The North Polar Spur (NPS) consists of the H I shell on the outside and the radio continuum shell RC NPS containing ionized gas and a region of diffuse X-ray enhancement. It was found that the parallel component of the magnetic field in the RC NPS is greater than 1.2 Gauss, and the electron density is below 0.4/cu cm. The ionization of the gas in RC NPS may be caused by hot stars inside the NPS or by X-ray emission; relativistic electrons are confined in Alfven waves by thy ionized gas. The model explains several features of the RC NPS including its forming only a portion of a complete loop adjacent to the H I NPS.
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Heiles et al. (1980) studied this question.