Jupiter’s aurora has an unexpected power source

New observations from NASA’s Juno mission suggest that Jupiter’s aurora is generated differently than Earth’s.

Earth’s auroras, the southern or northern lights, are generated by several different processes depending on their intensity. Because Jupiter’s auroras are much more powerful than Earth’s, researchers have long suspected the process behind them would be similar to that of Earth’s intense auroras. But NASA’s Juno mission’s flyovers now reveal that isn’t the case.

We spoke to Barry Mauk from Johns Hopkins University Applied Physics Laboratory (APL) about the study.

ResearchGate: How did this paper come about?

Barry Mauk: My institution, Johns Hopkins APL, participated in NASA’s Juno mission by designing and building the Jupiter Energetic Particle Detector Instrument (JEDI) to measure the higher energy particles that likely have a role in generating Jupiter’s aurora. During several of the Juno passes over Jupiter’s aurora, the JEDI instrument measured distinct signatures of auroral acceleration, and we are reporting some of those findings in this paper.

Auroral image of Jupiter’s southern aurora, used in the subject study, obtained by the Juno Ultraviolet Spectrograph (UVS) instrument on 2 February 2017. This image overlays three different UVS wavelength ranges and color codes them such that red, green, and blue indicate high, medium and low energy electrons impinging on the atmosphere, with color mixtures indicating a mixture of energies. The streak on the upper left is the auroral tail caused by Jupiter’s moon Io. Credit: G. Randy Gladstone.

RG: What are some of the challenges of gathering this data?

Mauk: We are thrilled to have been able to make these measurements over Jupiter’s polar regions. Johns Hopkins APL has been building these kinds of instruments for decades and has flown them on NASA missions to all of the planets in the solar system, with the exception of Mars. Jupiter’s polar regions presented to us perhaps the most challenging environment yet.  Juno flies over Jupiter’s poles at the astounding speed of over 100,000 miles per hour.  At that speed the spacecraft speeds past auroral forms in a matter of several seconds, and sometimes less.  JEDI had to be designed to gather all of the needed information, including all-sky angular information, in that brief time.

RG: What did you learn about Jupiter’s aurora?

Mauk: Electric potentials accelerate electrons downward onto Jupiter’s atmosphere, causing the emission of the light that constitutes Jupiter’s polar aurora. The electric potentials that were inferred are up to 400,000, a factor of 10 to 30 larger than the largest potentials observed in association with Earth’s aurora.

It was not a surprise that electric potentials are involved with the generation of Jupiter’s aurora.  Such electric potentials (albeit with much lower values) play a role in the generation of the very brightest aurora observed at Earth. The huge surprise is that, despite the magnitude of these potentials, and expectations from observations at Earth, they are not the source of the most intense aurora at Jupiter. Instead, Jupiter’s brightest auroras are caused by some kind of turbulent electron acceleration process that we do not understand very well. There are indications in our data that, as the power density of the generation process becomes stronger and stronger (much stronger than Earth’s) the auroral generation process becomes unstable, and a new acceleration processes comes into play.

A complete reconstruction of what the northern and southern auroral configurations looked like to the Juno Ultraviolet Spectograph (UVS) as Juno, in its large elliptical orbit, approached Jupiter, passed over the north pole, rapidly traveled to the southern hemisphere to pass over the southern pole, and finally receded from Jupiter. Jupiter itself is represented as a digital model as it might be observed in visible light, and the auroral configurations are static images in ultraviolet light measured by the UVS instrument. Credit: Bertrand Bonfond.

RG: How long have we known that Jupiter has an aurora? Did you expect it to behave different to Earths?

Mauk: Ground based radio wave observations of Jupiter revealed in the early 1950’s that Jupiter has a strong magnetic field and therefore a magnetosphere that might have similarities to Earth’s magnetosphere. Definitive measurements of Jupiter’s aurora were obtained by the Voyager spacecraft in 1979, and nearly contemporaneous measurements from Earth orbit by the International Ultraviolet Explorer mission. However, there were indications of the existence of Jupiter’s auora within ultraviolet measurements from rocket experiments in the early 1970’s.

RG: What does this new knowledge mean for our understanding of Jupiter?

Mauk: Because an outer layer of Jupiter’s atmosphere, its ionosphere, is electrically conducting, Jupiter, spinning as it does within its strong magnetosphere, acts as a gigantic electrical generator. The resulting electric currents that flow within the ionosphere close within the distant magnetosphere, many Jupiter diameters away from Jupiter itself. This process causes the electrified gases, called plasmas, within the distant magnetosphere to spin up in an attempt to match the spinning of Jupiter. Jupiter’s aurora is a powerful signature of this process, of Jupiter’s attempt to spin up its magnetosphere, and in the process to shed infinitesimal amounts of angular momentum. The fact that Jupiter’s aurora behaves differently than expected means that the space environment spinning up process, or the angular momentum shedding process, also behaves differently than expected. The use of magnetic fields to transport angular momentum is sometimes invoked in the study of distant astrophysical objects. Jupiter may have much to tell us about how this process works.

Featured image credit: G. Randy Gladstone (right image) and co-author Bertrand Bonfond (left image).