Over-darkening of pulsating aurora

Recent analyses of high-time resolution ground-based optical observations of pulsating aurora (PsA) have reported that the brightness of PsA sometimes decreases below the diffuse background level immediately after the ON phase of the main pulsation finishes. To date, however, the generation mechanism of such an “over-darkening PsA” is still unclarified. In this study, we investigated the characteristics of the over-darkening PsA by using simultaneous observations of PsA with an electron multiplying charge coupled device all-sky camera in Sodankylä, Finland and the Arase satellite. During one of the conjunction events in Scandinavia on March 29, 2017, almost all the PsA pulses showed clear over-darkening characteristics. By analyzing the 2D all-sky images at the times of over-darkening we discovered that over-darkening areas appeared in the trailing edge of PsA patches and moved in tandem with the poleward propagating patches. It was also found that similar over-decreasing characteristics were not seen in the chorus data from the wave instruments onboard Arase located at the magnetospheric counterpart of PsA. These results indicate that the over-darkening PsA is not caused by a temporal variation of chorus at a fixed point, but is produced by a propagation of over-darkening area with PsA patches. That is, the over-darkening PsA is a result of compounding effects of spatial structure and recurrent propagation of PsA. The mechanism creating the dark area is still unknown, but the existence of over-darkening PsA suggests that the temporal variation of PsA is not always a perfect copy of the modulation of lower-band chorus waves in the magnetosphere.