Large-Scale Features of the Radio Sky and a Model for Loop I
Open accessAbstract
The large-scale radio/microwave sky has been mapped over a range of frequencies from tens of MHz to tens of GHz, in intensity and polarization. The emission is primarily synchrotron radiation from cosmic ray electrons spiralling in the Galactic magnetic field, in addition to free–free radiation from warm ionized gas. Away from the Galactic plane, the radio sky is dominated by very large (tens of degrees) loops, arcs, spurs and filaments, including the well-known North Polar Spur (NPS), which forms part of Loop I with a diameter of ∼ 120 ∘ . In polarization data, such features are often more discernible due to their high polarization fractions suggesting ordered magnetic fields, while the polarization angles suggest fields that are parallel to the filament. The exact nature of these features are poorly understood. We give a brief review of these features, focussing on the NPS/Loop I, whose polarization directions can be explained using a simple expanding shell model, placing the centre of the shell at a distance of ∼100–200 pc. However, there is significant evidence for a larger distance in the range ∼500–1000 pc, while larger distances including the Galactic Centre are unlikely. We also briefly discuss other large-scale curiosities in the radio sky such as the microwave haze and anti-correlation of H α filaments and synchrotron polarized intensity.
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The shell expansion velocity is significantly slower than the H I/molecular outflows velocities observed at higher latitudes.
“The shell expansion velocity lies within -3 . 5 < v los < 3 . 5 km s -1 , significantly slower than the H i / molecular outflows velocities ( -180 ≲ v los < 280 km s -1 ) observed at higher latitudes (2 . 5 ◦ ≲ b ≲ 10 ◦ ; Dickinson 2018; Di Teodoro et al. 2020).”
From The Fermi/eROSITA bubbles: a look into the nuclear outflow from the Milky Way (2024) · cites this paper as Dickinson 2018
Section 3.1 describes an expanding shell model for Loop I but gives no shell expansion velocity. A search for the stated velocity ranges and higher-latitude H I/molecular outflows found no comparison in our copy; the velocities in Section 2.3 refer instead to Hα emission. (LLM, conf=0.73)
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Sarkar, K. C. (2019) · Monthly Notices of the Royal Astronomical Society · doi:10.1093/mnras/sty2944 · Cited by 27 · see citing works
Constraining the distance to the North Polar Spur with Gaia DR2
Das, K. K., Zucker, C., Speagle, J. S., Goodman, A. A., Green, G. M., & Alves, J. (2020) · Monthly Notices of the Royal Astronomical Society · doi:10.1093/mnras/staa2702 · Cited by 22 · see citing works
“E-mail: kaustavn@iitk.ac.in † E-mail: catherine.zucker@cfa.harvard.edu theory can be found in the review paper on NPS and Loop-I by Dickinson (2018).”
BackgroundResultInfluentialCOMAP Early Science. VI. A First Look at the COMAP Galactic Plane Survey
Rennie, T. J., Harper, S., Dickinson, C., Philip, L., Cleary, K., Bond, J. R., Borowska-Naguszewska, J., Breysse, P. C., Catha, M., Cepeda-Arroita, R., et al. (2022) · The Astrophysical Journal · doi:10.3847/1538-4357/ac63c8 · Cited by 18 · see citing works
“Furthermore, radio data provide unique information on the Galaxy not easily seen at other wavelengths such as large radio loops (Planck Collaboration et al. 2016a; Dickinson 2018).”
BackgroundSelf-citationLallement, R. (2022) · Comptes Rendus Physique · doi:10.5802/crphys.97 · Cited by 17 · see citing works
Asymmetric eROSITA bubbles as the evidence of a circumgalactic medium wind
Mou, G., Sun, D., Fang, T., Wang, W., Zhang, R., Yuan, F., Sofue, Y., Wang, T., & He, Z. (2023) · Nature Communications · doi:10.1038/s41467-023-36478-0 · Cited by 16 · see citing works
Morphological analysis of the polarized synchrotron emission with WMAP and Planck
Martire, F. A., Banday, A. J., Martínez-González, E., & Barreiro, R. B. (2023) · Journal of Cosmology and Astroparticle Physics · doi:10.1088/1475-7516/2023/04/049 · Cited by 11 · see citing works
“Filament XV is a bright structure at the center of the Galactic Haze [46].”
BackgroundTesting for spectral index variations in polarized CMB foregrounds
de Belsunce, R., Gratton, S., & Efstathiou, G. (2022) · Monthly Notices of the Royal Astronomical Society · doi:10.1093/mnras/stac2825 · Cited by 10 · see citing works
North Polar Spur: Gaseous plume(s) from star-forming regions ∼3–5 kpc from the Galactic Center?
Churazov, E., Khabibullin, I. I., Bykov, A. M., Чугай, Н. Н., Sunyaev, R., Утробин, В. П., & Зинченко, И. И. (2024) · Astronomy and Astrophysics · doi:10.1051/0004-6361/202451762 · Cited by 10 · see citing works
Synchrotron emission from virial shocks around stacked OVRO-LWA galaxy clusters
Hou, K.-C., Hallinan, G., & Keshet, U. (2023) · Monthly Notices of the Royal Astronomical Society · doi:10.1093/mnras/stad785 · Cited by 9 · see citing works
eROSITA narrowband maps at the energies of soft X-ray emission lines
Zheng, X., Ponti, G., Locatelli, N. T., Sanders, J. S., Merloni, A., Becker, W., Comparat, J., Dennerl, K., Freyberg, M. J., Maitra, C., et al. (2024) · Astronomy and Astrophysics · doi:10.1051/0004-6361/202449398 · Cited by 8 · see citing works
Signatures of Recent Cosmic-Ray Acceleration in the High-latitude Gamma-Ray Sky
Jóhannesson, G., & Porter, T. A. (2021) · The Astrophysical Journal · doi:10.3847/1538-4357/ac01c9 · Cited by 6 · see citing works
“The distances to the radio loop structure, in particular, the NPS and radio Loop I, have been the subject of debate in the literature since their discovery (e.g., Dickinson 2018; Kataoka et al. 2018).”
