JWST’s transformational observations of Giant Planet ionospheres

For more than three decades, thermalised emissions from the ionic molecule trihydrogen, H₃⁺, have been used to explore the ionospheres of the outer planets.  These observations, made by a combination of ground-based and in-orbit spacecraft telescopes, have allowed us to reveal both auroral and equatorial processes, understand the flow of energy from the aurora and even follow the ion winds within the auroral regions, revealing the driving currents that produce these features. But for every outer planet, these observatories have reached some fundamental limit, constrained by the limitations of Earth’s turbulent atmosphere, the low spectral resolution of instruments sent to these world and the lack of sensitivity to explore the weak emissions from transient thin gases at the very top of these atmospheres.

In the past three years, JWST has fundamentally revolutionised our understanding of the ionosphere of every one of these worlds. In a sequence of observations that includes GOT and ERS time at the very start of JWST’s mission, as well as a series of GO observations in Cycles 2, 3 and now 4, we have been garnering incredible, high sensitivity, highly-spatially resolved views at unprecedented detail.

At Jupiter, a familiar ionosphere has been bejewelled with intricate new small-scale details. Images of the non-auroral emissions capture the broad-scale ionospheric features in individual images at an equivalent detail to past maps combining 150 hours of telescope time, but between these broad features, small scale structures have been observed for the first time, suggestion a highly dynamic ionosphere with complex wave activity (Figure 1).  This complexity is now seen across the disk, in a sequence of observations that not only reveals these details (measurements that will be presented in detail within the Jupiter magnetosphere session), but also provides us with details of the vertical structures within Jupiter’s ionosphere, from pole to equator (again to be discussed in detail within the Jupiter session). We have also revealed complexity within Jupiter’s Io footprint, and across the northern auroral region.

Figure 1. Complex ionospheric wave activity above Jupiter’s Great Red Spot, revealed by Melin et al., 2024. The ionosphere is shown in red, with reflected sunlight from the underlying spot shown in blue.

At Saturn, the past constraints of weak emission and high background signal have greatly limited any spatially resolved measurements in either temperature or density, and have prevented any view of sub-auroral ionospheric structures. With JWST, we have revealed complex structures and dynamics, providing the first direct view of the thermal gradients that drives Saturn’s enigmatic planetary-period aurora. We have also discovered a string of sub-auroral beads that occur along the flank of the main auroral region, an unprecedented ionospheric feature that might be associated with Saturn’s unique alignment between the magnetic and rotational pole (Figure 2).

Figure 2. A polar map of Saturn’s sub-auroral ionosphere, imaged for the first time with JWST. Here, the transient weak auroral emission is saturated within this map, instead focusing on emission equatorward of the aurora. The sub-auroral region between 180-300^oW reveals a string of dark beads, fixed in longitude.

 

Past observations of Uranus have measured the planet-wide temperature and density for decades, but only the most recent ground-based measurements have begun to reveal auroral structures.  Similarly, Voyager II struggled to reveal the aurora in detail and Hubble Space Telescope observations can only observe the aurora of Uranus at their very brightest. Our past Earth-bound myopic views have been startlingly shattered by JWST, revealing a complex ionosphere and aurora for the first time. We are able to resolve detailed auroral structures and the first vertical profiles of the infrared ionosphere (to be discussed in detail within two separate presentations within the Uranus session), as well as previously unknown equatorial structure, sometimes directly associated with the planet’s magnetic field.

Finally, at Neptune, Voyager II provided the only views we have ever had of that planet’s aurora. Since 1989, ground-based observations have failed to detect any H₃⁺ emission from the ionosphere, and Hubble has failed to detect an auroral signature. JWST has lifted the darkness at Neptune, providing incredible views of the aurora, as well as complex ionospheric structures (Figure 3).

 

Figure 3. Neptune’s infrared ionosphere (Melin et al., 2024). Complex auroral and ionospheric structures are revealed in this detection of H₃⁺ at Neptune.