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

Abstract

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.

Seasonal and Solar Cycle Variations of Thermally Excited 630.0 nm Emissions in the Polar Ionosphere

“…A description of the ESR is given by Wannberg et al (1997). The handling of the ESR data for the period of interest in this paper is exactly the same way as compared to Kwagala et al (2018) and is described in detail therein and is quickly summarized here. The Naval Research Laboratory Mass Spectrometer and Incoherent Scatter Radar 2000 model (NRLMSISE-00; Picone et al, 2002) has been used to generate the number density of atomic oxygen.…”

“…Earlier literature (e.g., Kozyra et al, ; Lockwood et al, ) reports 3000 K as the electron temperature beyond which thermally excited emissions can become important. Kwagala et al () found an additional population of thermally excited emissions at lower temperatures (∼2300–3000 K) when extreme electron densities (>5 × 10 11 m −3 ) prevailed.…”

“…Thermally excited 630.0 nm emissions in the polar ionosphere arise when the ambient ionospheric electrons are rapidly heated by precipitating soft electrons and cooled via excitation of atomic oxygen to the 1‐D state, which then de‐excites via emission of the 630.0 nm line (Carlson, ; Carlson et al, ; Johnsen et al, ; Kozyra et al, ; Kwagala et al, , ; Lockwood et al, ). Kwagala et al () found that thermally excited emissions can contribute more than 50% of the observed 630.0 nm emission intensity in the cusp ionosphere.…”

“…In this paper, we therefore investigate seasonal and solar cycle variations of thermally excited emissions. This is the second paper in the series after Kwagala et al (2018). European Incoherent Scatter (EISCAT) Svalbard Radar (ESR) measurements are used to derive the thermal excitation component.…”

Fine-scale dynamics of fragmented aurora-like emissions

“…An electron temperature greater than about 2300 K is required to thermally excite significant OI 630.0 nm emission (Carlson et al, 2013;Kwagala et al, 2017), which has a much lower excitation threshold (1.96 eV) than the emission observed by ASK. The electron temperature during event 1 was below 2000 K, and therefore, there cannot be any significant thermally excited emission, suggesting the presence of a non-thermal (i.e.…”