Abstract
Abstract The internal mass source from the icy moon Enceladus in Saturn’s rapidly rotating magnetosphere drives electromagnetic dynamics in multiple spatial and temporal scales. The distribution and circulation of the internal plasma and associated energy are thus crucial in understanding Saturn’s magnetospheric environment. Magnetic reconnection is one of the key processes in driving plasma and energy transport in the magnetosphere, and also a fundamental plasma process in energizing charged particles. Recent works suggested that reconnection driven by Saturn’s rapid rotation might appear as a chain of microscale structures, named drizzle-like reconnection. The drizzle-like reconnection could exist not only in the nightside magnetodisk, but also in the dayside magnetodisk. Here, using in situ measurements from the Cassini spacecraft, we report multiple reconnection sites that were successively detected during a time interval longer than one rotation period. The time separation between two adjacently detected reconnection sites can be much less than one rotation period, implying that the reconnection processes are likely small-scale, or frequently repetitive. The spatial distribution of the identified long-standing multiple small reconnection site sequences shows no significant preference on local times. We propose that the small reconnection sites discussed in this Letter are rotationally driven and rotate with the magnetosphere. Since the reconnection process on Saturn can be long-durational, the rotational regime can cause these small-scale reconnection sites to spread to all local times, resulting in global release of energy and mass from the magnetosphere.
Highlights
Saturn’s magnetosphere receives mass and energy from internal sources and the solar wind
Plasmoids and dipolarizations triggered by reconnection processes during Dungey and Vasyliunas cycles are often associated with energetic particle injections which increase the intensity of energetic neutral atoms (ENA; Hill et al 2008)
The survey of the small-scale magnetic reconnection utilizes the measurements of magnetic field from the Cassini-MAG instrument (Dougherty et al 2004), electrons with energy ranges up to 28 keV from Cassini-CAPS/IMS/ELS (Young et al 2004), and energetic particles from the Low-Energy Magnetospheric Measurements System (LEMMS) and the Ion and Neutral Camera (INCA) on the Magnetosphere Imaging Instrument (MIMI) (Krimigis et al 2004)
Summary
Saturn’s magnetosphere receives mass and energy from internal sources (e.g., moons, rings, and Saturn’s atmosphere/ ionosphere) and the solar wind. The mass and energy are accumulated in the nightside magnetotail, which eventually results in magnetotail reconnection to release mass back to the interplanetary space This solar wind driven reconnection and plasma circulation are known as the Dungey cycle (Dungey 1961). Magnetic reconnection is expected to take place where the magnetic field is most stretched in the nightside, and is suggested to distribute further planetwards from dusk to dawn This large-scale internally driven reconnection, present at Jupiter, is known as the Vasyliunas cycle (Vasyliunas 1983; Kivelson & Southwood 2005). The DMR events in previous literature are believed to be driven by planetary rapid rotation and are suggested as “drizzle-like,” there is still a lack of adequate observational evidence for the drizzle nature of those small-scale reconnection processes, i.e., multiple small reconnection sites coexisting in the magnetosphere. The existence of multiple reconnection sites in Saturn’s magnetosphere would increase the corresponding mass-loss rate, manifesting the fundamentally different plasma processes in a rapid rotating magnetospheric environment compared to the Terrestrial magnetosphere
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