Loop I/NPS morphology predictions in the ultralong-wavelength band

Loop I/North Polar Spur (NPS) is the giant arc structure above the Galactic plane observed in radio sky. There is a long-standing debate about its origin. While many people believe it is created by nearby supernova remnants (SNRs), some others consider it is a giant bubble close to the Galactic Center (GC), associated with the Fermi Bubble and eROSITA X-ray bubble. In ultralong wavelength band (wavelength $\gtrsim 10$ m or frequency $\lesssim 30$ MHz), particularly below $\sim 10$ MHz, the free-free absorption of radio signal by the diffuse electrons in interstellar medium (ISM) becomes significant, resulting in sky morphology largely differs from higher frequencies. In this paper, we predict the Loop I/NPS morphology at ultralong wavelength band. We develop emissivity models for the two Loop I/NPS origin scenarios. We find that: For SNR model, the full Loop I/NPS is still a bright arc even at frequency as low as $\sim 1$ MHz. In GC model, while above $\sim 3$ MHz it is still visible; below this frequency it is visible only at Galactic latitude $b\gtrsim 30\degree$. At $b\lesssim 30 \degree$ the Loop I/NPS is invisible due to the absorption by ISM electrons between the GC and the Sun. Upcoming space ultralong wavelentgh projects such as DSL and FARSIDE can potentially distinguish these two scenarios and provide decisive information about the origin of Loop I/NPS.
Comment: 11 pages,5 figures