2,543 research outputs found
The electron beam instability revisited: Growth above and below f sub p
The growth of electrostatic waves near the plasma frequency (fp) due to an unstable electron beam is investigated by solving the unmagnetized electrostatic dispersion equation numerically. The numerical solutions are compared with analytic theories for reactive (or fluid-type) and kinetic versions of the beam instability, and for the O'Neil/Malmberg connection of the beam and Langmuir modes. Conditions for growth significantly above or below fp are given. Three general results are found: (1) The unstable waves do not grow on a mode with Langmuir dispersion except in the limit of a very dilute beam with growth on O'Neil/Malmberg's connected mode; (2) The properties of the unstable mode depend strongly on beam parameters such as beam density, speed and temperature; and (3) The frequency of maximum growth frequently lies significantly above or below fp, and differs significantly from that predicted by the Langmuir dispersion relation. Results imply important consequences for theories of strong turbulence and nonlinear wave-wave processes, and observational identification of fp from observed wave frequencies
Phase and Intensity Distributions of Individual Pulses of PSR B0950+08
The distribution of the intensities of individual pulses of PSR B0950+08 as a
function of the longitudes at which they appear is analyzed. The flux density
of the pulsar at 111 MHz varies strongly from day to day (by up to a factor of
13) due to the passage of the radiation through the interstellar plasma
(interstellar scintillation). The intensities of individual pulses can exceed
the amplitude of the mean pulse profile, obtained by accumulating 770 pulses,
by more than an order of magnitude. The intensity distribution along the mean
profile is very different for weak and strong pulses. The differential
distribution function for the intensities is a power law with index n = -1.1 +-
0.06 up to peak flux densities for individual pulses of the order of 160 Jy
Physics of magnetospheric boundary layers
The central ideas of this grant are that the magnetospheric boundary layers link disparate regions of the magnetosphere together, and the global behavior of the magnetosphere can be understood only by understanding the linking mechanisms. Accordingly the present grant includes simultaneous research on the global, meso-, and micro-scale physics of the magnetosphere and its boundary layers. These boundary layers include the bow shock, magnetosheath, the plasma sheet boundary layer, and the ionosphere. Analytic, numerical and simulation projects have been performed on these subjects, as well as comparison of theoretical results with observational data. Very good progress has been made, with four papers published or in press and two additional papers submitted for publication during the six month period 1 June - 30 November 1993. At least two projects are currently being written up. In addition, members of the group have given papers at scientific meetings. The further structure of this report is as follows: section two contains brief accounts of research completed during the last six months, while section three describes the research projects intended for the grant's final period
Are supernova remnants quasi-parallel or quasi-perpendicular accelerators
Observations of shock waves in the solar system which show a pronounced difference in the plasma wave and particle environment depending on whether the shock is propagating along or perpendicular to the interplanetary magnetic field are discussed. Theories for particle acceleration developed for quasi-parallel and quasi-perpendicular shocks, when extended to the interstellar medium suggest that the relativistic electrons in radio supernova remnants are accelerated by either the Q parallel or Q perpendicular mechanisms. A model for the galactic magnetic field and published maps of supernova remnants were used to search for a dependence of structure on the angle Phi. Results show no tendency for the remnants as a whole to favor the relationship expected for either mechanism, although individual sources resemble model remnants of one or the other acceleration process
Intrinsic Variability and Field Statistics for the Vela Pulsar: 2. Systematics and Single-Component Fits
Individual pulses from pulsars have intensity-phase profiles that differ
widely from pulse to pulse, from the average profile, and from phase to phase
within a pulse. Widely accepted explanations do not exist for this variability
or for the mechanism producing the radiation. The variability corresponds to
the field statistics, particularly the distribution of wave field amplitudes,
which are predicted by theories for wave growth in inhomogeneous media. This
paper shows that the field statistics of the Vela pulsar (PSR B0833-45) are
well-defined and vary as a function of pulse phase, evolving from Gaussian
intensity statistics off-pulse to approximately power-law and then lognormal
distributions near the pulse peak to approximately power-law and eventually
Gaussian statistics off-pulse again. Detailed single-component fits confirm
that the variability corresponds to lognormal statistics near the peak of the
pulse profile and Gaussian intensity statistics off-pulse. The lognormal field
statistics observed are consistent with the prediction of stochastic growth
theory (SGT) for a purely linear system close to marginal stability. The
simplest interpretations are that the pulsar's variability is a direct
manifestation of an SGT state and the emission mechanism is linear (either
direct or indirect), with no evidence for nonlinear mechanisms like
modulational instability and wave collapse which produce power-law field
statistics. Stringent constraints are placed on nonlinear mechanisms: they must
produce lognormal statistics when suitably ensemble-averaged. Field statistics
are thus a powerful, potentially widely applicable tool for understanding
variability and constraining mechanisms and source characteristics of coherent
astrophysical and space emissions.Comment: 11 pages, 12 figures. Accepted by Monthly Notices of the Royal
Astronmical Society in April 200
Intrinsic Variability and Field Statistics for the Vela Pulsar: 3. Two-Component Fits and Detailed Assessment of Stochastic Growth Theory
The variability of the Vela pulsar (PSR B0833-45) corresponds to well-defined
field statistics that vary with pulsar phase, ranging from Gaussian intensity
statistics off-pulse to approximately power-law statistics in a transition
region and then lognormal statistics on-pulse, excluding giant micropulses.
