22,000 research outputs found
Notes on drift theory
It is shown that there is a simpler way to derive the average guiding center drift of a distribution of particles than via the so-called single particle analysis. Based on this derivation it is shown that the entire drift formalism can be considerably simplified, and that results for low order anisotropies are more generally valid than is usually appreciated. This drift analysis leads to a natural alternative derivation of the drift velocity along a neutral sheet
Diffusive electron acceleration at SNR shock fronts and the observed SNR radio spectral indices
The radio synchrotron emission from relativistic electrons in shell supernova remnants (SNRs) provides a unique opportunity to probe the energy distribution of energetic electrons at their acceleration site (SNR shock fronts). This information provides insight into the acceleration mechanism(s). The implications of these observations for the diffusive (first-order Fermi) acceleration of electrons at the SNR shock fronts are discussed
Solutions to the tethered galaxy problem in an expanding universe and the observation of receding blueshifted objects
We use the dynamics of a galaxy, set up initially at a constant proper
distance from an observer, to derive and illustrate two counter-intuitive
general relativistic results. Although the galaxy does gradually join the
expansion of the universe (Hubble flow), it does not necessarily recede from
us. In particular, in the currently favored cosmological model, which includes
a cosmological constant, the galaxy recedes from the observer as it joins the
Hubble flow, but in the previously favored cold dark matter model, the galaxy
approaches, passes through the observer, and joins the Hubble flow on the
opposite side of the sky. We show that this behavior is consistent with the
general relativistic idea that space is expanding and is determined by the
acceleration of the expansion of the universe -- not a force or drag associated
with the expansion itself. We also show that objects at a constant proper
distance will have a nonzero redshift; receding galaxies can be blueshifted and
approaching galaxies can be redshifted.Comment: 8 pages including 6 figures, to appear in Am. J. Phys., 2003.
Reference added in postscrip
The Density Spike in Cosmic-Ray-Modified Shocks: Formation, Evolution, and Instability
We examine the formation and evolution of the density enhancement (density
spike) that appears downstream of strong, cosmic-ray-modified shocks. This
feature results from temporary overcompression of the flow by the combined
cosmic-ray shock precursor/gas subshock. Formation of the density spike is
expected whenever shock modification by cosmic-ray pressure increases strongly.
That occurence may be anticipated for newly generated strong shocks or for
cosmic-ray-modified shocks encountering a region of higher external density,
for example. The predicted mass density within the spike increases with the
shock Mach number and with shocks more dominated by cosmic-ray pressure. We
find this spike to be linearly unstable under a modified Rayleigh-Taylor
instability criterion at the early stage of its formation. We confirm this
instability numerically using two independent codes based on the two-fluid
model for cosmic-ray transport. These two-dimensional simulations show that the
instability grows impulsively at early stages and then slows down as the
gradients of total pressure and gas density decrease. Observational discovery
of this unstable density spike behind shocks, possibly through radio emission
enhanced by the amplified magnetic fields would provide evidence for the
existence of strongly cosmic-ray modified shock structures.Comment: 26 pages in Latex and 6 figures. Accepted to Ap
Continuing data analysis of the AS/E grazing incidence X-ray telescope experiment on the OSO-4 satellite
The work to correct and extend the calculation of the theoretical solar X-ray spectrum produced during earlier OSO-4 data analysis is reported along with the work to formulate models of active regions, and compare these models with the experimental values. An atlas of solar X-ray photographs is included, and solar X-ray observations are correlated with the solar wind
Fabrication and properties of gallium phosphide variable colour displays
The unique properties of single-junction gallium phosphide devices incorporating both red and green radiative recombination centers were investigated in application to the fabrication of monolithic 5 x 7 displays capable of displaying symbolic and alphanumeric information in a multicolor format. A number of potentially suitable material preparation techniques were evaluated in terms of both material properties and device performance. Optimum results were obtained for double liquid-phase-epitaxial process in which an open-tube dipping technique was used for n-layer growth and a sealed tipping procedure for subsequent p-layer growth. It was demonstrated that to prepare devices exhibiting a satisfactory range of dominant wavelengths which can be perceived as distinct emission colors extending from the red through green region of the visible spectrum involves a compromise between the material properties necessary for efficient red emission and those considered optimum for efficient green emission
An introductory guide to fluid models with anisotropic temperatures Part 1 -- CGL description and collisionless fluid hierarchy
We present a detailed guide to advanced collisionless fluid models that
incorporate kinetic effects into the fluid framework, and that are much closer
to the collisionless kinetic description than traditional magnetohydrodynamics.
Such fluid models are directly applicable to modeling turbulent evolution of a
vast array of astrophysical plasmas, such as the solar corona and the solar
wind, the interstellar medium, as well as accretion disks and galaxy clusters.
The text can be viewed as a detailed guide to Landau fluid models and it is
divided into two parts. Part 1 is dedicated to fluid models that are obtained
by closing the fluid hierarchy with simple (non Landau fluid) closures. Part 2
is dedicated to Landau fluid closures. Here in Part 1, we discuss the CGL fluid
model in great detail, together with fluid models that contain dispersive
effects introduced by the Hall term and by the finite Larmor radius (FLR)
corrections to the pressure tensor. We consider dispersive effects introduced
by the non-gyrotropic heat flux vectors. We investigate the parallel and
oblique firehose instability, and show that the non-gyrotropic heat flux
strongly influences the maximum growth rate of these instabilities.
Furthermore, we discuss fluid models that contain evolution equations for the
gyrotropic heat flux fluctuations and that are closed at the 4th-moment level
by prescribing a specific form for the distribution function. For the
bi-Maxwellian distribution, such a closure is known as the "normal" closure. We
also discuss a fluid closure for the bi-kappa distribution. Finally, by
considering one-dimensional Maxwellian fluid closures at higher-order moments,
we show that such fluid models are always unstable. The last possible non
Landau fluid closure is therefore the "normal" closure, and beyond the
4th-order moment, Landau fluid closures are required.Comment: Improved version, accepted to JPP Lecture Notes. Some parts were
shortened and some parts were expanded. The text now contains Conclusion
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