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

1. Introduction

Wieringa et al. (1993) were the first to note structure on arcminute scales in the linearly polarized component of the galactic radio background at 325 MHz, observed with the WSRT. The small-scale structure in the maps of polarized intensity P, (with polarized brightness temperatures of up to 10 K) does NOT have a counterpart in total intensity, or Stokes I, down to very low limits. Because the total Stokes I of the galactic radio background has an estimated of the order of 30 - 50 K at 325 MHz, which must be very smooth and therefore filtered out completely in the WSRT measurements, the apparent polarization percentage of the small-scale features can become very much larger than 100%.

The absence of corresponding small-scale structure in Stokes I led Wieringa et al. (ibid.) to propose that the small-scale structure in polarized intensity P is due to Faraday rotation modulation. In this picture, synchrotron radiation generated in the Galactic halo reaches us through a magneto-ionic screen, viz. the warm relatively nearby ISM. Structure in the electron density and/or magnetic field in the ISM causes spatial variations in the Rotation Measure (RM) of the screen. Hence, the angle of linear polarization of the synchrotron emission from the halo is rotated by different amounts along different lines of sight. Even if the polarized emission in the halo were totally smooth, in intensity as well as angle, the screen would produce structure in Stokes Q and U.

Small-scale structure in the polarized galactic radio background has recently been observed also at other frequencies. At 1420 MHz, Gray et al. (1998, 1999) used the DRAO synthesis telescope to study the phenomenon at 1´ resolution. Uyaniker et al. (1999) used the Effelsberg telescope at 1.4 GHz, to map the polarized emission at 9´ resolution over about 1100. Duncan et al. (1998) discuss radio polarization data at 1.4, 2.4 and 4.8 GHz with the Parkes radio telescope and the VLA, at resp. 5´, 10´ and 15´ resolution. All these observations support the interpretation in terms of modulation of emission originating at larger distances, by a relatively nearby Faraday screen.

The distributions of polarized intensity and angle may therefore be used to study the structure of the Faraday screen. In particular, polarization observations give information about the electron density, , and the component of the magnetic field parallel to the line of sight, , in the ISM on scales down to less than 0.5 pc ( 4´ at an assumed distance of 500 pc). The diffuse nature of the polarized radio background allows (almost) complete spatial mapping of RMs over large areas, provided one has observations at several frequencies. This gives a large advantage over RM determinations through individual objects, like pulsars or extra-galactic radio sources.

© European Southern Observatory (ESO) 2000

Online publication: March 28, 2000
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