Proton aurora observed from the ground

Auroral keV proton precipitation is a significant-energy particle input upon the high-latitude ionosphere, often dominating in the polar cusp and the dusk sector of the equatorward auroral oval. A unique signature of proton precipitation is the Doppler-shifted H Balmer lines (Hα, Hβ) observable from the ground. These lines are emitted by energetic H atoms produced within the proton beam through charge-exchange processes. Their observations allow one to assess the location, dynamic evolution especially during magnetospheric substorms, and spectral characteristics of the source regions of the energetic protons projected to the high-latitude ionosphere. They also allow to identify the associated magnetospheric processes and to evaluate ionospheric perturbations induced by the energetic protons. The source regions include the cusp, the low-latitude boundary layer, the mantle, and the plasma sheet, including its dayside extension. If qualitative studies of proton aurora morphology and time variability are possible with photometric observations of hydrogen lines, quantitative assessment of H-emission brightness, and incident proton mean energy and flux, requires spectroscopic measurements of the H-emission profile. In this review paper, we report on the tremendous progress made in the past 20 years in the observational capability applied to proton aurora and in the modeling of energetic proton transport in the upper atmosphere, which is needed for quantitative analysis of the spectroscopic measurements of H emission. The current issues in the field are also discussed and suggestions for future directions are proposed. They include the deployment of chains of instruments dedicated to proton aurora studies along magnetic local time and geomagnetic latitude, such as high-spectral-resolution-imaging spectrographs and spectral imagers. Such campaigns would improve our understanding of the topology and dynamics of the magnetosphere, and provide, at dayside, the azimuthal extent of the reconnection region. Magnetically conjugate experiments and optical instruments dedicated to proton aurora observations in Antarctica are greatly encouraged. The contribution of atmospheric scattering to the H-spectral profiles needs to be further assessed and additional laboratory measurements of differential cross sections are required for a comprehensive understanding of the physics of proton aurora.