1,979 research outputs found
Stochastic Acceleration in Relativistic Parallel Shocks
(abridged) We present results of test-particle simulations on both the first
and the second order Fermi acceleration at relativistic parallel shock waves.
We consider two scenarios for particle injection: (i) particles injected at the
shock front, then accelerated at the shock by the first order mechanism and
subsequently by the stochastic process in the downstream region; and (ii)
particles injected uniformly throughout the downstream region to the stochastic
process. We show that regardless of the injection scenario, depending on the
magnetic field strength, plasma composition, and the employed turbulence model,
the stochastic mechanism can have considerable effects on the particle spectrum
on temporal and spatial scales too short to be resolved in extragalactic jets.
Stochastic acceleration is shown to be able to produce spectra that are
significantly flatter than the limiting case of particle energy spectral index
-1 of the first order mechanism. Our study also reveals a possibility of
re-acceleration of the stochastically accelerated spectrum at the shock, as
particles at high energies become more and more mobile as their mean free path
increases with energy. Our findings suggest that the role of the second order
mechanism in the turbulent downstream of a relativistic shock with respect to
the first order mechanism at the shock front has been underestimated in the
past, and that the second order mechanism may have significant effects on the
form of the particle spectra and its evolution.Comment: 14 pages, 11 figures (9 black/white and 2 color postscripts). To be
published in the ApJ (accepted 6 Nov 2004
Stochastic Particle Acceleration in Parallel Relativistic Shocks
We present results of test-particle simulations on both the first- and the
second-order Fermi acceleration for relativistic parallel shock waves. Our
studies suggest that the role of the second-order mechanism in the turbulent
downstream of a relativistic shock may have been underestimated in the past,
and that the stochastic mechanism may have significant effects on the form of
the particle spectra and its time evolution.Comment: Poster at "The XXII Texas Symposium on Relativistic Astrophysics",
Stanford, USA, December 2004, (TSRA04), 6 pages, LaTeX, 5 ps/eps figure
Particle acceleration in thick parallel shocks with high compression ratio
We report studies on first-order Fermi acceleration in parallel modified
shock waves with a large scattering center compression ratio expected from
turbulence transmission models. Using a Monte Carlo technique we have modeled
particle acceleration in shocks with a velocity ranging from nonrelativistic to
ultrarelativistic and a thickness extending from nearly steplike to very wide
structures exceeding the particle diffusion length by orders of magnitude. The
nonrelativistic diffusion approximation is found to be surprisingly accurate in
predicting the spectral index of a thick shock with large compression ratio
even in the cases involving relativistic shock speeds.Comment: 4 pages, 2 figures, accepted to A&
Turbulence transmission in parallel modified shocks using ray tracing
We apply a semi-classical approach of handling waves as quasiparticle gas in
a slowly varying flow -- analogous to ray tracing -- to calculate the Alfven
wave transmission parameters, the resulting cross-helicity of the waves and the
scattering-centre compression ratio, for cases where the shock thickness is
large enough for the turbulent waves in the plasma to see the transition of the
background flow parameters as smooth and slowly varying. For nonrelativistic
shocks the wave transmission produces similar effects on the downstream
turbulence and the scattering-centre compression ratio as does the transmission
through a step shock: the downstream Alfven waves propagate predominantly
towards the shock in the local plasma frame and, thus, the scattering-centre
compression ratio is larger than the gas compression ratio. For thick
relativistic shocks, however, we find qualitative differences with respect to
the step-shock case: for low-Alfvenic-Mach-number shocks the downstream waves
propagate predominantly away from the shock, and the scattering-centre
compression ratio is lower than that of the gas. Thus, when taken into account,
the Alfven wave transmission can decrease the efficiency of the first-order
Fermi acceleration in a thick relativistic shock.Comment: 6 pages, 5 figures, accepted to A&
Double resonant absorption measurement of acetylene symmetric vibrational states probed with cavity ring down spectroscopy
A novel mid-infrared/near-infrared double resonant absorption setup for
studying infrared-inactive vibrational states is presented. A strong
vibrational transition in the mid-infrared region is excited using an idler
beam from a singly resonant continuous-wave optical parametric oscillator, to
populate an intermediate vibrational state. High output power of the optical
parametric oscillator and the strength of the mid-infrared transition result in
efficient population transfer to the intermediate state, which allows measuring
secondary transitions from this state with a high signal-to-noise ratio. A
secondary, near-infrared transition from the intermediate state is probed using
cavity ring down spectroscopy, which provides high sensitivity in this
wavelength region. Due to the narrow linewidths of the excitation sources, the
rovibrational lines of the secondary transition are measured with sub-Doppler
resolution. The setup is used to access a previously unreported symmetric
vibrational state of acetylene, in the
normal mode notation. Single-photon transitions to this state from the
vibrational ground state are forbidden. Ten lines of the newly measured state
are observed and fitted with the linear least-squares method to extract the
band parameters. The vibrational term value was measured to be at 9775.0018(45)
, the rotational parameter was 1.162222 ,
and the quartic centrifugal distortion parameter was 3.998(62), where the numbers in the parenthesis are one-standard
errors in the least significant digits
Why is solar cycle 24 an inefficient producer of high-energy particle events?
The aim of the study is to investigate the reason for the low productivity of
high-energy SEPs in the present solar cycle. We employ scaling laws derived
from diffusive shock acceleration theory and simulation studies including
proton-generated upstream Alfv\'en waves to find out how the changes observed
in the long-term average properties of the erupting and ambient coronal and/or
solar wind plasma would affect the ability of shocks to accelerate particles to
the highest energies. Provided that self-generated turbulence dominates
particle transport around coronal shocks, it is found that the most crucial
factors controlling the diffusive shock acceleration process are the number
density of seed particles and the plasma density of the ambient medium.
Assuming that suprathermal populations provide a fraction of the particles
injected to shock acceleration in the corona, we show that the lack of most
energetic particle events as well as the lack of low charge-to-mass ratio ion
species in the present cycle can be understood as a result of the reduction of
average coronal plasma and suprathermal densities in the present cycle over the
previous one
Preparation and execution of teeth clenching and foot muscle contraction influence on corticospinal hand-muscle excitability
Contraction of a muscle modulates not only the corticospinal excitability (CSE) of the contracting muscle but also that of different muscles. We investigated to what extent the CSE of a hand muscle is modulated during preparation and execution of teeth clenching and ipsilateral foot dorsiflexion either separately or in combination. Hand-muscle CSE was estimated based on motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) and recorded from the first dorsal interosseous (FDI) muscle. We found higher excitability during both preparation and execution of all the motor tasks than during mere observation of a fixation cross. As expected, the excitability was greater during the execution phase than the preparation one. Furthermore, both execution and preparation of combined motor tasks led to higher excitability than individual tasks. These results extend our current understanding of the neural interactions underlying simultaneous contraction of muscles in different body parts.Peer reviewe
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