How Often Do Thermally Excited 630.0 nm Emissions Occur in the Polar Ionosphere?

• Published in: Journal of geophysical research. Space physics Vol. 123; no. 1; pp. 698 - 710
• Main Authors: Kwagala, Norah Kaggwa, Oksavik, Kjellmar, Lorentzen, Dag A., Johnsen, Magnar G.
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 01-01-2018; American Geophysical Union
• Subjects: Atmospheric models; Atomic oxygen; cusp; Electron density; Electron temperatures; Electrons; Emissions; ESR; Fysikk: 430; Geofag: 450; Geosciences: 450; Interplanetary magnetic field; Ionosphere; Magnetic fields; Magnetic reconnection; Matematikk og Naturvitenskap: 400; Mathematics and natural science: 400; Oxygen; Petroleum geology and petroleum geophysics: 464; Petroleumsgeologi og -geofysikk: 464; Physics: 430; Polar ionosphere; Radar; Solar magnetic field; Solar wind; thermally excited 630 nm; VDP
• ISSN: 2169-9380; 2169-9402; 2169-9402
• DOI: 10.1002/2017JA024744

This paper studies thermally excited emissions in the polar ionosphere derived from European Incoherent Scatter Svalbard radar measurements from the years 2000–2015. The peak occurrence is found around magnetic noon, where the radar observations show cusp‐like characteristics. The ionospheric, interplanetary magnetic field and solar wind conditions favor dayside magnetic reconnection as the dominant driving process. The thermal emissions occur 10 times more frequently on the dayside than on the nightside, with an average intensity of 1–5 kR. For typical electron densities in the polar ionosphere (2 × 1011 m−3), we find the peak occurrence rate to occur for extreme electron temperatures (>3000 K), which is consistent with assumptions in literature. However, for extreme electron densities (>5 × 1011 m−3), we can now report on a completely new population of thermal emissions that may occur at much lower electron temperatures (∼2300 K). The empirical atmospheric model (NRLMSISE‐00) suggests that the latter population is associated with enhanced neutral atomic oxygen densities.
Key Points
Thermally excited emissions maximize in the cusp around magnetic noon
Peak occurrence at electron temperatures exceeding 3000 K
Peak occurrence at electron densities exceeding 5 × 1011 m−3 for lower electron temperatures (2300–3000 K)