BackgroundExcess cataloged X-ray and radio sources at galaxy-cluster virial shocks
Ilani, G., Hou, K.-C., & Keshet, U. (2024) · Journal of Cosmology and Astroparticle Physics · doi:10.1088/1475-7516/2024/10/008 · Cited by 5 · see citing works
The inside-out quenching of the MHONGOOSE galaxy NGC 1371
Veronese, S., de Blok, W. J. G., Fraternali, F., Maccagni, F. M., Healy, J., Kleiner, D., Oosterloo, T. A., & Morganti, R. (2025) · Astronomy and Astrophysics · doi:10.1051/0004-6361/202555735 · Cited by 3 · see citing works
The TeV Spectral Bump of Cosmic-Ray Protons and Helium Nuclei: The Role of Nearby Supernova Remnants
Bhadra, S., Thoudam, S., Nath, B. B., & Sharma, P. (2025) · The Astrophysical Journal · doi:10.3847/1538-4357/ade796 · Cited by 3 · see citing works
On the Hα faintness of the North Polar Spur
Sofue, Y., Kataoka, J., & Iwashita, R. (2023) · Monthly Notices of the Royal Astronomical Society · doi:10.1093/mnras/stad1985 · Cited by 2 · see citing works
Loop I/NPS Morphology Predictions in the Ultralong-wavelength Band
Cong, Y., Yue, B., Xu, Y., Deng, F., Zhang, J., & Chen, X. (2025) · The Astrophysical Journal · doi:10.3847/1538-4357/adc9a4 · Cited by 1 · see citing works
“Because of its huge angular size ( ∼ 120 °, Dickinson 2018), many people consider it to be a nearby object within ≲ 1 kpc, but the exact distance and geometry are still uncertain.”
InfluentialIrfan, M. O., Bull, P., Santos, M. G., Wang, J., Grainge, K., Li, Y., Carucci, I. P., Spinelli, M., & Cunnington, S. (2021) · Monthly Notices of the Royal Astronomical Society · doi:10.1093/mnras/stab3346
Marins, A. R., Elcio Abdalla, A., Vieira, J., Formigari, L., Abdalla, F. B., de Queiroz, A. R., & Wang, B. (2024) · Springer eBooks · doi:10.1007/978-3-031-55098-0_10
Multiband Studies of The Interstellar Medium
Зинченко, И. И. (2025) · Physics of Space: All-Russian Student Scientific Conference with International Participation · doi:10.15826/b978-5-7996-3986-0.03
Diffuse Continuum Emission and Large Extended Sources at MeV Energies
Ackermann, M., Malyshev, D., & Malyshev, D. V. (2025) · Space Science Reviews · doi:10.1007/s11214-025-01215-0 · Cited by 1 · see citing works
“One of the main arguments in favor of a local origin of Loop I is the polarization of light from stars behind a synchrotron emitting region (Vidal et al. 2015; Dickinson 2018).”
Hot corinos : the early organic molecular enrichment of the planet formation zones
Simone, M. D. (2022) · theses.fr (ABES) · doi:10.70675/b8a0df50zb30ez413az9086z73aeed100a90
Anonymous, Bao, Y. W., & Liu, Y. (2026) · Physical review. D/Physical review. D. · doi:10.1103/sys3-nzl5
- Authors
- Dickinson, C.
- Affiliations
- University of Manchester (GB)
- Corresponding author
- Dickinson, Clive
- ORCID
- Dickinson, Clive
- Venue
- Galaxies
- Publisher
- MDPI AG
- Published
- 2018-05-22
- Volume
- 6
- Issue
- 2
- Pages
- 56
- ISSN
- 2075-4434 (ISSN-L)
- OpenAlex
- W2803112055
- Cited by
- 31
- References
- 72
- Citations by year
- 2026: 1 · 2025: 5 · 2024: 5 · 2023: 5 · 2022: 6 · 2021: 5 · 2020: 1 · 2019: 2 · 2018: 1
- FWCI
- 1.19
- Type
- journal article
- Language
- en
- Open access
- gold
- doi.org
- Licence
- Locations
- Galaxies (publishedVersion, cc-by)
- Research Explorer (The University of Manchester) (submittedVersion, other-oa)
- Funders
- Science and Technology Facilities Council (ST/P000649/1)
- Topics
- Physical Sciences › Physics and Astronomy › Nuclear and High Energy Physics › Astrophysics and Cosmic Phenomena
- Physical Sciences › Physics and Astronomy › Astronomy and Astrophysics › Astrophysics and Star Formation Studies
- Physical Sciences › Physics and Astronomy › Astronomy and Astrophysics › Astrophysical Phenomena and Observations
- Keywords
- Loop I, North Polar Spur, synchrotron radiation, cosmic ray electrons, galactic magnetic fields, radio polarization, free-free emission, warm ionized gas, H-alpha filaments, Galactic Center
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