16,435 research outputs found
The solar wind structures associated with cosmic ray decreases and particle acceleration in 1978-1982
The time histories of particles in the energy range 1 MeV to 1 GeV at times of all greater than 3 percent cosmic ray decreases in the years 1978 to 1982 are studied. Essentially all 59 of the decreases commenced at or before the passages of interplanetary shocks, the majority of which accelerated energetic particles. We use the intensity-time profiles of the energetic particles to separate the cosmic ray decreases into four classes which we subsequently associate with four types of solar wind structures. Decreases in class 1 (15 events) and class 2 (26 events) can be associated with shocks which are driven by energetic coronal mass ejections. For class 1 events the ejecta is detected at 1 AU whereas this is not the case for class 2 events. The shock must therefore play a dominant role in producing the depression of cosmic rays in class 2 events. In all class 1 and 2 events (which comprise 69 percent of the total) the departure time of the ejection from the sun (and hence the location) can be determined from the rapid onset of energetic particles several days before the shock passage at Earth. The class 1 events originate from within 50 deg of central meridian. Class 3 events (10 decreases) can be attributed to less energetic ejections which are directed towards the Earth. In these events the ejecta is more important than the shock in causing a depression in the cosmic ray intensity. The remaining events (14 percent of the total) can be attributed to corotating streams which have ejecta material embedded in them
The Formation of CIRs at Stream-Stream Interfaces and Resultant Geomagnetic Activity
Corotating interaction regions (CIRs) are regions of compressed plasma formed at the leading edges of corotating high-speed solar wind streams originating in coronal holes as they interact with the preceding slow solar wind. Although particularly prominent features of the solar wind during the declining and minimum phases of the 11-year solar cycle, they may also be present at times of higher solar activity. We describe how CIRs are formed, and their geomagnetic effects, which principally result from brief southward interplanetary magnetic field excursions associated with Alfven waves. Seasonal and long-term variations in these effects are briefly discussed
Particle Acceleration and Magnetic Field Generation in Electron-Positron Relativistic Shocks
Shock acceleration is an ubiquitous phenomenon in astrophysical plasmas.
Plasma waves and their associated instabilities (e.g., Buneman, Weibel and
other two-stream instabilities) created in collisionless shocks are responsible
for particle (electron, positron, and ion) acceleration. Using a 3-D
relativistic electromagnetic particle (REMP) code, we have investigated
particle acceleration associated with a relativistic electron-positron jet
front propagating into an ambient electron-positron plasma with and without
initial magnetic fields. We find small differences in the results for no
ambient and modest ambient magnetic fields. New simulations show that the
Weibel instability created in the collisionless shock front accelerates jet and
ambient particles both perpendicular and parallel to the jet propagation
direction. Furthermore, the non-linear fluctuation amplitudes of densities,
currents, electric, and magnetic fields in the electron-positron shock are
larger than those found in the electron-ion shock studied in a previous paper
at the comparable simulation time. This comes from the fact that both electrons
and positrons contribute to generation of the Weibel instability. Additionally,
we have performed simulations with different electron skin depths. We find that
growth times scale inversely with the plasma frequency, and the sizes of
structures created by the Weibel instability scale proportional to the electron
skin depth. This is the expected result and indicates that the simulations have
sufficient grid resolution. The simulation results show that the Weibel
instability is responsible for generating and amplifying nonuniform,
small-scale magnetic fields which contribute to the electron's (positron's)
transverse deflection behind the jet head.Comment: 18 pages, 8 figures, revised and accepted for ApJ, A full resolution
of the paper can be found at
http://gammaray.nsstc.nasa.gov/~nishikawa/apjep1.pd
Particle Acceleration in Relativistic Jets due to Weibel Instability
Shock acceleration is an ubiquitous phenomenon in astrophysical plasmas.
Plasma waves and their associated instabilities (e.g., the Buneman instability,
two-streaming instability, and the Weibel instability) created in the shocks
are responsible for particle (electron, positron, and ion) acceleration. Using
a 3-D relativistic electromagnetic particle (REMP) code, we have investigated
particle acceleration associated with a relativistic jet front propagating
through an ambient plasma with and without initial magnetic fields. We find
only small differences in the results between no ambient and weak ambient
magnetic fields. Simulations show that the Weibel instability created in the
collisionless shock front accelerates particles perpendicular and parallel to
the jet propagation direction. While some Fermi acceleration may occur at the
jet front, the majority of electron acceleration takes place behind the jet
front and cannot be characterized as Fermi acceleration. The simulation results
show that this instability is responsible for generating and amplifying highly
nonuniform, small-scale magnetic fields, which contribute to the electron's
transverse deflection behind the jet head. The ``jitter'' radiation (Medvedev
2000) from deflected electrons has different properties than synchrotron
radiation which is calculated in a uniform magnetic field. This jitter
radiation may be important to understanding the complex time evolution and/or
spectral structure in gamma-ray bursts, relativistic jets, and supernova
remnants.Comment: ApJ, in press, Sept. 20, 2003 (figures with better resolution:
http://gammaray.nsstc.nasa.gov/~nishikawa/apjweib.pdf
Particle Acceleration and Radiation associated with Magnetic Field Generation from Relativistic Collisionless Shocks
Shock acceleration is an ubiquitous phenomenon in astrophysical plasmas.
