Radio emission from close-in exoplanets: Can we extend the radio-magnetic scaling law to the sub-Alfvénic stellar wind regime?

Observations of exoplanetary radio aurora can directly probe planetary magnetic fields and magnetic star-planet interactions. However, the search for exoplanetary radio aurora has been without confirmed detections despite favorable predictions based on extrapolations of the radio-magnetic scaling law (RMSL). The RMSL is based on solar system planets in the super-Alvenic solar wind and it is unclear whether the RMSL holds for close-in exoplanets in more magnetic, sub-Alfvenic winds. We aim to test whether the relation can be extended to sub-Alfvenic stellar winds. We employ 3D magnetohydrodynamic simulations of the magnetosphere of a Jupiter-like planet at various distances from the Sun to study the expected radio power, considering atmospheric photoionization, a solar wind model with nominal and enhanced magnetic field. Our radio predictions match the RMSL in the super-Alfvenic solar wind regime. We find that the RMSL overestimates the planetary radio power by one order of magnitude in sub-Alfvenic stellar winds. This discrepancy is significantly enhanced with a more magnetic wind due to a strong decrease in wind-magnetosphere energy transfer efficiency. We expect the overestimation by the RMSL to increase further with more magnetic cool stars. Furthermore, due to atmospheric photoionization and resulting high ionospheric electron densities, favorable conditions for generation and escape of electron-cyclotron maser instability (ECMI) driven radio emission are confined to larger orbits (> 0.1au) and to the planetary nightside in the solar wind case. This further decreases our predicted radio powers by up to one order of magnitude. In more magnetic winds, enhanced open planetary magnetic flux and reconnection driven outflows cause magnetospheric electron depletion, resulting in improved ECMI conditions, eliminating the mitigating effect of photoionization on ECMI emission.

Source: arXiv:2607.07704v1 - http://arxiv.org/abs/2607.07704v1 PDF: https://arxiv.org/pdf/2607.07704v1 Original Link: http://arxiv.org/abs/2607.07704v1