Early Evidence of Isolated Auroral Structures in the 100 km Height Regime Observed at Subauroral Latitudes by the Aurora Pioneer Carl Størmer

Recently, Hunnekuhl & MacDonald (2020) reported on early observations of isolated aurora-like structures by the aurora pioneer Carl Størmer classified by him as feeble homogeneous arcs of great altitude. His descriptions and height measurements are in good agreement with recent observations of STEVE (Strong Thermal Emission Velocity Enhancement). It is hardly known that Størmer did also report on pulsating auroral structures observed isolated from the main auroral activity at subauroral latitudes in the 100 km height regime, some of them stationary relative to ground over periods of ~ 1 hour. He published detailed descriptions as well as height and position measurements in the less known Norwegian journal Geofysiske Publiskasjoner in 1942 and in his book The Polar Aurora in 1955. His descriptions show strong similarities with descriptions of detached subauroral arcs and patches first reported in the modern literature by Anger et al. (1977) and Moshupi et al. (1978, 1979) based on ISIS-2 data. This work reminds Størmer's early work on isolated aurorae in the 100 km height regime and put it in context with more recent observations of detached subauroral arcs and patches including those from citizen scientists hardly considered in the current research on subauroral aurorae.