# Shallow-water Magnetohydrodynamics for Westward Hotspots on Hot Jupiters

## TL;DR

This paper proposes a magnetic wave-driven mechanism that can reverse equatorial winds on hot Jupiters like HAT-P-7b, explaining observed westward hotspots through the influence of planetary magnetic fields.

## Contribution

It introduces a simplified model linking magnetic fields to wind reversals on hot Jupiters, supported by simulations and estimates of magnetic field strengths.

## Key findings

- Magnetic fields can cause equatorial wind reversals on hot Jupiters.
- The model estimates a minimum dipole field of ~6 G for HAT-P-7b to reverse winds.
- Wind reversals on CoRoT-2b likely involve other physical phenomena.

## Abstract

Westward winds have now been inferred for two hot Jupiters (HJs): HAT-P-7b and CoRoT-2b. Such observations could be the result of a number of physical phenomena such as cloud asymmetries, asynchronous rotation, or magnetic fields. For the hotter HJs magnetic fields are an obvious candidate, though the actual mechanism remains poorly understood. Here we show that a strong toroidal magnetic field causes the planetary-scale equatorial magneto-Kelvin wave to structurally shear as it travels, resulting in westward tilting eddies, which drive a reversal of the equatorial winds from their eastward hydrodynamic counterparts. Using our simplified model we estimate that the equatorial winds of HAT-P-7b would reverse for a planetary dipole field strength $B_{\text{dip},\text{HAT-P-7b}} \gtrsim 6 \, \mathrm{G} $, a result that is consistent with three-dimensional magnetohydrodynamic simulations and lies below typical surface dipole estimates of inflated HJs. The same analysis suggests the minimum dipole field strength required to reverse the winds of CoRoT-2b is $B_{\text{dip},\text{CoRoT-2b}} \gtrsim 3 \, \mathrm{kG}$, which considerably exceeds estimates of the maximum surface dipole strength for HJs. We hence conclude that our magnetic wave-driven mechanism provides an explanation for wind reversals on HAT-P-7b; however, other physical phenomena provide more plausible explanations for wind reversals on CoRoT-2b.