Title: 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$(^{1}$D) 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 $10^{11}$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$(^{1}$D) 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.