Statistical properties of internal modulation in pulsating aurora

Abstract. We statistically investigated the occurrence characteristics of internal modulation (IM), a rapid intensity modulation of a few hertz embedded in pulsating aurora (PsA), using high-speed ground-based optical data obtained with qCMOS cameras. For datasets acquired at Kiruna, Sweden, and Skibotn, Norway, we applied a two-step hierarchical machine-learning classification, first identifying the presence of PsA and then determining whether IM was present within the PsA. This enabled IM, which has previously been examined mainly through case studies, to be quantified for the first time in a systematic and statistically comparable manner. The results show that the fraction of PsA accompanied by IM increases toward the morning-sector magnetic local time (MLT) sector and tends to be higher at the lower-latitude site, Kiruna, than at the higher-latitude site, Skibotn. Representative examples from the classification results show that, on the midnight sector, IM commonly appears as a hierarchical structure superposed on clear main pulsations, whereas on the morning sector, the main pulsation tends to be relatively weak and the IM component often becomes dominant, suggesting that the morphology of IM itself also depends on MLT. These results suggest that IM is not merely an apparent subtype accompanying PsA, but rather a diagnostic feature that may provide insight into the wave--particle interaction underlying PsA.