Ground‐based observations of diffuse auroral frequencies in the context of whistler mode chorus

Abstract

We present a case study of an active auroral event containing diffuse auroral structures. These data were collected on 04 February 2009 during a ground‐based observational campaign at Poker Flat, Alaska (L ∼ 6), from January through April 2009. They were used to extract the frequencies of pulsating auroral structures that lasted many hours after substorm breakup. There are two main frequency ranges reported, coincident with the spatial resolution capabilities of the two different fields of view of the optical imagers used. The all‐sky imager clearly revealed the pulsations of the large patches, ranging from approximately 50 to 500 mHz. The narrowfield imager measured the pulsations within the finer structure to be in the range of 0.5 to 15 Hz. Both observations are consistent with the frequencies expected from theoretical, modeling and observational work on whistler mode chorus and ECH waves. In particular, the repetition frequencies observed in the visual aurora closely match the observed repetition rate of discrete chorus elements previously described in the literature. There are three significant additional insights gained through these data. The frequency of the pulsations correlates to the intensity of the aurora, with the brighter aurora containing higher frequencies. The scale size of the structures relates to the frequency of the pulsations, with the smaller structures occurring with higher frequency pulsations. The intensity profile of the pulsating structures is not symmetric, with the intensity increasing at a slower rate than it decreases.