Abstract
While the pulsating auroral phenomena have been recognized and studied for decades, our understating of their generation mechanisms remains incomplete to date. In one main class of pulsating auroras which is termed “patchy pulsating auroras” (PPA), the auroral patches are found to basically maintain their shape and size over many pulsation cycles. Also, PPAs are repeatedly found to essentially co-move with the ExB convection drift. The above properties led many researchers to hypothesize that PPA might connect to a structure of enhanced cold plasma in the magnetosphere. In this study, we review the existing evidence, and provide new perspective and support, of the low-energy plasma structure potentially associated with PPA. Based on observations from both the magnetosphere and the topside ionosphere, we suggest that ionospheric auroral outflows might constitute one possible source mechanism of the flux tubes with enhanced low-energy plasma that connect to the PPA. We also review the existing theories of pulsating auroras, with particular focus on the role of low-energy plasma in these theories. To date, none of the existing theories are complete and mature enough to offer a quantitatively satisfactory explanation of pulsating auroras. At last, we suggest a few future research directions to advance our understanding of pulsating auroras: a) more accurate measurements of the cold plasma density, b) more developed theories of the underlying mechanisms of ELF/VLF wave modulation, and c) auxiliary processes in the topside ionosphere or near-Earth region accompanying pulsating auroras.
Highlights
Pulsating aurora is an auroral form that undergoes quasi-periodic fluctuations in intensity (Johnstone, 1978)
One notable advantage of Takahashi et al (2019) study over the above-mentioned studies using SuperDARN radar lies in that, Takahashi et al (2019)’s multi-instrumental measurements offered height information of ion velocities. Their results showed that the electric field estimated from the drift speed of auroral patches approximately corresponded with the convective electric field derived from European incoherent scatter (EISCAT) and Kilpisjärvi Atmospheric Imaging Receiver Array (KAIRA), indicating that the motion of these auroral patches was overall governed by the magnetospheric convection, though height-dependent subtle variations due to the polarization electric field generated within the patch exist
Given the notion that the pulsating auroras could be the direct consequence of the quasi-periodic modulation of the whistler-mode chorus, their studies are still deemed relevant to our research objective, though it is impossible to check whether the hypothesized pulsating auroras are of patchy pulsating auroras (PPA) or amorphous pulsating auroras (APA) types
Summary
Pulsating aurora is an auroral form that undergoes quasi-periodic fluctuations in intensity (Johnstone, 1978). One other area of important new findings of pulsating auroras in the last decade, thanks to the deployment of high-timeresolution imagers, is the details of rapid modulation embedded in each on-time, and the subtleties and high variabilities in the on- and off-time durations, of pulsating auroras [e.g., Samara et al (2010); Nishiyama et al (2014; 2016); Humberset et al (2016); Dahlgren et al (2017)] Though those fine-scale temporal characteristics are not of the core interest of this review, some of the results may shed light on the possible role of low-energy plasma in pulsating auroras.
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