First ground-based optical analysis of H_β Doppler profiles close to local noon in the cusp

Abstract. Observations of hydrogen emissions along the magnetic zenith at Longyearbyen (78.2 N, 15.8 E geographic) are used to investigate the energy and source of protons precipitating into the high latitude region. During the hours around local solar noon (11:00 UT), measurements of the hydrogen Balmer β line are severely affected by sunlight, such that most data until now have been disregarded during these times. Here we use a simple technique to subtract sunlight contamination from such spectral data. An example is shown in which the removal of twilight contamination reveals a brightening of H_β aurora over Svalbard on 27 November 2000 between 08:00 UT and 10:00 UT, which is centred on magnetic noon (08:48 UT). These data were measured by the High Throughput Imaging Echelle Spectrograph (HiTIES), one instrument on the Southampton-UCL Spectrographic Imaging Facility (SIF). Data from the IMAGE satellite confirms the location of a cusp "spot" over Svalbard at the time of the ground-based measurements, which moved in response to changes in the IMF conditions. A coincident pass of the DMSP F12 satellite provided input spectra for modelling studies of the H_β profiles, which confirm that the method for removing the twilight contamination is robust. The results described here are the first ground-based optical measurements of H_β Doppler profiles from the cusp region close to local solar noon, when scattered sunlight swamps the raw data.