Undulations on the equatorward edge of the diffuse proton aurora: TIMED/GUVI observations

Undulations on the equatorward edge of the diffuse proton aurora have been identified by using TIMED/GUVI auroral images in the far ultraviolet wavelengths. While undulations have been previously reported on the duskside (Lui et al., 1982), GUVI observations show the undulation also occurs in the dayside, nightside, and morningside. The GUVI proton auroral images provide direct optical evidence that the undulations occur in the proton aurora. It is also the first detection of the undulation in the dayside indicating strong convection shear in the region. The undulation in the nightside, a wavy structure in the whole diffuse proton aurora, is significantly different from those in the duskside and dayside. While almost all of the undulation events are observed during magnetic storms (Dst \< -60 nT), one exceptional case shows undulation in the dayside with Dst = 30 nT. However, the case is associated with a large solar wind speed (650 km/s) and a high dynamic pressure (14 nPa). Coincident DMSP SSIES observations suggest that both large ion drift velocity (\>1000 m/s) and strong velocity shear (\>0.1 s-1) within the diffuse aurora oval are necessary conditions for the undulation to occur. The SSIES data also indicate the areas with large ion drift velocity and shear move to higher latitudes in the MLT sectors toward midnight. This may explain why the undulation is rarely detected in the nightside.