Evaluation of a method to derive ionospheric conductivities using two auroral emissions (428 and 630 nm) measured with a photometer at Tromsø (69.6°N)

Abstract

This paper mainly aims at evaluating capabilities of derivation of ionospheric conductivities using two principal auroral emissions (427.8 and 630 nm). We have evaluated a photometric method of derivation of ionospheric conductivities based on simultaneous observations of a photometer (field of view = ~1.2°), a digital camera, and the EISCAT UHF radar (field of view = ~0.7°) operated at Tromsø, Norway (69.6°N, 19.2°E), for two nights on October 10 and 11, 2002. We have compared height-integrated Pedersen and Hall conductivities with a post-integration time of 10 s derived from EISCAT UHF radar observations and photometer observations with wavelengths of 427.8 and 630.0 nm. Sky images taken with the digital camera are utilized for distinguishing types of auroras in the views of the EISCAT UHF radar and the photometer. In general, a good agreement of temporal variations of the height-integrated Pedersen and Hall conductivities was found between EISCAT and photometer values. In cases of auroral arcs passing by in the field of view, however, differences in derived values between the two methods were found. Possible causes of the differences are discussed. We conclude that (1) the photometric method using 427.8 and 630 nm can capture temporal variations of the conductivities well, but unavoidable underestimations of the Pedersen (about 30-40%) and the Hall (about 50-60%) conductivities are involved, and (2) care is necessary for using photometric observational data when auroral arcs appear in the field of view.

A new five-wavelength photometer operated in Tromsø (69.6°N, 19.2°E)

“…7, 630.0, 777.4, and 844.6 nm. As already mentioned, the 427.8 nm emission is directly related to the electron flux (Gustavsson et al 2001;Rees and Luckey 1974), and a combination of the 427.8 nm with any of the 630, 777.4, or 844.6 nm emissions will enable deriving a characteristic energy of precipitating electrons (Adachi et al 2017;Lanchester et al 2009). The 557.7 nm emission is the brightest line among those, about five times more intense than that of the 427.8 nm emission.…”

“…5), the incident light is split by dichroic mirrors (No. 6,7,8,9); the first, second, and fourth dichroic mirrors are long-pass filter types, and the third one is a short-pass filter type. Five sets of bandpass filters (No.…”

“…The altitude of the lower border and the altitude of maximum emission depend strongly on the highest and typical energy levels of the precipitating electrons (Gustavsson et al 2001). For example, the emission intensities at wavelengths of 427.8 nm (N 2 + first negative band) and 630.0 nm (metastable atomic state O( 1 D)) have been used to infer the characteristic energies and energy fluxes of them (Adachi et al 2017;Niciejewski et al 1989;Strickland et al 1989;Rees and Luckey 1974). The absolute emission rate of 427.8 nm radiation is related to the electron flux (Gustavsson et al 2001;Rees and Luckey 1974), and the intensity ratio of 427.8-630.0 nm is related to the characteristic energy (e.g., Vondrak and Sears 1978).…”

“…A four-wavelength photometer was operated at the EISCAT (Folkestad et al 1983) Tromsø site (69. 6°N, 19.2°E) between 2002 and 2016, and data were used for derivation of the conductivities (e.g., Adachi et al 2017;Oyama et al 2013) and monitoring the auroral activity (e.g., Takahashi et al 2015). Adachi et al (2017) compared height-integrated conductivities derived from the EISCAT radar measurements and the photometer operated at Tromsø.…”

“…6°N, 19.2°E) between 2002 and 2016, and data were used for derivation of the conductivities (e.g., Adachi et al 2017;Oyama et al 2013) and monitoring the auroral activity (e.g., Takahashi et al 2015). Adachi et al (2017) compared height-integrated conductivities derived from the EISCAT radar measurements and the photometer operated at Tromsø. They used 427.8 nm and 630 nm emissions for derivation of characteristic energy and flux of the auroral particle electrons and derived ionospheric conductivities.…”