Successive Dipolarization Fronts With a Stepwise Electron Acceleration During a Substorm in Saturn's Magnetotail

Abstract

Studies on substorms originated from the discovery of consistent patterns of auroral activity in Earth's ionosphere (Akasofu, 1964). After decades of intense studies, several features of Earth's substorms have become well known (e.g., Haerendel, 2015). At the beginning of a substorm, dayside magnetopause reconnection occurs, and the corresponding magnetic flux is transported to the lobes and loaded into the magnetotail (Dungey, 1961). Then, the increased magnetic pressure within the lobes thins the central current sheet and promotes magnetic reconnection in the magnetotail (Coroniti & Kennel, 1972). Moreover, a pair of connected X-and O-lines form in the plasma sheet (Russell & McPherron, 1973). As the rate of magnetotail reconnection increases, the O-line evolves to the plasmoid and will be released from the central magnetotail. Magnetic dipolarization is generated near the earthward X-line, and then transports the energetic particles into the inner magnetosphere (Russell & McPherron, 1973). Saturn's space environment is different from Earth's, suggesting that Saturn has a different substorm driving mechanism. Compared with Earth (1 AU), Saturn is farther away from the Sun (9.58 AU), and the solar wind near Saturn is relatively weak, with a dynamic pressure of 0.015 nPa (1.7 nPa for Earth). Although magnetopause reconnections occasionally occur at Saturn's magnetopause (Desroche et al., 2013;Masters et al., 2014;Fuselier et al., 2014), the solar wind is not the most important driver of magnetopause reconnection at Saturn. The water-group neutrals released from Enceladus are ionized and serve as the internal plasma source in Saturn's inner magnetosphere (Bagenal & Delamere, 2011). Moreover, due to Saturn's rapid rotation, the magnetic field configuration and the plasmas in Saturn's magnetosphere present the characteristic of rotation. The rotational energy arises from the corotational kinetic energy that planetary rotation exerts on magnetospheric plasma (Ge Abstract Substorms are a fundamental phenomenon in planetary magnetospheric systems. Using Cassini measurements, we report a typical event in which four successive dipolarization fronts (DFs) were observed within 1 hour during the substorm in Saturn's magnetotail. The last three DFs caused a type of stepwise electron acceleration and generated energetic electrons. The pitch angle distributions of the electrons show evidence of the Fermi acceleration mechanism behind these DFs. Therefore, we infer the magnetotail dynamics process during Saturn's substorm: The stepwise acceleration by successive DFs powerfully accelerates the field-aligned electrons and generates field-aligned energetic electrons. These high-energy electrons are injected into the inner magnetosphere and become an important trigger of Saturn's aurora. Our results show an efficient acceleration mechanism for the electrons caused by successive DFs and confirm an important source of energetic particles during the substorm in Saturn, and these findings improve our understanding of Saturn's substorm ...

MAVEN Observations of Tailward Reconnection Front in the Martian Magnetotail

“…RFs play an important role in magnetotail dynamics (e.g., Angelopoulos et al, 2013;Artemyev et al, 2020;Huang et al, 2015a;Jackman et al, 2014;Sundberg et al, 2012). Ion acceleration (e.g., Li et al, 2014;Zhou et al, 2015) as well as electron acceleration (Xu et al, 2020) associated with TRFs have also been investigated. As for our case, the electrons within the 251-905 eV energy range gathered around 90° behind the TRF and the FPR is a growing type, which probably results from betatron acceleration (Fu et al, 2011(Fu et al, , 2012b.…”

“…One possible mechanism to form TRFs is the magnetic reconnection between the crustal field and the draped field in the magnetotail. Using the method suggested by Xu et al (2017), the magnetic field topology is identified as shown in Figure S3 of Supporting Information S1. It seems that there are mostly draped fields around the current sheet in our event.…”

“…RFs in magnetized planets' magnetotail have been widely studied (e.g., Angelopoulos et al., 2013; Artemyev et al., 2020; Fu, Khotyaintsev, Vaivads, André, & Huang, 2012, 2013; Huang, Fu, Vaivads, et al., 2015; Huang, Fu, Yuan, et al., 2015; Huang, Yuan,et al., 2015; Huang et al., 2019; Jackman et al., 2014; Jiang et al., 2020; Liu et al., 2018; Sundberg et al., 2012; Xu et al., 2021, 2022; Zhou et al., 2009, 2010). However, RFs or TRFs in unmagnetized planets (like Mars and Venus) have not been reported until now.…”

“…DFs also contribute to particle acceleration (Artemyev et al, 2012;Ashour-Abdalla et al, 2011;Birn et al, 2014;Deng et al, 2010;Fu et al, 2011;Huang, Zhou, et al, 2012;Huang, Fu, Vaivads, et al, 2015;Huang, Fu, Yuan, et al, 2015;Zhou et al, 2010). Besides on Earth, DFs are also observed in other magnetized planets like Mercury (e.g., Dewey et al, 2017;Sundberg et al, 2012), Saturn (e.g., Jackman et al, 2014;Yao et al, 2017;Xu et al, 2021Xu et al, , 2022, and Jupiter (e.g., Artemyev et al, 2020).…”

Observations of Whistler-mode Waves and Large-amplitude Electrostatic Waves Associated with a Dipolarization Front in the Bursty Bulk Flow

“…DFs are believed to efficiently lead to particle heating and acceleration (Fu et al 2011;Liu et al 2017;Zhong et al 2019a;Ma et al 2020). In Saturnʼs magnetotail, Xu et al (2022) demonstrated that electrons are stepwise accelerated by successive DFs during substorms. The DF has also been suggested to be associated with electron and ion heating in the Jovian magnetotail (Kasahara et al 2011).…”