Unusual plasma waves detected above Jupiter's north pole may finally have explanation
Charles Blue
contributing writer
Sadie Harley
scientific editor
Andrew Zinin
lead editor
Recent observations of Jupiter's powerful magnetic field by NASA's Juno spacecraft have uncovered a never-before-documented variety of plasma waves that seem to defy our current understanding of planetary magnetospheres.
A new published in Â鶹ÒùÔºical Review Letters provides a compelling explanation of these intriguing phenomena and proposes they form one class of plasma waves and morph into a completely different variety.
Like their ocean-water counterparts, plasma waves are ripples or oscillations propagating through a "sea" of charged particles in a planet's magnetosphere.
They traditionally fall into one of two categories: rapid, high-frequency oscillations of negatively charged electrons, which are known as Langmuir waves; and lower and slower oscillations of relatively heavy ions (atoms stripped of one or more electrons) known as Alfvén waves.
The electrons that create high-frequency Langmuir waves oscillate parallel to Jupiter's magnetic-field lines. This is a planetary magnetosphere approximation of soundwaves emanating from an oscillating guitar string.
Ions, however, behave quite differently. Rather than being parts of a freely floating sea of electrically charged plasma, positively charged ions remain bound to Jupiter's powerful magnetic field lines, twirling around them at a fixed rate known as their gyrofrequency. This rate puts an upper limit on the frequency of Alfvén waves.
The results from Juno appear to blur the line between these two phenomena. The data show that at Jupiter's high northern latitudes, where Jupiter's magnetic field dips to a mere 40 times that of Earth's, the plasma frequencies were much lower than the ion gyrofrequency, the opposite of what would normally be observed.
To shed light on these anomalous readings, a team of researchers led by Robert Lysak of the University of Minnesota, identified a potential mechanism whereby large numbers of Alfvén waves could transition to Langmuir waves.
By studying the data from Juno as its decaying orbit brought it closer to Jupiter's northern latitudes (and its final plunge into the planet's thick atmosphere), the researchers compared the relationships between plasma wave frequency and wave number. The further north the spacecraft traveled, the lower the density of the magnetosphere it measured, which also corresponded with lower electron concentrations.
Lysak and his team suggest that, in these highly unusual magnetosphere conditions near Jupiter's north pole, there is a potential pathway for large numbers of Alfvén waves to transform into Langmuir waves.
This metamorphosis could be catalyzed, they propose, by another unusual phenomenon previously observed by Juno in 2016: powerful upward-traveling beams of electrons packing energies approaching 100 thousand electron volts.
The researchers note that their results "… indicate the existence of a new type of plasma wave mode occurring in the unusual conditions of high magnetic field strength and low plasma density at high latitudes and low altitudes in Jupiter's magnetosphere."
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More information: R. L. Lysak et al, New Plasma Regime in Jupiter's Auroral Zones, Â鶹ÒùÔºical Review Letters (2025).
Journal information: Â鶹ÒùÔºical Review Letters
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