Estimation of Auroral Radio Emissions in Ultracool Dwarfs Based on 3D Global Magnetohydrodynamic Simulations

Auroral emissions occur when charged particles are accelerated along magnetic field lines and precipitate into an upper atmosphere, so their detection implies both a magnetic field and an atmosphere. Because auroral radio emission is strongly circularly polarized (Wu & Lee, 1979), radio observations are a promising way to detect exoplanetary auroras. Previous studies using magnetosphere-ionosphere (M-I) coupling models and global 3D MHD simulations estimated typical auroral radio powers of hot Jupiters to be about 10^15 W, but only one detection claim has been reported, so these models still need observational tests.
Ultracool dwarfs (UCDs) are useful analogs for such tests. Rapidly rotating, strongly magnetized UCDs show intense, highly circularly polarized radio bursts with powers of ~10^16 W, likely produced by auroral processes (e.g., Hallinan et al., 2008). Earlier M-I coupling studies reproduced these powers (Nichols et al., 2012; Turnpenney et al., 2017), but key inputs, such as plasma angular velocity and mass loading, remain uncertain because the plasma environment of UCDs is poorly constrained.
Here, we use a global 3D MHD model originally developed for solar system planets (e.g., Fukazawa et al., 2005), restricting inputs to relatively well-constrained UCD parameters such as rotation period and magnetic field strength. This reduces assumptions and provides a more self-consistent estimate of auroral currents and radio power. For LSR J1835+3259, we obtained a total auroral current of ~7×10^9 A, about 10^3 times that of Jupiter (Kamran et al., 2022). Comparison with VLA observations (Hallinan et al., 2008) indicates that a Pedersen conductance of ~0.5-5 mho is required to reproduce the observed radio power of 10^15-10^16 W. This range is comparable to both Jupiter's inferred conductance (0.1-10 mho; Gérard et al., 2021) and a recent independent M-I coupling estimate for UCDs (0.1-10 mho; Satyagraha et al., JpGU-AGU 2026). These results support the plausibility of our MHD framework for UCD auroral processes. In future work, we will extend this approach to hot Jupiters and Earth-like exoplanets to further test the model and support future observations.