LHS 1678 given its low photometric variability, and can serve as targets for radio transit experiments. Our SPI non-detections may be due to radio beaming geometry, a sub-GHz maximum emission frequency, or undetectable flux density. If the last case applies, then flux density upper limits constrain the exoplanet magnetosphere. GJ 367 b has the tightest constraints – no extended magnetosphere and an exoplanet field <0.8 G – although these results depend strongly Corresponding author: Jackie Villadsen j.villadsen@bucknell.edu 2 Villadsen et al.

on unknown stellar wind parameters inferred from stellar rotation period. Due to their small orbital distance, our non-detection systems a priori appear to have more favorable conditions for SPI than most radio-detected SPI candidate systems in the literature, a tension that can either be resolved by favorable wind/geometry conditions on the detected candidates or by a non-SPI (stellar activity) explanation for those candidate detections. Our results favor the approach of sub-GHz searches for radio SPI, especially with the sensitivity of new/upcoming facilities such as MeerKAT, and underscore the need for observational and theoretical work to constrain the magnetized stellar wind parameters.
1. INTRODUCTION Exoplanet magnetic fields provide a unique tracer of planets’ internal structure and modulate their atmospheric loss (Brain et al. 2024). This valuable tracer is difficult to detect directly, with searches for cyclotron emission from planetary aurorae remaining inconclusive (Turner et al. 2024) and terrestrial fields of ∼1 G requiring space-based MHz observatories (Burns 2021; Knapp et al. 2024). In the meantime, indirect detection of exoplanet magnetospheres through star-planet interaction (SPI) may provide initial estimates of exoplanet magnetic field strengths, especially as TESS and ground-based radial velocity experiments rapidly expand the known population of nearby, close-in planets. Close-in planets move through the stellar wind slower than the local Alfv´en speed, allowing energy transfer back to the host star along the star-planet magnetic flux tube. In such sub-Alfv´enic SPI, the power transferred back to the star depends on the size of the exoplanet magnetosphere and thus the planet’s magnetic field strength. The transferred energy should produce orbitally-modulated stellar activity signals. Such orbital modulation has been observed in chromospheric activity lines (Cauley et al. 2019) and optical flaring (Ilin et al. 2025b), and proposed for gyrosynchrotron radio flaring (Osten & Wolk 2025) and auroral radio emission. Sub-Alfv´enic radio SPI is expected to manifest as bursts of circularly-polarized radio emission analogous to Jupiter’s auroral radio bursts caused by Io and Ganymede (Zarka et al. 2018), where the emission mechanism is the electron cyclotron maser (ECM). The range of cyclotron frequencies along the star-planet flux tube determines the emission band, which should drop off at a maximum ECM cutoff frequency corresponding to the stellar surface field. ECM amplification causes a beaming pattern shaped like a wide, hollow cone, such that bursts of emission occur at orbital phases when the beam points at Earth (Hess & Zarka 2011; Kavanagh & Vedantham 2023). Solar system-based models (Zarka 2007; Saur et al. 2013) predict that sub-Alfv´enic SPI from nearby exoplanetary systems may be detectable by current radio facilities at MHz to GHz frequencies (Turnpenney et al. 2018). The search for radio SPI can be divided into two steps: identifying candidate radio-emitting systems, and confirming orbital modulation of the radio signal, which has not been definitively accomplished for any radio SPI candidate to date. Testing for orbital periodicity can require hundreds of hours of radio monitoring. In identifying targets for long-term monitoring, studies may choose to focus on either young magnetically-active stars or older inactive stars. Young strongly-magnetized stars offer the benefit of a high predicted SPI luminosity, enabling sensitivity to weak planetary fields, which makes the recent radio non-detection of two rapidly-rotating M dwarf planet hosts particularly surprising (Ortiz Ceballos et al. 2025).