Plasma waves and their associated instabilities (e.g., the Buneman instability,
two-streaming instability, and the Weibel instability) created in the shocks
are responsible for particle (electron, positron, and ion) acceleration. Using
a 3-D relativistic electromagnetic particle (REMP) code, we have investigated
particle acceleration associated with a relativistic jet front propagating
through an ambient plasma with and without initial magnetic fields. We find
only small differences in the results between no ambient and weak ambient
magnetic fields. Simulations show that the Weibel instability created in the
collisionless shock front accelerates particles perpendicular and parallel to
the jet propagation direction. The simulation results show that this
instability is responsible for generating and amplifying highly nonuniform,
small-scale magnetic fields, which contribute to the electron's transverse
deflection behind the jet head. The ``jitter'' radiation from deflected
electrons has different properties than synchrotron radiation which is
calculated in a uniform magnetic field. This jitter radiation may be important
to understanding the complex time evolution and/or spectral structure in
gamma-ray bursts, relativistic jets, and supernova remnants.Comment: 4 pages, 1 figure, submitted to Proceedings of 2003 Gamma Ray Burst
Conferenc
Particle acceleration, magnetic field generation, and emission in relativistic pair jets
Shock acceleration is a ubiquitous phenomenon in astrophysical plasmas.
Plasma waves and their associated instabilities (e.g., Buneman, Weibel and
other two-stream instabilities) created in collisionless shocks are responsible
for particle (electron, positron, and ion) acceleration. Using a 3-D
relativistic electromagnetic particle (REMP) code, we have investigated
particle acceleration associated with a relativistic jet front propagating into
an ambient plasma. We find that the growth times of Weibel instability are
proportional to the Lorentz factors of jets. Simulations show that the Weibel
instability created in the collisionless shock front accelerates jet and
ambient particles both perpendicular and parallel to the jet propagation
direction.Comment: 4 pages, 2 figures, submitted to Il nuovo cimento (4th Workshop
Gamma-Ray Bursts in the Afterglow Era, Rome, 18-22 October 2004
Development of a carbon fibre composite active mirror: Design and testing
Carbon fibre composite technology for lightweight mirrors is gaining
increasing interest in the space- and ground-based astronomical communities for
its low weight, ease of manufacturing, excellent thermal qualities and
robustness. We present here first results of a project to design and produce a
27 cm diameter deformable carbon fibre composite mirror. The aim was to produce
a high surface form accuracy as well as low surface roughness. As part of this
programme, a passive mirror was developed to investigate stability and coating
issues. Results from the manufacturing and polishing process are reported here.
We also present results of a mechanical and thermal finite element analysis, as
well as early experimental findings of the deformable mirror. Possible
applications and future work are discussed.Comment: Accepted by Optical Engineering. Figures 1-7 on
http://www.star.ucl.ac.uk/~sk/OEpaper_files
Particle acceleration in electron-ion jets
Weibel instability created in collisionless shocks is responsible for
particle (electron, positron, and ion) acceleration. Using a 3-D relativistic
electromagnetic particle (REMP) code, we have investigated particle
acceleration associated with a relativistic electron-ion jet fronts propagating
into an ambient plasma without initial magnetic fields with a longer simulation
system in order to investigate nonlinear stage of the Weibel instability and
its acceleration mechanism. The current channels generated by the Weibel
instability induce the radial electric fields. The z component of the Poynting
vector (E x B) become positive in the large region along the jet propagation
direction. This leads to the acceleration of jet electrons along the jet. In
particular the E x B drift with the large scale current channel generated by
the ion Weibel instability accelerate electrons effectively in both parallel
and perpendicular directions.Comment: 2 pages, 1 figure, Proceedings for Astrophysical Sources of High
Energy Particles and Radiation, AIP proceeding Series, eds . T. Bulik, G.
Madejski and B. Ruda
Convergence towards an asymptotic shape in first-passage percolation on cone-like subgraphs of the integer lattice
In first-passage percolation on the integer lattice, the Shape Theorem
provides precise conditions for convergence of the set of sites reachable
within a given time from the origin, once rescaled, to a compact and convex
limiting shape. Here, we address convergence towards an asymptotic shape for
cone-like subgraphs of the lattice, where . In particular, we
identify the asymptotic shapes associated to these graphs as restrictions of
the asymptotic shape of the lattice. Apart from providing necessary and
sufficient conditions for - and almost sure convergence towards this
shape, we investigate also stronger notions such as complete convergence and
stability with respect to a dynamically evolving environment.Comment: 23 pages. Together with arXiv:1305.6260, this version replaces the
old. The main results have been strengthened and an earlier error in the
statement corrected. To appear in J. Theoret. Proba
- …