464 research outputs found
Solar cycle evolution of the solar wind in three dimensions
Measurements of the solar wind speed both in and out of the ecliptic are presented for 1971-82. The speed estimates, which were made with the interplanetary scintillation system at UC San Diego, have been compared to in situ for large, slowly evolving structures, and thus such structures can be studied up to 60 degrees north and south heliographic latitude. Annual average wind speeds are presented versus latitude for an entire solar cycle. Fast wind streams from the poles persisted through declining and low solar activity, but were closed off during four years of high activity. This evolution follows that of the polar coronal holes, as displayed by comparing averaged speed and coronal density over latitude and longitude. The most recent data (1982) show the reestablishment of large tilted polar holes and associated fast streams. Coronal magnetic field data show that the neutral sheet is confined to low latitudes at solar minimum and extends to high latitudes at solar maximum; thus the slow solar wind comes from the same latitude range as that of the neutral sheet
Radio Scintillation due to Discontinuities in the Interstellar Plasma Density
We develop the theory of interstellar scintillation as caused by an irregular
plasma having a power-law spatial density spectrum with a spectral exponent of
4 corresponding to a medium with abrupt changes in its density. An ``outer
scale'' is included in the model representing the typical scale over which the
density of the medium remains uniform. Such a spectrum could be used to model
plasma shock fronts in supernova remnants or other plasma discontinuities. We
investigate and develop equations for the decorrelation bandwidth of
diffractive scintillations and the refractive scintillation index and compare
our results with pulsar measurements. We consider both a medium concentrated in
a thin layer and an extended irregular medium. We conclude that the
discontinuity model gives satisfactory agreement for many diffractive
measurements, in particular the VLBI meaurements of the structure function
exponent between 5/3 and 2. However, it gives less satisfactory agreement for
the refractive scintillation index than does the Kolmogorov turbulence
spectrum. The comparison suggests that the medium consists of a pervasive
background distribution of turbulence embedded with randomly placed discrete
plasma structures such as shocks or HII regions. This can be modeled by a
composite spectrum following the Kolmogorov form at high wavenumbers and
steepening at lower wavenumbers corresponding to the typical (inverse) size of
the discrete structures. Such a model can also explain the extreme scattering
events. However, lines of sight through the enhanced scattering prevalent at
low galactic latitudes are accurately described by the Kolmogorov spectrum in
an extended medium and do not appear to have a similar low-wavenumber
steepening.Comment: Accpeted for ApJ vol 531, March 200
IPS observations of the solar wind speed out of the ecliptic
Interplanetary scintillation observations from 1971-1975 show that the average solar wind speed increases away from the solar equator, with a mean gradient of 2.1 km/s per degree. These results are compared with spacecraft observations over the + or - 7 deg attainable in the ecliptic and with those deduced from comet tails. The role of temporal variations, especially those caused by latitude dependent solar wind streams, is emphasized, and this points to the need for extensive ecliptic and ground-based observations during an out-of-the-ecliptic spacecraft mission
Interstellar Scintillations of Polarization of Compact Sources
We demostrate that the measurement of fluctuations of polarization due to the
galactic interstellar scintillations may be used to study the structure of the
radiation field at compact radio sources. We develop a mathematical formalism
and demonstrate it on a simple analytical model in which the scale of the
polarization variation through the source is comparable to the source size. The
predicted amplitude of modulation of the polarized radiation flux is ~20% x
(pi_s) x (m_sc), where (pi_s) is the characteristic degree of polarization of
radiation at the source and (m_sc) is the typical modulation index due to
scattering, i.e., (m_sc)~1 for diffractive scintillations and (m_sc)<1 for
refractive scintillations.Comment: 5 pages, 2 figures, emilateapj.sty. Submitted to ApJ
Interstellar Scintillation Observations of 146 Extragalactic Radio Sources
From 1979--1996 the Green Bank Interferometer was used by the Naval Research
Laboratory to monitor the flux density from 146 compact radio sources at
frequencies near 2 and 8 GHz. We filter the ``light curves'' to separate
intrinsic variations on times of a year or more from more rapid interstellar
scintilation (ISS) on times of 5--50 d. Whereas the intrinsic variation at 2
GHz is similar to that at 8 GHz (though diminished in amplitude), the ISS
variation is much stronger at 2 than at 8 GHz. We characterize the ISS
variation by an rms amplitude and a timescale and examine the statistics of
these parameters for the 121 sources with significant ISS at 2 GHz. We model
the scintillations using the NE2001 Galactic electron model assuming the
sources are brightness-limited.
