27 research outputs found
Filling factors and scale heights of the DIG in the Milky Way
The combination of dispersion measures of pulsars, distances from the model
of Cordes and Lazio (2002) and emission measures from the WHAM survey enabled a
statistical study of electron densities and filling factors of the diffuse
ionized gas (DIG) in the Milky Way. The emission measures were corrected for
absorption and contributions from beyond the pulsar distance. For a sample of
157 pulsars at |b| > 5 degrees, mainly located in interarm regions within about
3 kpc from the Sun, we find that: (1) The average volume filling factor along
the line of sight is inversely proportional to the mean electron density in
clouds. (2) The average volume filling factor increases towards larger
distances from the Galactic plane. (3) The local volume filling factor may
reach a maximum near |z| = 0.9 kpc, whereas the local electron density
continues to decrease at higher |z|, thus causing the observed flattening in
the distribution of dispersion measures perpendicular to the plane above this
height. (4) The scale heights of the electron density, the volume filling
factor and the emission measure are the same and in the range 250-500 pc.Comment: 16 pages, 13 figures. Accepted for publication in A
Densities and filling factors of the DIG in the Solar neighbourhood
For the first time we have combined dispersion measures and emission measures
towards 38 pulsars at KNOWN distances from which we derived the mean electron
density in clouds, N_c, and their volume filling factor, F_v, averaged along
the line of sight. The emission measures were corrected for absorption by dust
and contributions from beyond the pulsar distance. Results: The scale height of
the electron layer for our sample is 0.93+/-0.13 kpc and the midplane electron
density is 0.023+/-0.004 cm^-3, in agreement with earlier results. The average
density along the line of sight is = 0.018+/-0.002 cm^-3 and nearly
constant. Since = F_v N_c, an inverse relationship between F_v and N_c is
expected. We find F_v(N_c) = (0.011+/-0.003) N_c^{-1.20+/-0.13}, which holds
for the ranges N_c = 0.05-1 cm^-3 and F_v = 0.4-0.01. Near the Galactic plane
the dependence of F_v on N_c is significantly stronger than away from the
plane. F_v does not systematically change along or perpendicular to the
Galactic plane, but the spread about the mean value of 0.08+/-0.02 is
considerable. Conclusions: The inverse F_v-N_c relation is consistent with a
hierarchical, fractal density distribution in the diffuse ionized gas (DIG)
caused by turbulence. The observed near constancy of then is a signature
of fractal structure in the ionized medium, which is most pronounced outside
the thin disk.Comment: 9 pages, 9 figures. Accepted for publication in A&
Interstellar Plasma Turbulence Spectrum Toward the Pulsars PSR B0809+74 and B0950+08
Interstellar scintillations of pulsars PSR B0809+74 and B0950+08 have been
studied using observations at low frequencies (41, 62, 89, and 112 MHz).
Characteristic temporal and frequency scales of diffractive scintillations at
these frequencies have been determined. The comprehensive analysis of the
frequency and temporal structure functions reduced to the same frequency has
shown that the spectrum of interstellar plasma inhomogeneities toward both
pulsars is described by a power law. The exponent of the spectrum of
fluctuations of interstellar plasma inhomogeneities toward PSR B0950+08 (n =
3.00 +- 0.05) appreciably differs from the Kolmogorov exponent. Toward PSR
B0809+74 the spectrum is a power law with an exponent n = 3.7 +- 0.1. A strong
angular refraction has been detected toward PSR B0950+08. The distribution of
inhomogeneities along the line of sight has been analyzed; it has been shown
that the scintillations of PSR B0950+08 take place on a turbulent layer with
enhanced electron density, which is localized at approximately 10 pc from the
observer. For PSR B0809+74 the distribution of inhomogeneities is
quasi-uniform. Mean-square fluctuations of electron density on inhomogeneities
with a characteristic scale rho_0 = 10^7 m toward four pulsars have been
estimated. On this scale the local turbulence level in the 10-pc layer is 20
times higher than in an extended region responsible for the scintillations of
PSR B0809+74.Comment: 13 pages, 11 figure
The Asymmetry Coefficient for Interstellar Scintillation of Extragalactic Radio Sources
Comparing the asymmetry coefficients and scintillation indices for observed
time variations of the intensity of the radiation of extragalactic sources and
the predictions of theoretical models is a good test of the nature of the
observed variations. Such comparisons can be used to determine whether
flux-density variations are due to scintillation in the interstellar medium or
are intrinsic to the source. In the former case, they can be used to estimate
the fraction of the total flux contributed by the compact component (core)
whose flux-density variations are brought about by inhomogeneities in the
interstellar plasma. Results for the radio sources PKS 0405-385, B0917+624, PKS
1257-336, and J1819+3845 demonstrate that the scintillating component in these
objects makes up from 50% to 100% of the total flux, and that the intrinsic
angular sizes of the sources at 5 GHz is 10-40 microarcseconds. The
characteristics of the medium giving rise to the scintillations are presented