Rayleigh‐Taylor type instability in auroral patches

Based on observations by a high‐resolution narrow field‐of‐view CCD camera, we found small‐scale (5–25 km) finger‐like structures at the western boundary of auroral patches in images obtained at Gillam (geomagnetic latitude: 65.5°N), Canada, in January 2008. Since shear motion was not observed along the boundary of the patches, we suggest that these structures are formed by macroscopic Rayleigh‐Taylor type plasma instability arising in the magnetospheric equatorial plane from the force balancing of the (eastward) magnetic tension force and the (westward) pressure gradient force.