Could a plasma wave really change its identity mid-flight? Around Jupiter’s poles, the answer appears to be yes and the transformation is rewriting the physics of planetary auroras.
NASA’s Juno mission, in its special polar orbit since 2016, has been flying low over Jupiter’s high-latitude areas, where the magnetic field of the gas giant is strongest in the solar system. It has provided scientists with a unique perspective to investigate the auroral regions of the planet with greater accuracy than ever. The Waves instrument on Juno “listens” for electromagnetic signatures of charged particles that spiral along magnetic field lines and indicates the faint oscillations passing through Jupiter’s plasma environment.
In the vast majority of astrophysical plasmas, wave behavior is in neatly categorizable classes. Alfvén waves low-frequency wobbles of ions that are anchored to magnetic field lines are capped by an upper frequency controlled by the ion gyrofrequency. Langmuir waves, on the other hand, are high-frequency wobbles of electrons traveling parallel to those same lines, their frequency controlled by the plasma frequency. Juno’s measurements, however, over Jupiter’s north pole overturned those rules. In this case, the plasma frequency was below the ion gyrofrequency, a flip of the terrestrial standard, and the waves observed never broke that plasma frequency.
Analysis by Robert Lysak of the University of Minnesota demonstrates that within the strongly magnetized, tenuous polar plasma, Alfvén waves may evolve into Langmuir modes as their wavelength decreases. The driver could be energetic upward-propagating electron beams, with energies ranging from 1 keV to 2 MeV, first reported by Juno in 2016. This process is a new plasma regime that also might happen on magnetized exoplanets or even on some stars.
The physical environment for this event is as unusual as the wave itself. Jupiter’s polar magnetosphere is threaded with a tangled combination of open and closed magnetic field lines, sculpted by the planet’s fast 10-hour rotation, sluggish dayside magnetic reconnection, and mass loading by volcanic Io. Global magnetohydrodynamic simulations indicate that only about 9% of the polar cap’s magnetic flux is truly open to the solar wind, forming crescent-shaped regions largely devoid of auroral emissions. The rest is closed, with helical field lines looping through the outer magnetosphere and back, enabling intense particle precipitation directly into the polar cap.
This topology explains why Jupiter’s auroras differ so starkly from Earth’s. On Earth, the most intense displays are an oval around a fairly dark polar cap with electrons accelerated by electric potential drops along field lines. On Jupiter, particles stream directly into the poles creating bright, disorganized ultraviolet and infrared emissions over the cap. Juno’s in situ traversals of these regions have demonstrated that the most powerful auroras do not fall along the planet’s strongest electric potentials, as on Earth, but potentially are fueled by turbulent wave–particle interactions precisely the processes that can cause the Alfvén-to-Langmuir transition.
The magnetospheric environment amplifies these effects. The magnetic field at Jupiter’s high latitudes reaches 40 times the strength of Earth’s, and the plasma density is very low there, lowering the plasma frequency and modifying dispersion relations controlling wave propagation. In this environment, even basic plasma behavior such as friction acts differently: intense magnetization can divert frictional forces transverse to particle motion, shifting the radius and frequency of their helical trajectories. These microphysical alterations cascade up, affecting how energy and momentum are conducted through the magnetosphere.
The discovery, reported in Physical Review Letters, is of far more than interest to Jovian physics. It provides a novel diagnostic for extreme plasma environments, where ordinary models based on Earth observations break down. As Ali Sulaiman, co-author of the study, explained, “The James Webb Space Telescope has given us some infrared images of the aurora, but Juno is the first spacecraft in a polar orbit around Jupiter.” Each subsequent pass over the poles could photograph additional instances of this wave shape-shifting and improving our knowledge of how magnetic fields and plasma interact on worlds far beyond our own.
