Whistler-mode Waves observed during Mercury flybys by BepiColombo/Mio PWI

The Plasma Wave Investigation (PWI) aboard BepiColombo/Mio spacecraft will deploy all sensors after the Mercury orbit insertion at the end of 2025. The full observational capabilities of electric fields, plasma waves, and radio waves enables the measurement of electron density and temperature, electromagnetic turbulences, and radio waves in and around the Hermean magnetosphere and exosphere, in the electric field from DC to 10 MHz along the spin plane and in the magnetic field from 0.3 Hz to 20 kHz in three-axis and from 2.5 kHz to 640 kHz in one-axis.

Unfortunately, during the cruising phase from its launch in 2018, long wire antennas (15-m x 4) for electric fields and the boom (4.5-m) for magnetic fields are not deployed. For electric field, one of the antenna pairs, WPT, is exposed to space, but with the length of few cm and grounded to the spacecraft body with several k-ohm. For magnetic field, the search-coils are active but under large noise environment from the spacecraft body. Although under such severe conditions, the PWI saw plasma waves around Mercury during the 1^st, 2^nd, and 3^rd  flybys (Oct. 2021, June 2022, and June 2023).

In this paper, we show the summary of the first historical detections of Hermean whistler waves and their interpretations. We saw magnetic field turbulences in several 10s kHz in the dawn side magnetosphere after the closest approach. It can be interrupted as the first whistler-wave detection. Whistler-mode chorus waves are natural electromagnetic emissions known to play a key role in electron acceleration and loss mechanisms via wave–particle interactions in planetary magnetospheres. Mio’s search coil magnetometers measured chorus waves with tens of pT in the dawn sector, while no clear wave activity was observed in the night sector. This dawn-dusk asymmetry could be explained by the impact of background magnetic field inhomogeneities on the nonlinear wave generation process. In the 1^st and 2^nd flybys the BepiColombo orbit is not close to the magnetic equator, but the 3^rd flyby seems just there. Potential direct comparisons with electron data will be discussed using this data, in or close to the source region of Whistler waves.

We are now preparing the antenna deployment starting at the end of 2025 and initial observation plans from spring of 2026.  We will also report the current status of the data production plans.