Auroral emissions induced by ion precipitation during the 2000 Bastille Day storm

The great geomagnetic storm of 15-16 July, 2000, presented a rare opportunity to study the Sun-Earth system under extreme conditions. Some of the most vivid manifestations of a geomagnetic storm are the polar aurorae that are particularly prominent at ultraviolet wavelengths. We present measurements of middle ultraviolet aurorae taken by the Ionospheric Spectroscopy and Atmospheric Chemistry (ISAAC) experiment on 16 July during the recovery phase of this Bastille Day storm. ISAAC was launched on the Advanced Research and Global Observation Satellite (ARGOS) in a 850~km circular, 0230/1430~LT Sun-synchronous orbit and obtained spectra of Earth's limb that include emissions from O⁺, N⁺, N₂ and NO. Measurements taken over the nighttime southern pole early in the recovery phase of the storm show strong auroral emissions in the O~II 247.0~nm line that are consistent with proton precipitation and charge exchange with thermospheric atomic oxygen. These precipitating protons are also most likely the same ion population mirroring at low altitudes that are responsible for energetic neutral atoms (ENAs) observed from high altitude orbit. We corroborate these ISAAC measurements with simultaneous far ultraviolet spectra of Earth's limb obtained by the Low Resolution Airglow and Aurora Spectrograph (LORAAS) that was also flown on ARGOS.