Dr Licia Ray, Senior Researcher in Space and Planetary Physics, has been exploring how Jupiter's vast magnetosphere connects to its atmosphere via magnetic waves called Alfvén waves. Alfvén waves are essential for transferring energy and angular momentum between Jupiter and circulating plasma from its moon, Io, a process that generates bright aurora. Yet beyond a specific distance, known as the Alfvén radius, this connection weakens, and the planet and plasma are decoupled.

Dr Ray has recently published research showing that the location of Jupiter’s Alfvén radius depends on local planetary time. Surprisingly, the Alfvén radius on Jupiter’s nightside is close to the planet, occurring at distances 30-35 times the radius of Jupiter away. At this distance, the plasma quickly decouples from the planet, limiting the influence of Jupiter’s atmosphere and explaining the mysterious sharp boundary in nightside auroral forms and strong radial currents inferred from magnetic field measurements.

In contrast, on the Sun-facing side of the planet, the plasma remains coupled to the planet over larger distances (out to at least 60 times the radius of Jupiter) due to a stronger magnetic field and faster Alfvén waves.

Previous studies that described Jupiter’s Alfvén radius assumed that the magnetosphere was symmetric out to large distances. In contrast, this research incorporated a local-time-dependent magnetic field model based on decades of spacecraft data. While the Alfvén radius's exact location is dynamic, shifting in response to plasma circulation and solar wind changes, the day/night asymmetry will remain. Changes in the Alfvén radius location will influence the brightness and location of Jupiter’s nightside aurora and can be linked to dawn auroral storms.

Observations of Jupiter’s aurora have captured images of distinct, local time-fixed auroral features. Physical explanations of these features have not considered Alfvénic coupling, however the Alfvén radius location is remarkably consistent with observations. For example, Juno mission observations show a ‘polar collar’, a ring of emission where the aurora is narrow with a sharp boundary from dusk through dawn. This directly maps to the locations within Jupiter’s magnetosphere where plasma is decoupled from the atmosphere.

Dr Ray said: “I’m very excited about this research which provides new interpretation for auroral observations that addresses decades-long mysteries in local-time fixed auroral structures. The asymmetric Alfvén radius also offers new insight into the physical drivers behind Jupiter’s post-midnight currents, super-corotating plasma flows, and provides critical boundaries for future studies of the Jovian system.”

In summary, the study redefines how Jupiter's magnetosphere should be viewed by presenting a more complex picture of Alfvén wave travel and local time structures. Current models of magnetosphere-ionosphere coupling, which traditionally assume symmetry, should now consider local time differences to accurately interpret auroral emissions and plasma dynamics.