Differentiating Diffuse Aurora

Pulsating aurora is a pervasive early morning auroral display. Perhaps because of the awkward viewing hours, it received comparatively little attention in past years and is often discussed as if it is just one phenomenon. However, pulsating aurora can be differentiated into at least three types based on the extent of their pulsation and structuring (Grono and Donovan, 2018). Amorphous pulsating aurora (APA) is characterized by extensive pulsation and a lack of persistent structuring. In contrast, patchy pulsating aurora (PPA) and patchy aurora (PA) features can persist for tens of minutes and follow ionospheric convection (Grono et al., 2017). While PPA structures pulsate over much of their area, PA is mostly non-pulsating. The most common pulsating aurora is APA, which is nearly ubiquitous during the early morning and can appear earlier than PPA and PA (Grono and Donovan, 2019b). Pulsating auroras do not occur poleward of the proton aurora, only within or equatorward of it (Grono and Donovan, 2019a). PPA and PA appear predominantly equatorward of the boundary between stably trapped protons and those scattered by tight magnetic field curvature. This suggests that the processes responsible for the patchiness of PPA and PA are constrained to the mostly dipolar inner magnetosphere. Nishimura et al. (2010) demonstrated that there can be a nearly one-to-one correspondence between plasma wave power and auroral brightness. Consequently, auroral brightness should be able to act as a proxy for wave power, and wave structuring should indicate which type of pulsating aurora a spacecraft is observing. PA should be associated with unmodulated plasma waves persisting for the length of time it takes for a spacecraft to transit the source region of a patch. The location of such wave structuring coincides with where PA occurs (Grono and Donovan, 2019c). Sunrise prevents optical observations of the aurora from continuing past dawn, making it difficult to assess how long pulsating aurora can persist for. However, these plasma wave observations indicate that the processes responsible for PA and PPA structuring may continue into the afternoon