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Astron. Astrophys. 356, L13-L16 (2000)


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Letter to the Editor

Structure in the local Galactic ISM on scales down to 1 pc, from multi-band radio polarization observations

M. Haverkorn 1, P. Katgert 1 and A.G. de Bruyn 2,3

1 Sterrewacht Leiden, P.O. Box 9513, 2300 RA Leiden, The Netherlands (haverkrn,katgert@strw.leidenuniv.nl)
2 ASTRON, P.O. Box 2, 7990 AA Dwingeloo, The Netherlands (ger@nfra.nl)
3 Kapteyn Institute, P.O. Box 800, 9700 AV Groningen, The Netherlands

Received 14 February 2000 / Accepted 7 March 2000

Abstract

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 10 K) 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 [FORMULA] across the deepest `canals' is in general close to [FORMULA] (or [FORMULA] etc.). These very abrupt changes in [FORMULA], 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 [FORMULA] in [FORMULA], 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 ([FORMULA] pc) ISM. Therefore, the abrupt changes of RM across the `canals' provide evidence for very thin ([FORMULA] 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.

Key words: polarization – ISM: magnetic fields – ISM: structure – radio continuum: ISM

Send offprint requests to: haverkrn@strw.leidenuniv.nl

© European Southern Observatory (ESO) 2000

Online publication: March 28, 2000

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