Distributed Observations and Modeling of Auroral Dayside Doppler-shifted Hydrogen Profiles Along a Geomagnetic Meridian

Optical ground-based observations of auroral Balmer excited hydrogen atoms at Hα (λ656- nm) and Hβ (λ486-nm) were observed with Ebert-Fastie spectrometers at Ny Ålesund (76.26°N 110.98°E geomagnetic) and Longyearbyen, Svalbard (75.31°N 111.88°E geomagnetic), respectively. These emissions are well known to originate from precipitating protons whose charge exchanges with the atmosphere lead to a diffuse, Doppler-shifted emission region. The instruments are located 118 km apart on a line of roughly constant geomagnetic longitude, thus making this configuration suitable for studying the variation of proton energy with geomagnetic latitude, the "velocity filter" effect. Using the two datasets, analysis of the Hα/Hβ ratio was performed and compared with Monte- Carlo simulations of proton precipitation utilizing in situ particle data as input. In one case, proton energies determined from the blue-shifted wing of the Doppler profiles show an inverse relationship with magnetic latitude, consistent with local ionospheric convection and the velocity filter concept.