical study of Kelvin-Helmholtz instability and its impact on synthetic emission from magnetized jets
Non-thermal emission from Active Galactic Nuclei (AGN) jets extends up-to
large scales in-spite of them being prone to a slew of magneto-hydrodynamic
instabilities. The main focus of this study is to understand the impact of MHD
instabilities on the non-thermal emission from large-scale AGN jets. We perform
high-resolution three-dimensional numerical magneto-hydrodynamic simulations of
a plasma column to investigate the dynamical and emission properties of jet
configurations at kilo-parsec scales with different magnetic field profiles,
jet speeds, and density contrast. We also obtain synthetic non-thermal emission
signatures for different viewing angles using an approach that assumes static
particle spectra and that obtained by evolving the particle spectra using
Lagrangian macro-particles incorporating the effects of shock acceleration and
radiative losses. We find that the shocks due to Kelvin-Helmholtz (KH)
instability in the axial magnetic field configurations can strongly affect the
jet dynamics. Additionally, we also find the presence of weak biconical shocks
in the under-dense jet columns. The inclusion of a helical magnetic field
hinders the vortex growth at the shear surface thereby stabilizing the jet
column. With the evolving particle spectra approach, the synthetic SEDs
obtained for cases with strong KH instability show the presence of multiple
humps ranging from radio to TeV gamma-ray band. We conclude that the
high-energy electrons accelerated in the vicinity of freshly formed shocks due
to KH instability, result in high X-ray emission.Comment: 18 pages, 12 figures, Accepted for publication in A&
Slow down of a globally neutral relativistic beam shearing the vacuum
The microphysics of relativistic collisionless sheared flows is investigated
in a configuration consisting of a globally neutral, relativistic beam
streaming through a hollow plasma/dielectric channel. We show through
multidimensional PIC simulations that this scenario excites the Mushroom
instability (MI), a transverse shear instability on the electron-scale, when
there is no overlap (no contact) between the beam and the walls of the
hollow plasma channel. The onset of the MI leads to the conversion of the
beam's kinetic energy into magnetic (and electric) field energy, effectively
slowing down a globally neutral body in the absence of contact. The
collisionless shear physics explored in this configuration may operate in
astrophysical environments, particularly in highly relativistic and supersonic
settings where macroscopic shear processes are stable
Quantitatively consistent computation of coherent and incoherent radiation in particle-in-cell codes - a general form factor formalism for macro-particles
Quantitative predictions from synthetic radiation diagnostics often have to
consider all accelerated particles. For particle-in-cell (PIC) codes, this not
only means including all macro-particles but also taking into account the
discrete electron distribution associated with them. This paper presents a
general form factor formalism that allows to determine the radiation from this
discrete electron distribution in order to compute the coherent and incoherent
radiation self-consistently. Furthermore, we discuss a memory-efficient
implementation that allows PIC simulations with billions of macro-particles.
The impact on the radiation spectra is demonstrated on a large scale LWFA
simulation.Comment: Proceedings of the EAAC 2017, This manuscript version is made
available under the CC-BY-NC-ND 4.0 licens
Jupiter's X-ray and EUV auroras monitored by Chandra, XXM-Newton, and Hisaki satellite
Jupiter's X-ray auroral emission in the polar cap region results from particles which have undergone strong field-aligned acceleration into the ionosphere. The origin of precipitating ions and electrons and the time variability in the X-ray emission are essential to uncover the driving mechanism for the high-energy acceleration. The magnetospheric location of the source field line where the X-ray is generated is likely affected by the solar wind variability. However, these essential characteristics are still unknown because the long-term monitoring of the X-rays and contemporaneous solar wind variability has not been carried out. In April 2014, the first long-term multiwavelength monitoring of Jupiter's X-ray and EUV auroral emissions was made by the Chandra X-ray Observatory, XMM-Newton, and Hisaki satellite. We find that the X-ray count rates are positively correlated with the solar wind velocity and insignificantly with the dynamic pressure. Based on the magnetic field mapping model, a half of the X-ray auroral region was found to be open to the interplanetary space. The other half of the X-ray auroral source region is magnetically connected with the prenoon to postdusk sector in the outermost region of the magnetosphere, where the Kelvin-Helmholtz (KH) instability, magnetopause reconnection, and quasiperiodic particle injection potentially take place. We speculate that the high-energy auroral acceleration is associated with the KH instability and/or magnetopause reconnection. This association is expected to also occur in many other space plasma environments such as Saturn and other magnetized rotators
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