Magnetosphere-ionosphere coupling at Jupiter-like exoplanets with internal plasma sources: implications for detectability of auroral radio emissions

In this paper we provide the first consideration of magnetosphere-ionosphere coupling at Jupiter-like exoplanets with internal plasma sources such as volcanic moons. We estimate the radio power emitted by such systems under the condition of near-rigid corotation throughout the closed magnetosphere, in order to examine the behaviour of the best candidates for detection with next generation radio telescopes. We thus estimate for different stellar X-ray-UV (XUV) luminosity cases the orbital distances within which the ionospheric Pedersen conductance would be high enough to maintain near-rigid corotation, and we then consider the magnitudes of the large-scale magnetosphere-ionosphere currents flowing within the systems, and the resulting radio powers, at such distances. We also examine the effects of two key system parameters, i.e. the planetary angular velocity and the plasma mass outflow rate from sources internal to the magnetosphere. In all XUV luminosity cases studied, a significant number of parameter combinations within an order of magnitude of the jovian values are capable of producing emissions observable beyond 1 pc, in most cases requiring exoplanets orbiting at distances between ~1 and 50 AU, and for the higher XUV luminosity cases these observable distances can reach beyond ~50 pc for massive, rapidly rotating planets. The implication of these results is that the best candidates for detection of such internally-generated radio emissions are rapidly rotating Jupiter-like exoplanets orbiting stars with high XUV luminosity at orbital distances beyond ~1 AU, and searching for such emissions may offer a new method of detection of more distant-orbiting exoplanets.

💡 Research Summary

This paper presents the first systematic study of magnetosphere‑ionosphere (M‑I) coupling on Jupiter‑like exoplanets that possess internal plasma sources such as volcanic moons. Building on the well‑established current system that powers Jupiter’s main auroral oval, the authors adapt the model to exoplanetary environments to evaluate the radio power generated by the associated field‑aligned currents. The analysis assumes that a steady plasma outflow (mass loss rate Ṁ) from an internal moon is picked up by the planetary magnetic field, forced into corotation, and then diffuses radially outward. As the plasma slows, a torque is transferred via Pedersen currents in the ionosphere, which can enforce near‑rigid corotation provided the ionospheric Pedersen conductance ΣP* is sufficiently high. ΣP* is taken to scale with the stellar X‑ray/UV (XUV) flux, which itself depends on the host star’s XUV luminosity and the planet’s orbital distance.

The magnetic field model combines an inner dipole with an outer current‑sheet component, reproducing Jupiter’s equatorial Bz and flux function F. The planetary magnetic moment is assumed to scale as M ∝ Ωp3/4, where Ωp is the planetary rotation rate; thus faster rotation yields stronger fields and larger fluxes. The authors derive analytic expressions for the equatorial magnetic field, flux function, and the mapping between equatorial and ionospheric latitudes. Using these, they compute the field‑aligned current density J‖ as a function of Ṁ, Ωp, and ΣP*.

Radio power is estimated from the product of current I and the field‑aligned voltage ΔV, multiplied by an efficiency factor η≈1 % (consistent with Jupiter’s auroral radio emission). The study explores three representative stellar XUV luminosities (solar, 10× solar, 100× solar) and a range of planetary parameters: orbital distances from ~0.5 to 50 AU, rotation periods from 10 h to 30 h, and plasma mass outflow rates from 10^2 to 10^4 kg s⁻¹.

Key findings are:

1. Pedersen Conductance and Corotation Radius – For high‑XUV stars, ΣP* remains above the critical value out to tens of AU, allowing near‑rigid corotation across the entire closed magnetosphere. For solar‑type XUV, the corotation‑maintaining region shrinks to ≲1 AU.

2. Effect of Plasma Outflow – Increasing Ṁ by an order of magnitude raises the current density and radio power roughly proportionally. At Ṁ≈10^4 kg s⁻¹, radio powers reach ~10^12–10^13 W.

3. Rotation Rate Influence – Faster rotation (period ≤10 h) boosts the magnetic moment (∝Ωp3/4) and compresses the current system, leading to current densities up to several μA m⁻² and radio powers exceeding 10^13 W.

4. Detectability – Assuming a LOFAR‑like detection threshold of 1 mJy, planets with the combination of high XUV illumination, rapid rotation, and strong plasma outflow can be detected out to 10–50 pc. Even with modest parameters, many configurations are detectable beyond 1 pc.

The authors argue that, unlike previous “hot‑Jupiter” models that rely on stellar wind‑planet reconnection or Io‑type Alfvénic interactions, internally‑driven M‑I coupling depends primarily on planetary intrinsic properties and the presence of an active moon. Consequently, radio searches can probe a new class of exoplanets—those on wider orbits (≥1 AU) around active stars, which are otherwise difficult to detect with transit or radial‑velocity methods.

In conclusion, the paper demonstrates that Jupiter‑like exoplanets with vigorous internal plasma sources can sustain strong magnetosphere‑ionosphere currents and generate auroral radio emissions powerful enough to be observable with next‑generation low‑frequency arrays. Rapidly rotating, massive planets orbiting XUV‑bright stars at distances of a few to several tens of AU emerge as the most promising targets for future radio‑based exoplanet surveys.