Planetary researchers from NASA’s Juno mission made the first detection beyond Earth of an internal magnetic field that changes over time — a phenomenon called secular variation. They also determined Jupiter’s secular variation is most likely driven by its atmospheric (or zonal) winds.
This image of Jupiter was taken on February 12, 2019, as Juno performed its 18th close flyby of the planet. Image credit: NASA / JPL-Caltech / SwRI / MSSS / Kevin M. Gill.
“Determination of secular variation of a planet’s magnetic field provides a window into understanding the dynamo responsible for generating its field,” the scientists said.
“However, of the six Solar System planets with active dynamos, secular variation has been firmly established only for Earth.”
“This phenomenon has been on the wish list of planetary scientists for decades,” said Juno principal investigator Dr. Scott Bolton, from the Southwest Research Institute.
“Our discovery could only take place due to Juno’s extremely accurate science instruments and the unique nature of Juno’s orbit, which carries it low over the planet as it travels from pole to pole.”
Dr. Bolton and colleagues compared magnetic field observations of Jupiter from NASA’s Pioneer 10 and 11, Voyager 1 and Ulysses spacecraft (acquired 1973-1992) to a new model of the gas giant’s magnetic field (called JRM09).
The JRM09 model was based on data collected during Juno’s first eight science passes of Jupiter using its magnetometer, an instrument capable of generating a detailed 3D map of the magnetic field.
What the team found is that from the first Jupiter magnetic field data provided by the Pioneer spacecraft through to the latest data provided by Juno, there were small but distinct changes to the field.
“Finding something as minute as these changes in something so immense as Jupiter’s magnetic field was a challenge,” said Juno scientist Dr. Kimee Moore, from Harvard University.
“Having a baseline of close-up observations over four decades long provided us with just enough data to confirm that Jupiter’s magnetic field does indeed change over time.”
Once the researchers proved secular variation did occur, they sought to explain how such a change might come about.
The operation of Jupiter’s zonal winds best explained the changes in its magnetic field.
These winds extend from the planet’s surface to over 1,860 miles (3,000 km) deep, where the planet’s interior begins changing from gas to highly conductive liquid metal. They are believed to shear the magnetic fields, stretching them and carrying them around the planet.
Nowhere was Jupiter’s secular variation as large as at the planet’s Great Blue Spot, an intense patch of magnetic field near the planet’s equator.
The combination of the Great Blue Spot and strong zonal winds at this latitude result in the largest secular variations in the field on the Jovian world.
“It is incredible that one narrow magnetic hot spot, the Great Blue Spot, could be responsible for almost all of Jupiter’s secular variation, but the numbers bear it out,” Dr. Moore said.
“With this new understanding of magnetic fields, during future science passes we will begin to create a planet-wide map of Jupiter’s secular variation. It may also have applications for scientists studying Earth’s magnetic field, which still contains many mysteries to be solved.”
The research was published in the journal Nature Astronomy.
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K.M. Moore et al. Time variation of Jupiter’s internal magnetic field consistent with zonal wind advection. Nature Astronomy, published online May 20, 2019; doi: 10.1038/s41550-019-0772-5
