Dune aurora: Statistical survey from a citizen science database

Abstract. Auroral forms can provide information not only on the state of near-Earth space but also on conditions in the lower-thermosphere–ionosphere. The so-called dune aurora, consisting of brighter stripes forming a wave-like pattern in the dim, diffuse green aurora, has been hypothesised as being an optical signature revealing the presence of large-scale atmospheric waves above or near the mesopause. However, only a few dune aurora events have been studied to date, leaving many open questions regarding the nature of this phenomenon. We carry out the first statistical analysis of dune aurora events by collecting citizen science observations of the dunes since 2000 using the Skywarden (https://taivaanvahti.fi) database of observations. From a total of 289 dune aurora observations made during 56 different events by citizen scientists from Northern Europe, North America, Australia, and New Zealand, we investigate the distribution of dune events as a function of location, month, local time, solar wind and interplanetary magnetic field (IMF) conditions, and geomagnetic activity. We compare those distributions to that of all the aurora observations reported in Skywarden since 2000. We also estimate the duration of dune events based on the available observations, and we investigate a possible relationship between dune aurora and equivalent current patterns derived from ground-based magnetometer measurements. We find that the vast majority of dune observations take place near the equatorward boundary of the auroral oval, in the dusk sector earlier than the peak in all auroral report distribution, and in association with strong (in most cases eastward but occasionally westward) auroral electrojet signatures. The dune observations are often associated with elevated solar wind density and IMF magnitude, and the IMF B_y component may play a role in their formation. Finally, their monthly distribution peaks in March and October, which could be the result of a combination of geomagnetic, atmospheric, darkness, and cloudiness conditions needed for them to form.