Abstract
Abstract Magnetically driven hotspot variations (which are tied to atmospheric wind variations) in hot Jupiters are studied using nonlinear numerical simulations of a shallow-water magnetohydrodynamic (SWMHD) system and a linear analysis of equatorial SWMHD waves. In hydrodynamic models, mid-to-high-latitude geostrophic circulations are known to cause a net west-to-east equatorial thermal energy transfer, which drives hotspot offsets eastward. We find that a strong toroidal magnetic field can obstruct these energy transporting circulations. This results in winds aligning with the magnetic field and generates westward Lorentz force accelerations in hotspot regions, ultimately causing westward hotspot offsets. In the subsequent linear analysis we find that this reversal mechanism has an equatorial wave analogy in terms of the planetary-scale equatorial magneto-Rossby waves. We compare our findings to three-dimensional MHD simulations, both quantitatively and qualitatively, identifying the link between the mechanics of magnetically driven hotspot and wind reversals. We use the developed theory to identify physically motivated reversal criteria, which can be used to place constraints on the magnetic fields of ultra-hot Jupiters with observed westward hotspots.
Highlights
In recent years the field of exoplanetary research has greatly developed its understanding of exoplanet characterisation both observationally and theoretically
Magnetically-driven hotspot variations in hot Jupiters are studied using non-linear numerical simulations of a shallow-water magnetohydrodynamic (SWMHD) system and a linear analysis of equatorial SWMHD waves
Observational measurements of hot Jupiters (e.g., Harrington et al 2006; Cowan et al 2007; Knutson et al 2007, 2009; Charbonneau et al 2008; Swain et al 2009; Crossfield et al 2010; Wong et al 2016), find that these planets have equatorial temperature maxima located eastward of their substellar points. This is consistent with both hydrodynamic simulations (e.g., Showman & Guillot 2002; Shell & Held 2004; Cooper & Showman 2005, 2006; Langton & Laughlin 2007; Dobbs-Dixon & Lin 2008; Menou & Rauscher 2009; Rauscher & Menou 2010; Dobbs-Dixon et al 2010; Perna et al 2010; Heng et al 2011; Perez-Becker & Showman 2013) and hydrodynamic theory (Showman & Polvani 2011; Debras et al 2020) of synchronously rotating hot Jupiters, which predict that such hotspots are driven eastward by the interaction between midto-high latitude geostrophic circulations and equatorial jets
Summary
In recent years the field of exoplanetary research has greatly developed its understanding of exoplanet characterisation both observationally and theoretically. Observational measurements of hot Jupiters (e.g., Harrington et al 2006; Cowan et al 2007; Knutson et al 2007, 2009; Charbonneau et al 2008; Swain et al 2009; Crossfield et al 2010; Wong et al 2016), find that these planets have equatorial temperature maxima (hotspots) located eastward of their substellar points This is consistent with both hydrodynamic simulations (e.g., Showman & Guillot 2002; Shell & Held 2004; Cooper & Showman 2005, 2006; Langton & Laughlin 2007; Dobbs-Dixon & Lin 2008; Menou & Rauscher 2009; Rauscher & Menou 2010; Dobbs-Dixon et al 2010; Perna et al 2010; Heng et al 2011; Perez-Becker & Showman 2013) and hydrodynamic theory (Showman & Polvani 2011; Debras et al 2020) of synchronously rotating hot Jupiters, which predict that such hotspots are driven eastward by the interaction between midto-high latitude geostrophic circulations and equatorial jets. Helling et al (2019) recently ruled out cloud asymmetries as the explanation for westward brightspots on HAT-P-7b
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