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Shallow-water Magnetohydrodynamics for Westward Hotspots on Hot Jupiters

Lookup NU author(s): Alexander Hindle, Dr Paul Bushby, Professor Tamara Rogers

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This is the authors' accepted manuscript of an article that has been published in its final definitive form by Institute of Physics, 2019.

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Abstract

Westward winds have now been inferred for two hot Jupiters: 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 hot Jupiters magnetic fields are an obvious candidate, though the actual physical 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 Bdip,HAT-P-7b > 6G, a result that is consistent with three-dimensional magnetohydrodynamic simulations and lies below typical surface dipole estimates of inflated hot Jupiters. The same analysis suggests that the minimum dipole field strength required to reverse the winds of CoRoT-2b is Bdip,CoRoT-2b > 3kG, which considerably exceeds estimates of the maximum surface dipole strength for hot Jupiters. 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.


Publication metadata

Author(s): Hindle AW, Bushby PJ, Rogers TM

Publication type: Article

Publication status: Published

Journal: The Astrophysical Journal Letters

Year: 2019

Volume: 872

Issue: 2

Online publication date: 21/02/2019

Acceptance date: 10/02/2019

Date deposited: 11/02/2019

ISSN (print): 2041-8205

ISSN (electronic): 2041-8213

Publisher: Institute of Physics

URL: https://doi.org/10.3847/2041-8213/ab05dd

DOI: 10.3847/2041-8213/ab05dd


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