Statistical study of auroral fragmentation into patches

Auroral complex shapes are formed through the connection of the ionosphere and magnetosphere by the geomagnetic field lines, projecting disturbances in the magnetosphere onto the ionosphere through auroral particles. Thus, the study of auroral dynamics is important for the understanding of magnetospheric disturbances. Shiokawa et al. [JGR, 2010] reported observations of small-scale finger-like auroral structures which appeared on the west side of auroral patches, at Gillam (GLAT=65.5°N), Canada, suggesting a pressure-driven plasma instability in the magnetosphere. However, statistical characteristics of this phenomenon have not been investigated yet. Using an all-sky imager at Tromsoe (MLAT=67.1°N), Norway, from January 2009 to November 2012, we made a statistical analysis of the occurrence conditions of 19 events of auroral structures that seemed to be caused by pressure-driven instability. We found 14 large-scale finger-like structures, developed from auroral arcs, and 6 small-scale finger-like structures which appeared in auroral patches. The large-scale structures were seen from midnight to dawn MLT and small-scale structures were seen mainly at dawn. Large and small-scale structures tend to appear at the beginning and late recovery phase of substorms, respectively. Their scale sizes are larger than the gyro-radius of the ions in the magnetospheric equatorial plane, indicating that the finger-like structures are caused by MHD instabilities. However, the size of the small-scale structures is roughly two times that of the gyro-radius of the ions, suggesting that the ion finite Larmor radius effects may play a role in the shape of small-scale structures [Hiraki and Sakaguchi, 2010]. Additionally, the eastward propagation speeds of the finger-like structures are slower than the typical midnight auroral drift speed, indicating that the low-energy plasma may be the source of these structures. These results could contradict the idea that the high-energy particles lead to a pressure-driven instability.