Recurrent magnetic dipolarization at Saturn: revealed by Cassini

Planetary magnetospheres receive plasma and energy from the Sun or moons of planets and consequently stretch magnetic field lines. The process may last for varied timescales at different planets. From time to time, energy is rapidly released in the magnetosphere and subsequently precipitated into the ionosphere and upper atmosphere. Usually, this energy dissipation is associated with magnetic dipolarization in the magnetosphere. This process is accompanied by plasma acceleration and field-aligned current formation, and subsequently auroral emissions are often significantly enhanced. Using measurements from multiple instruments on board the Cassini spacecraft, we reveal that magnetic dipolarization events at Saturn could reoccur after one planetary rotation and name them as recurrent dipolarizations. Three events are presented, including one from the dayside magnetosphere, which has no known precedent with terrestrial magnetospheric observations. During these events, recurrent energizations of plasma (electrons or ions) were also detected, which clearly demonstrate that these processes shall not be simply attributed to modulation of planetary periodic oscillation, although we do not exclude the possibility that the planetary periodic oscillation may modulate other processes (e.g., magnetic reconnection) which energizes particles. We discuss the potential physical mechanisms for generating the recurrent dipolarization process in a comprehensive view, including aurora and energetic neutral atom emissions. Plain Language Summary Using measurements from the Cassini spacecraft, we reveal a new feature of magnetic dipolarization at Saturn, that is, the magnetic signature repeat after one planetary rotation, which is named recurrent dipolarization. Up to hundreds of kiloelectron volt electrons and ions are identified for the recurrent dipolarization events, suggesting that these particles have experienced efficient acceleration and cannot be purely due to planetary modulation. It remains a mystery why the magnetic dipolarization process associated with energetic ions and electrons could reoccur after one planetary rotation. Moreover, dipolarization process in Saturn's dayside magnetosphere is reported for the first time at Saturn, which has no known precedent with terrestrial or other planetary magnetospheric observations. The results demonstrate that magnetosphere-ionosphere coupling dynamics at Saturn and Earth have fundamental similarities and differences.