My Thesis in 400 words: Raphaël Hardy

Raphaël Hardy. (Credit: É. Artigau)

Raphaël Hardy, an iREx student at Université de Montréal, finished his master degree in 2020. Here, he summarizes the research project he carried out during his studies:

Hot Jupiters are exoplanets with unique features. Due to their proximity to their host stars, they show remarkable non-symmetry. This proximity with the star causes tidal locking, meaning one side of the planet is always exposed to intense radiation from its host and the other side is immersed in a perpetual night.

This geometry means there is a difference of temperature of up to 500 K from the day to night side. This gradient in temperature induces zonal winds that can reach the order of 1 km s1 to redistribute heat to the night side. The hot spot is the hottest spot of the planet and is a telltale of these strong winds. In the absence of winds, the hottest region on the planet should be the closest region to the star, the substellar point. Observations and hydrodynamic numerical simulations show that zonal winds on these planets go eastward. However, two recent observations are showing westward winds. These planets are CoRoT-2 b and HAT-P-7 b. The most common explanation to this contradiction is that the magnetic field, which is interacting with the partially ionized atmosphere, can reverse these winds. It was previously shown that a magnetic diffusivity varying in space can locally generate magnetic fields when its gradient aligns correctly with the electric current density.

I present in my thesis a one-dimensional magnetohydrodynamic model with a temperature dependant magnetic diffusivity in the equatorial plane in the context of hot Jupiters. The simulations develop growing torsional alfvenic oscillations due to the non-linear coupling of the magnetohydrodynamics and the temperature partial differential equations and the temperature dependant magnetic diffusivity. A parameter space is explored and the influence of the parameters on the oscillations is studied. A local model is developed in order to derive analytical equations characterizing these waves and compare its results with the results of the one-dimensional model. I end my thesis by calculating the corresponding periods and longitudinal displacement of the one-dimension model oscillations for a Jupiter-sized planet. The periods correspond to an interval from 225 to 473 days and the displacements range from a few degrees up to 40degrees. This means that the oscillations could be observed with a few orbits.