Reconnection acceleration in Saturn's dayside magnetodisc:a multicase study with Cassini

Recently, rotationally driven magnetic reconnection was firstly discovered in Saturn’s dayside magnetosphere (Guo et al. 2018). This newly confirmed process could potentially drive bursty phenomena at Saturn, i.e., pulsating energetic particles and auroral emissions. Using Cassini’s measurements of magnetic fields and charged particles, we investigate particle acceleration features during three magnetic reconnection events observed in Saturn’s dayside magnetodisc. The results suggest that the rotationally driven reconnection process plays a key role in producing energetic electrons (up to 100 keV) and ions (several hundreds of keV). In particular, we find that energetic oxygen ions are locally accelerated at all three reconnection sites. Isolated, multiple reconnection sites were recorded in succession during an interval lasting for much less than one Saturn rotation period. Moreover, a secondary magnetic island is reported for the first time at the dayside, collectively suggesting that the reconnection process is not steady and could be ‘drizzle-like’. This study demonstrates the fundamental importance of internally driven magnetic reconnection in accelerating particles in Saturn’s dayside magnetosphere, and likewise in the rapidly rotating Jovian magnetosphere and beyond

Electron pulsations generated by rotating magnetospheric dynamics at Saturn

Quasi-periodic pulsations of energetic electrons have been frequently observed in Saturn's magnetosphere. The mechanisms for the electron pulsations are far from conclusive, although generally believed to be associated with field-line resonance due to their similar periodicities. Here we report an electron pulsation event that is related to aurora beads and energetic neutral atom (ENA) emissions. The perturbation of the magnetic field indicates that Cassini spacecraft encountered a series of field-aligned currents (FACs) connected to the aurora beads. The fluxes of energetic electrons were enhanced when the spacecraft crossed the FACs. Both aurora beads and ENA emission were rotating. Given that the FACs interconnect the aurora beads and the active region at equator, a hot plasma population associated with the ENA enhancement, we conclude that the FACs were rotating with Saturn and had finite extent in the azimuthal direction. The periodic features also manifested in the whistler-mode auroral hiss emissions in the same event. We proposed that the electron pulsations we studied are spatial effect of the rotating magnetosphere. The rotation of the whole magnetosphere transfers the spatial effect to the temporal effect, i.e., the pulsation sequences observed by Cassinis multiple instruments

Long-standing Small-scale Reconnection Processes at Saturn Revealed by Cassini

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