Juno Spots Faint Auroral Rings on Jupiter
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Planetary scientists using data from the Ultraviolet Spectrograph (UVS) aboard NASA’s Juno spacecraft have discovered expanding emission circles of ultraviolet (UV) brightness near the polar dark region of Jupiter.

Hue et al. discovered faint aurora features likely triggered by charged particles coming from the edge of Jupiter’s massive magnetosphere; this occurrence, shown in the false color series of images recorded 30 seconds apart (red panels), displays the characteristically ring-like emissions, expanding rapidly over time. Image credit: NASA / SWRI / JPL-Caltech / V. Hue / G.R. Gladstone / B. Bonfond.

“We think these newly discovered faint UV features originate millions of miles away from Jupiter, near the Jovian magnetosphere’s boundary with the solar wind,” said study lead author Dr. Vincent Hue, a researcher at the Southwest Research Institute.

“The solar wind is a supersonic stream of charged particles emitted by the Sun. When they reach Jupiter, they interact with its magnetosphere in a way that is still not well understood.”

Jupiter’s magnetic field is 20,000 times stronger than Earth’s and creates a magnetosphere so large it begins to deflect the solar wind between 2 and 4 million miles before it reaches the gas giant.

“Despite decades of observations from Earth combined with numerous in-situ spacecraft measurements, scientists still do not fully understand the role the solar wind plays in moderating Jupiter’s auroral emissions,” said study co-author Dr. Thomas Greathouse, also from the Southwest Research Institute.

“Jupiter’s magnetospheric dynamics, the motion of charged particles within its magnetosphere, is largely controlled by Jupiter’s 10-hour rotation, the fastest in the Solar System. The solar wind’s role is still debated.”

Previous observations with the NASA/ESA Hubble Space Telescope and Juno have allowed scientists to determine that most of Jupiter’s powerful auroras are generated by internal processes, that is the motion of charged particles within the magnetosphere.

However, on numerous occasions, Juno’s UVS instrument detected a faint type of aurora, characterized by rings of emissions expanding rapidly with time.

“The high-latitude location of the rings indicates that the particles causing the emissions are coming from the distant Jovian magnetosphere, near its boundary with the solar wind,” said study co-author Dr. Bertrand Bonfond, a researcher at the Université de Liège.

In this region, plasma from the solar wind often interacts with the Jovian plasma in a way that is thought to form Kelvin-Helmholtz instabilities.

These phenomena occur when there are shear velocities, such as at the interface between two fluids moving at different speeds.

Potential candidates to produce the rings are dayside magnetic reconnection events, where oppositely directed Jovian and interplanetary magnetic fields converge, rearrange and reconnect.

Both of these processes are thought to generate particle beams that could travel along the Jovian magnetic field lines, to eventually precipitate and trigger the ring auroras on Jupiter.

“Although this study does not conclude what processes produce these features, the Juno extended mission will allow us to capture and study more of these faint transient events,” Dr. Hue said.

The study was published in the Journal of Geophysical Research: Space Physics.

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V. Hue et al. 2021. Detection and Characterization of Circular Expanding UV-Emissions Observed in Jupiter’s Polar Auroral Regions. Journal of Geophysical Research: Space Physics 126 (3); doi: 10.1029/2020JA028971

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