Temporal characteristics and energy deposition of pulsating auroral patches

Abstract

We present a careful statistical analysis of pulsating aurora (PA) using all‐sky green line (557.7 nm) images obtained at 3.3 Hz. Six well‐defined individual PA patches are identified and extracted using a contouring technique. Quantitative parameters such as the patch duration (on‐time and off‐time), peak intensity, and integrated intensity are determined for each patch and each pulsation. The resulting characteristics serve as strict observational constraints that any of the many competing theories attempting to explain PA must predict. The purpose of this paper is to determine the characteristics of PA patches in order to provide better observational constraints on the suggested mechanisms. All aspects of the temporal behavior of the individual patches appear to be erratic. Historically, PA has been defined very loosely and we argue that the use of the term “pulsating” is inappropriate since our findings and other published results are not regularly periodic and thus a more appropriate term may be fluctuating aurora. Further, we find that the observational constraints do not fit well with the flow cyclotron maser theory, which in particular is suggested to create PA patches. There is no clear candidate of the suggested mechanisms and drivers to explain the observational constraints set by the PA patches in a satisfactory manner.

Variations in energy, flux, and brightness of pulsating aurora measured at high time resolution

“…They speculated that the asymmetry was the result of the relaxation oscillator, where the longer fall time is due to diffusion of electrons at adjacent pitch angles during their bounce periods, which becomes less and less effective. A similar feature was also observed by Humberset et al (2016). The main difference between the pulsations with longer fall times seen here and those reported by Jaynes et al (2013) is the repetition rate; the sawtooth pattern covered many pulsation periods, whereas ASK only detected sporadic pulsations with a longer fall time.…”

“…The central question of what causes the (quasi-) periodicity of pulsating aurora is thus still under debate. The pulsations during both events exhibit a shorter average duration of the ON period compared to earlier similar statistical studies, where the duration distribution peaked at about 6 s (Humberset et al, 2016;Yamamoto, 1988). This could be attributed to the separation of rise time, ON time, and fall time in this study, where the average rise and fall time is more than a second each.…”

Observation of pulsating aurora signatures in cosmic noise absorption data

“…While it has been highlighted that the characteristic period of pulsations differs from one beam to another (i.e., from one patch to another), it must also be pointed out that the period of pulsations of a single patch varies within time scales as short as a few minutes. This irregularity in optical pulsation period can be very clearly seen in the ASC data shown in Figure ; it agrees with results obtained by Humberset et al [] based on a study of pulsating patches observed with an all‐sky imager on 1 March 2012 over Poker Flat (Alaska).…”

“…The pulsating patches are not necessarily in phase with each other and may have different periods [e.g., Johnstone, 1978;Smith et al, 1980]. Generally, the fluctuations are characterized by quasiperiodic on and off times [Humberset et al, 2016]. In addition, the pulsating structures tend to have an east-west drift up to about 1 km/s [Davis, 1978].…”

Microscopic Observations of Pulsating Aurora Associated With Chorus Element Structures: Coordinated Arase Satellite‐PWING Observations

“…The overall relationship between various spatial behaviors of pulsating auroras and chorus wave generation is still unclear. There seems to be no clear theoretical and analytical studies to explain this observational constraint that few-hertz modulations of pulsating aurora are caused by the spatial scaling of pulsating auroral patch (see Table 2 in Humberset et al, 2016, for a brief summary of theories of pulsating aurora). Although a rapid motion of a pulsating auroral shape at a few-hertz range was reported by Nishiyama et al (2016) and Fukuda et al (2016), the spatial variations in their studies showed somewhat different characteristics.…”