A study of Io's sodium jet with TRAPPIST

Since 2014, the TRAPPIST telescopes have been used to observe and monitor the neutral-sodium structures around Io, including notably the sodium jet, a transient and localised region structure of sodium emission that emanates from Io. The physical processes involved in the escape of particles – for example, how the volcanoes of Io provide directly or indirectly material to the Jovian plasma torus – are not yet fully understood, and the behaviour of the various neutral sodium structures, as intermediaries between Io’s atmosphere and the plasma torus, may help to bridge this gap. Observations were carried out over 17 nights in 2014–2015, 30 nights in 2021, and 23 nights in 2022–2023 with the TRAPPIST (TRAnsiting Planets and PlanetesImals Small Telescope) telescopes, in which particular attention was paid to the sodium jet and the quantification of its physical properties (length and brightness). It was found that these properties can vary greatly from one observation to another and independently of the position of Io in its orbit. Additionally, the delay between the detection of a strong jet and the subsequent contemporaneous brightening of the plasma torus and Io’s extended sodium nebula was too long to be understood as a simple injection of material by the jet, given the expected velocities of the jet particles, which indicates that jets do not straightforwardly contribute to the population of these two structures. We will also show the latest observations, including those concurrent to the last two Io flybys by Juno. This work also demonstrates the advantage of the on-going monitoring of Io provided by the TRAPPIST telescopes in understanding the variability of the sodium jet and presents a large corpus of jet detections against which work in related fields may compare.