Auroral Radio Emission from Ultracool Dwarfs and Exoplanets: A Unified M–I Coupling Model

    Auroral radio emission provides a powerful diagnostic of magnetic activity in magnetized astrophysical objects, enabling constraints on magnetic field strength. In ultracool dwarfs (UCDs), intense coherent radio emission has been widely observed and is thought to arise from magnetosphere–ionosphere (M–I) coupling, similar to solar system planets. In exoplanetary systems, analogous emission is expected from M–I coupling and star–planet interactions, although no exoplanetary auroral radio emission has yet been conclusively detected, apart from a marginal detection (Turner et al., 2021). Because these emissions are highly circularly polarized and occur near the cyclotron frequency, they provide a direct probe of magnetic field strength.
    We develop an analytical model based on a simplified M–I coupling framework to predict auroral radio power across various systems. The model uses the magnetospheric plasma velocity profile as the primary driver of energy transfer, avoiding poorly constrained parameters. From the M-I velocity difference, we derive electric fields, FACs, and ionospheric Joule heating.
    Validation with Jupiter and Saturn reproduces observed radio power within a factor of 3. For three cases of UCDs (LSR J1835, TVLM 513, 2M J0036), observed radio luminosities (~10¹⁵ W) are reproduced with conductances of ~1-10 mho. Application to τ Boo b (Turner et al., 2021) constrains the magnetospheric scale through the plasma velocity profile in the M–I coupling model; interpreting this scale as the magnetopause standoff distance and assuming pressure balance implies stellar wind pressures consistent with ZDI and MHD estimates.
    These results demonstrate a unified framework for auroral radio emission across planets and cool stars, providing a predictive tool for future low-frequency observations.