Séminaires

Semi-analytical study of stellar high-order gravity modes
Yoshiki Hatta (Graduate School of Advanced Studies, SOKENDAI, Tokyo)

Salle de réunion AGO (local -1/14), Institut d'Astrophysique et de Géophysique
Bâtiment B5c, Quartier Agora, Allée du 6 Août, 19C, B-4000 Liège 1 (Sart-Tilman)

Modern space-borne missions such as CoRoT, Kepler, and TESS have enabled us to find a few hundreds of intermediate-mass (1.4-8.0 solar mass) main-sequence stars that are pulsating with gravity (g) modes whose restoring force is the buoyancy. The measured g-mode periods are quite informative on internal properties of the stars, but they are difficult to interpret because of a rather complex relation between the g-mode period and internal dynamics and structure of the stars. In this talk, I present semi-analytical studies of high-order g modes, where relations between the g-mode period and internal properties, namely, the Brunt-Väisälä (BV) frequency profile and internal rotation rate, have been established. Such semi-analytical studies of high-order g modes will possibly allow us to put further constraints on, e.g., mixing processes inside intermediate-mass main-sequence g-mode pulsators such as beta Cep, SPB, and gamma Dor stars that have been principal targets in asteroseismology and will be observed by the PLATO telescope. 

Plasma conditions at the orbits of Io, Europa, and Ganymede derived from the lead angle of the satellite auroral footprints observed by Juno-UVS
Shinnosuke Satoh (Tohoku University)

Salle de réunion AGO (local -1/14), Institut d'Astrophysique et de Géophysique
Bâtiment B5c, Quartier Agora, Allée du 6 Août, 19C, B-4000 Liège 1 (Sart-Tilman)

Io, Europa, and Ganymede act as an obstacle to the corotating plasma in the Jovian magnetosphere. Through the electrodynamic interaction at the moons (e.g., Kivelson et al., 2004), Alfvén waves are launched and propagate along the magnetic field. Auroral electrons are accelerated toward/away from Jupiter's atmosphere by the Alfvén waves and ultimately induce multiple satellite auroral footprints and a diffuse auroral tail in Jupiter's atmosphere (e.g., Clarke et al., 2002; Bonfond et al., 2008).

The Alfvén velocity depends on the local magnetic field magnitude and the local plasma mass density. The position of satellite auroral footprints is detemined by the Alfvén wave propagation time. The angular separation between the satellite body and the auroral footprint is called the lead angle. The footprint lead angle has been proven to be useful to investigate temporal variations of plasma parameters in the Io plasma torus (Moirano et al., 2023) and the plasma disc at Europa's orbit (Satoh et al., 2024). Both studies traced the Alfvén waves from the moon to the MAW spot in the plasma sheet with various plasma parameters and estimated the lead angle to find the best fit parameters. Estimation of the plasma parameters at the orbits of the icy moons is important because the magnetospheric plasma controls source and loss of neutral atmospheres in the icy moons.

Now, using the same fitting procedure as Satoh et al. (2024), we’re trying to derive the three ion parameters (atomic mass, number density, and temperature) at the orbits of Io, Europa, and Ganymede, from the footprint lead angle measured by Juno-UVS. In addition to the MAWs, we also use the lead angle of the Transhemispheric Electron Beam (TEB) spot (Bonfond et al., 2008). The TEB spot is generated by the electrons accelerated away from Jupiter in the other hemisphere. Hence, the TEB spot in one hemisphere is strongly associated with the other hemisphere's MAW spot. Using the TEB spot, we can trace the Alfvén waves that have different propagation paths than the ones corresponding to the MAW spot observed in the same hemisphere at the same time, which is expected to add another constraint for the estimation of ion parameters.