On the Persistent Shape and Coherence of Pulsating Auroral Patches

Abstract

The pulsating aurora covers a broad range of fluctuating shapes that are poorly characterized.The purpose of this paper is therefore to provide objective and quantitative measures of the extent to which pulsating auroral patches maintain their shape, drift and fluctuate in a coherent fashion. We present results from a careful analysis of pulsating auroral patches using all-sky cameras. We have identified four well-defined individual patches that we follow in the patch frame of reference. In this way we avoid the space-time ambiguity which complicates rocket and satellite measurements. We find that the shape of the patches is remarkably persistent with 85-100% of the patch being repeated for 4.5-8.5 min. Each of the three largest patches has a temporal correlation with a negative dependence on distance, and thus does not fluctuate in a coherent fashion. A time-delayed response within the patches indicates that the so-called streaming mode might explain the incoherency. The patches appear to drift differently from the SuperDARN-determined ⃗ E × ⃗ B convection velocity. However, in a nonrotating reference frame the patches drift with 230-287 m/s in a north eastward direction, which is what typically could be expected for the convection return flow.

Patch Size Evolution During Pulsating Aurora

“…The individually tracked patches are a sample of a size spectrum for more stable or long-lived patches. The lifetimes of individual patches analyzed by Humberset et al (2018) were indeed 7-10 min. Furthermore, three of their four analyzed patches keep at least 85% of their shape during their lifetime.…”

“…Tracking methods used by Grono et al (2017) and Humberset et al (2016) produce good estimates of the patch propagation, but the patch detection and tracking is limited to stable structures. More recently, Humberset et al (2018) investigated the morphology of four individual patches. They reported patch sizes of 1,000-5,000 km 2 assuming an emission height of 110 km (horizontal scale sizes of some 10 km).…”

Diffuse and Pulsating Aurora

“…This suggests that convection of cold plasma is not the only process that controls pulsating aurora dynamics. Some patches indeed do not follow the E×B drift speed (Grono et al, 2017, Grono andHumberset et al, 2018).…”

“…Each patch generally shows a series of pulsations that typically lasts for several minutes, and the pulse amplitude and shape stay constant for the duration (Yamamoto, 1988;Humberset et al, 2018). Pulsation of each patch is incoherent with neighboring patches; the pulsation period and phase of a patch are independent of those at other patches (Scourfield et al, 1972).…”

Constraining the Source Regions of Pulsating Auroras

“…Pulsating auroras typically have an irregular, patchy structure (Royrvik & Davis, 1977) that constantly evolves (Shiokawa et al, 2010). Individual features Geophysical Research Letters 10.1029/2019GL084611 have been measured to have diameters between 1 and 100 km (Royrvik & Davis, 1977), areas between 100 and 4,800 km 2 (Humberset et al, 2018), and thicknesses between 15 and 25 km (Jones et al, 2009).…”