On the contribution of thermal excitation to the total 630.0 nm emissions in the northern cusp ionosphere

Abstract

Direct impact excitation by precipitating electrons is believed to be the main source of 630.0 nm emissions in the cusp ionosphere. However, this paper investigates a different source, 630.0 emissions caused by thermally excited atomic oxygen O(1D) when high electron temperature prevail in the cusp. On 22 January 2012 and 14 January 2013, the European Incoherent Scatter Scientific Association (EISCAT) radar on Svalbard measured electron temperature enhancements exceeding 3000 K near magnetic noon in the cusp ionosphere over Svalbard. The electron temperature enhancements corresponded to electron density enhancements exceeding 1011 m−3 accompanied by intense 630.0 nm emissions in a field of view common to both the EISCAT Svalbard radar and a meridian scanning photometer. This offered an excellent opportunity to investigate the role of thermally excited O(1D) 630.0 nm emissions in the cusp ionosphere. The thermal component was derived from the EISCAT Radar measurements and compared with optical data. For both events the calculated thermal component had a correlation coefficient greater than 0.8 to the total observed 630.0 nm intensity which contains both thermal and particle impact components. Despite fairly constant solar wind, the calculated thermal component intensity fluctuated possibly due to dayside transients in the aurora.

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

“…Based on a statistical study, Zhou et al () reported the cusp to be located between 0800 and 1600 MLT, which is coincident with the highest occurrence rate in our study. This also backs up the fact that all the cases reported in literature (e.g., Carlson et al, ; Kwagala et al, ; Wickwar & Kofman, ) were in the cusp region, since our results suggest higher probability of such events at this location compared to the other magnetic local times. This may also be linked to the difference between the typical average energies of precipitating electrons on the nightside and dayside in the polar ionosphere, which are hard and soft, respectively (e.g., Newell et al, ).…”

“…This is very similar to the emission rates' altitudes in the model presented by Figure 6), where the peak volume emission rate 10.1002/2017JA024744 for thermal excitation was at ∼350 km altitude. This peak emission altitude (∼350 km) for thermally excited emission also agrees with other studies (e.g., Egeland et al, 1992;Kwagala et al, 2017;Wickwar and Kofman, 1984), however, a little lower than 400-500 km from some studies (e.g., Carlson et al, 2013;Kozyra et al, 1990).…”

“…Some studies have reported underestimates of 20–30% during an active high‐latitude ionosphere (e.g., Forbes et al, ; Liu & Lühr, ). When Kwagala et al () accounted for the 30% underestimate, the calculated thermal component increased by 10–15% and therefore concluded that the neutral density from the model gives a lower limit of the thermal component. On a general view of the NRLMSISE‐00 model compared to the other atmospheric models, Pardini et al () investigated uncertainty and bias in upper atmosphere models during the solar cycle 23 and found that the NRLMSISE‐00 model had the minimum biases at all altitudes among the models that were analyzed.…”

“…reported detection of thermal 630.0 nm emissions in the cusp using time/space agreement between calculations and observations. Using the same formulae,Kwagala et al (2017) presented a direct comparison between the calculations and 10Occurrence rate of the strong thermally excited emissions at different electron temperatures, densities, modeled neutral atomic oxygen densities, emission rates, and altitudes. Electron temperature versus density for (a) electrons and (b) atomic oxygen.…”

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

“…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. On the basis of existing knowledge, we have done a series of statistical studies of thermally excited 630.0 nm emissions in the polar ionosphere.…”