We find the observed rms amplitude to be in general agreement with the model,
provided that the compact components of the sources have about 50% of their
flux density in a component with maximum brightness temperatures
--K. Thus our results are consistent with cm-wavelength VLBI
studies of compact AGNs, in that the maximum brightness temperatures found are
consistent with the inverse synchrotron limit at K, boosted
in jet configurations by Doppler factors up to about 20. The average of the
observed 2 GHz ISS timescales is in reasonable agreement with the model at
Galactic latitudes above about 10\de. At lower latitudes the observed
timescales are too fast, suggesting that the transverse plasma velocity
increases more than expected beyond about 1 kpc.Comment: 32 pages, 16 figures. Submitted to Ap
Interstellar Scintillation of the Polarized Flux Density in Quasar, PKS 0405-385
The remarkable rapid variations in radio flux density and polarization of the
quasar PKS 0405-385 observed in 1996 are subject to a correlation analysis,
from which characteristic time scales and amplitudes are derived. The
variations are interpreted as interstellar scintillations. The cm wavelength
observations are in the weak scintillation regime for which models for the
various auto- and cross-correlations of the Stokes parameters are derived and
fitted to the observations. These are well modelled by interstellar
scintillation (ISS) of a 30 by 22 micro-as source, with about 180 degree
rotation of the polarization angle along its long dimension. This success in
explaining the remarkable intra-day variations (IDV)in polarization confirms
that ISS gives rise to the IDV in this quasar. However, the fit requires the
scintillations to be occurring much closer to the Earth than expected according
to the standard model for the ionized interstellar medium (IISM). Scattering at
distances in the range 3-30 parsec are required to explain the observations.
The associated source model has a peak brightness temperature near 2.0
10^{13}K, which is about twenty-five times smaller than previously derived for
this source. This reduces the implied Doppler factor in the relativistic jet,
presumed responsible to 10-20, high but just compatible with cm wavelength VLBI
estimates for the Doppler factors in Active Galactic Nuclei (AGNs).Comment: 43 pages 15 figures, accepted for ApJ Dec 200
Modeling of Interstellar Scintillation Arcs from Pulsar B1133+16
The parabolic arc phenomenon visible in the Fourier analysis of the
scintillation spectra of pulsars provides a new method of investigating the
small scale structure in the ionized interstellar medium (ISM). We report
archival observations of the pulsar B1133+16 showing both forward and reverse
parabolic arcs sampled over 14 months. These features can be understood as the
mutual interference between an assembly of discrete features in the scattered
brightness distribution. By model-fitting to the observed arcs at one epoch we
obtain a ``snap-shot'' estimate of the scattered brightness, which we show to
be highly anisotropic (axial ratio >10:1), to be centered significantly off
axis and to have a small number of discrete maxima, which are coarser the
speckle expected from a Kolmogorov spectrum of interstellar plasma density. The
results suggest the effects of highly localized discrete scattering regions
which subtend 0.1-1 mas, but can scatter (or refract) the radiation by angles
that are five or more times larger.Comment: 14 pages, 4 figures, submitted to Astrophysical Journa
Dual-Frequency Observations of 140 Compact, Flat-Spectrum Active Galactic Nuclei for Scintillation-Induced Variability
The 4.9 GHz Micro-Arcsecond Scintillation-Induced Variability (MASIV) Survey
detected a drop in Interstellar Scintillation (ISS) for sources at redshifts z
> 2, indicating an apparent increase in angular diameter or a decrease in flux
density of the most compact components of these sources, relative to their
extended emission. This can result from intrinsic source size effects or
scatter broadening in the Intergalactic Medium (IGM), in excess of the expected
(1+z)^0.5 angular diameter scaling of brightness temperature limited sources
due to cosmological expansion. We report here 4.9 GHz and 8.4 GHz observations
and data analysis for a sample of 140 compact, flat-spectrum sources which may
allow us to determine the origin of this angular diameter-redshift relation by
exploiting their different wavelength dependences. In addition to using ISS as
a cosmological probe, the observations provide additional insight into source
morphologies and the characteristics of ISS. As in the MASIV Survey, the
variability of the sources is found to be significantly correlated with
line-of-sight H-alpha intensities, confirming its link with ISS. For 25
sources, time delays of about 0.15 to 3 days are observed between the
scintillation patterns at both frequencies, interpreted as being caused by a
shift in core positions when probed at different optical depths. Significant
correlation is found between ISS amplitudes and source spectral index; in
particular, a large drop in ISS amplitudes is observed at spectral indices of <
-0.4 confirming that steep spectrum sources scintillate less. We detect a
weakened redshift dependence of ISS at 8.4 GHz over that at 4.9 GHz, with the
mean variance at 4-day timescales reduced by a factor of 1.8 in the z > 2
sources relative to the z < 2 sources, as opposed to the factor of 3 decrease
observed at 4.9 GHz. This suggests scatter broadening in the IGM.Comment: 30 pages, 14 figures, accepted for publication in the Astronomical
Journa
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