SCUBA polarimetry has been enormously successful in mapping magnetic fields in sources ranging from pre-stellar cores to the Galactic Centre. With this wealth of information, a question arose: is there such a thing as a 'typical' interstellar magnetic field? We decided to try and pick a region in our Galaxy that is subject to all the important processes, i.e. in the vicinity of a supernova remnant, buffeted by a major star-formation region, exposed to interstellar radiation, etc. In summer 2001 we had time to observe exactly such a region, the AFGL 333 cloud filament - this lies between the W4 SNR and W3 star-forming complex, but as yet has no young stars itself.

Figure 1 shows the 850 micron polarimetry results, superimposed on a scan map of the AFGL 333 filament, which is about 8 arcmin long (about 6 parsecs at 2.5 kpc distance). The vector lengths are proportional to polarization, and the directions point along the magnetic field. Two things were immediately very surprising about this image. Firstly, the polarization is high, with a mean of 6.5% - this exceeds the 3.5-5% observed in nearby star-forming clouds (see the series of recent papers by Brenda Matthews et al.). A high polarization suggests the magnetic field is unusually organised, even though the greater cloud distance means we are observing over larger scales. Secondly, there is a very clear dominant field direction, running south-east to north-west - but where there is a brighter peak the vectors start to curve. This shows up especially well for the 'magnetic wrapper' around the central double core. So our observation of a 'generic' molecular cloud has in fact shown a magnetic field structure quite different from all the gallery we have for other clouds!

Figure 2 shows the results in a wider context - the colour scale shows the IRAS 100 micron image over about 130 pc, including the huge shell of the SNR W4, and the W3 complex (in white, saturated on this scale. The blue 'worm' is the outline of the filament mapped with SCUBA, and the single vector is the average of all the vectors shown in the previous figure. It's obvious that this points very close to the centre of the SNR, which suggests that the supernova explosion was the dominant force establishing the magnetic field in our 'generic' filament. A model for a magnetic SNR-cloud interaction was discussed by Jun & Jones (1999; ApJ 511, 774), who found that the field is dominantly radial where it is stretched out along expanding fingers produced by Rayleigh-Taylor instabilities. Although strictly this is for a specific initial situation (diffuse cloud, linear field…), there is striking resemblence of some of their model results (Figure 3, showing magnetic intensity) to our magnetic field map!

So is the field initially present in a cloud a highly-organised one established by the very large-scale environment? And how does this turn into the much more structured fields seen in star-forming cores? In AFGL 333 the field seems to be streaming out from the supernova origin and maybe getting wrapped around denser (starless) cores, but we need to revisit the theoretical models to see if this agrees with the energetics involved. Observations are the key, however, in piecing together the still largely mysterious role played by magnetic fields in the clouds forming the next generations of stars.

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