These data are analyzed here in terms of two superposed wave populations, using
a new calculation for the amplitude statistics of two vectorially-combined
transverse fields. Detailed analyses show that the approximately power-law and
lognormal distributions observed are fitted well at essentially all on-pulse
phases by Gaussian-lognormal and double-lognormal combinations, respectively.
These good fits, plus the smooth but significant variations in fit parameters
across the source, provide strong evidence that the approximately power-law
statistics observed in the transition region are not intrinsic. Instead, the
data are consistent with normal pulsar emission having lognormal statistics at
all phases. This is consistent with generation in an inhomogeneous source
obeying stochastic growth theory (SGT) and with the emission mechanism being
purely linear (either direct or indirect). A nonlinear mechanism is viable only
if it produces lognormal statistics when suitably ensemble-averaged. Variations
in the SGT fit parameters with phase imply that the radiation is relatively
more variable near the pulse edges than near the center, as found in earlier
work. In contrast, Vela's giant micropulses come from a very restricted phase
range and have power-law statistics with indices () not
inconsistent with nonlinear wave collapse. These results imply that normal
pulses have a different source and generation mechanism than giant micropulses,
as suggested previously on other grounds.Comment: 10 pages and 14 figures. Accepted by Monthly Notices of the Royal
Astronomical Society in April 200
Statistics of Auroral Langmuir Waves
The Physics of Auroral Zone Electrons II (PHAZE II) sounding rocket was launched in February 1997 into active pre-midnight aurora. The resulting high frequency wave data are dominated by Langmuir waves. Consistent with many previous observations the Langmuir waves are sporadic, occurring in bursts lasting up to a few hundred ms. We compute statistics of the electric field amplitudes of these Langmuir waves, with two results. First, the shape of the distribution of running averages of the electric field amplitudes remains approximately stationary for a large range of widths of running average less than ~0.3 ms and for a large range of widths exceeding about 1 ms. The interpretation of this transition timescale is unclear but appears unlikely to be of instrumental origin. Second, for 2.6-ms running averages, corresponding to the latter range, the distribution of the logarithm of electric field amplitudes matches a Gaussian form very well for all nine cases studied in detail, hence the statistics are lognormal. These distributions are consistent with stochastic growth theory (SGT)
Electrostatic Decay of Plasma Turbulence
The study of the evolution of a suprathermal electron beam traveling through
a background plasma is relevant for the physics of solar flares and their
associated type III solar radio bursts. As they evolve guided by the coronal
magnetic field-lines, these beams generate Langmuir turbulence. The
beam-generated turbulence is in turn responsible for the emission of radio
photons at the second harmonic of the local plasma frequency, which are
observed during type III solar radio bursts. To generate the radio emission,
the beam-aligned Langmuir waves must coalesce, and therefore a process capable
of re-directioning the turbulence in an effective fashion is required.
Different theoretical models identify the electrostatic (ES) decay process L1
-> L2 + S (L: Langmuir wave; S: Ion-acoustic wave) as the re-directioning
mechanism for the L waves. Two different regimes have been proposed to play a
key role: the back-scattering and the diffusive (small angle) scattering. This
paper is a comparative analysis of the decay rate of the ES decay for each
regime, and of the different observable characteristics that are expected for
the resulting ion-acoustic waves.Comment: 14 pages, 8 Figures. AAS LaTeX Macros v5.0. To appear in The
Astrophysical Journa
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