274 research outputs found
Parsec-scale structure in the warm ISM from polarized galactic radio background observations
We present multi-frequency polarization observations of the diffuse radio
synchrotron background modulated by Faraday rotation, in two directions of
positive latitude. No extended total intensity I is observed, which implies
that total intensity has no structure on scales smaller than approximately a
degree. Polarized intensity and polarization angle, however, show abundant
small-scale structure on scales from arcminutes to degrees. Rotation Measure
(RM) maps show coherent structure over many synthesized beams, but also abrupt
large changes over one beam. RM's from polarized extragalactic point sources
are correlated over the field in each of the two fields, indicating a galactic
component to the RM, but show no correlation with the RM map of the diffuse
radiation. The upper limit in structure in I puts constraints on the random and
regular components of the magnetic field in the galactic interstellar medium
and halo. The emission is partly depolarized so that the observed polarization
mostly originates from a nearby part of the medium. This explains the lack of
correlation between RM from diffuse emission and from extragalactic point
sources as the latter is built up over the entire path length through the
medium.Comment: To appear in "Astrophysical Polarized Backgrounds", Conference
Proceedings, eds S. Cecchini, S. Cortiglioni, R. Sault and C. Sbarra, in
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Structure in the local Galactic ISM on scales down to 1 pc, from multi-band radio polarization observations
We discuss observations of the linearly polarized component of the diffuse
galactic radio background. These observations, with an angular resolution of
4', were made with the Westerbork Synthesis Radio Telescope (WSRT) in 5
frequency bands in the range 341-375 MHz. The linearly polarized intensity P
(with polarized brightness temperature going up to 10K) shows a `cloudy'
structure, with characteristic scales of 15'-30', which contains relatively
long, but very narrow `canals' (essentially unresolved) in which P is only a
small fraction of that in the neighbouring beams.
These `canals' are generally seen in more than one frequency band, although
their appearance changes between bands. They are probably due to depolarization
within the synthesized beam, because the change in polarization angle across
the deepest `canals' is in general close to 90 degrees (or 270 etc.). These
very abrupt changes in polarization angle, which are seen only across the
`canals', seem to be accompanied by abrupt changes in the Rotation Measure
(RM), which may have the right magnitude to create the difference of close to
90 degrees in polarization angle, and thereby the `canals'.
The structure in the polarization maps is most likely due to Faraday rotation
modulation of the probably smooth polarized radiation emitted in the halo of
our Galaxy by the fairly local ISM (up to 500 pc). Therefore, the abrupt
changes of RM across the `canals' provide evidence for very thin (about 1 pc),
and relatively long transition regions in the ISM, across which the RM changes
by as much as 100%. Such drastic RM changes may well be due primarily to abrupt
changes in the magnetic field.Comment: 4 pages, 4 figures, to be published in A&A Letter
The Jamming Perspective on Wet Foams
Amorphous materials as diverse as foams, emulsions, colloidal suspensions and
granular media can {\em jam} into a rigid, disordered state where they
withstand finite shear stresses before yielding. The jamming transition has
been studied extensively, in particular in computer simulations of
frictionless, soft, purely repulsive spheres. Foams and emulsions are the
closest realizations of this model, and in foams, the (un)jamming point
corresponds to the wet limit, where the bubbles become spherical and just form
contacts. Here we sketch the relevance of the jamming perspective for the
geometry and flow of foams --- and also discuss the impact that foams studies
may have on theoretical studies on jamming.
We first briefly review insights into the crucial role of disorder in these
systems, culminating in the breakdown of the affine assumption that underlies
the rich mechanics near jamming. Second, we discuss how crucial theoretical
predictions, such as the square root scaling of contact number with packing
fraction, and the nontrivial role of disorder and fluctuations for flow have
been observed in experiments on 2D foams. Third, we discuss a scaling model for
the rheology of disordered media that appears to capture the key features of
the flow of foams, emulsions and soft colloidal suspensions. Finally, we
discuss how best to confront predictions of this model with experimental data.Comment: 7 Figs., 21 pages, Review articl
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