The Radio Continuum Brightness Minimum near Polaris: A Hole in the Interstellar Magnetic Field?
Large-scale radio continuum observations at decimetre wavelengths reveal an extended remarkable nonthermal brightness minimum of 10° in diameter close to the equatorial north pole. Its positional coincidence with an enormous arch of neutral hydrogen gas and a similar structure seen in the FIR (l00µ) is indicative of it being due to a shell-type phenomenon.
It is shown that a thick shell characterized by an inner and outer radius r and R (where r/R ⋍ 0.5) with only a small enhancement of the magnetic field strength (⋍ 20%) in the shell will account for a distinct brightness minimum with respect to the surrounding. The expected shell emission is much less pronounced and may therefore become undetectable due to fluctuations of the total emission along the whole line of sight.
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