Uranus possesses an intrinsic magnetic field that encircles the ice giant and influences the local space environment. The solar wind plasma, made up of charged particles, flows away from the Sun and interacts with Uranus’ magnetic field to form what is called a planetary magnetosphere. In a new study, a duo of space physicists at NASA’s Goddard Space Flight Center analyzed high-resolution magnetic field data collected by Voyager 2 during the Uranus flyby in 1986 and found that the spacecraft flew through a plasmoid — a giant bubble filled with planetary plasma — in the tail of Uranus’ magnetosphere.
Uranus in natural colors. Image credit: NASA / ESA / Hubble Team / Erich Karkoschka, University of Arizona.
Little known at the time of Voyager 2’s flyby, plasmoids have since become recognized as an important way planets lose mass.
These giant bubbles of plasma pinch off from the end of a planet’s magnetotail, the part of its magnetic field blown back by the Sun like a windsock.
With enough time, escaping plasmoids can drain the ions from a planet’s atmosphere, fundamentally changing its composition.
They had been observed at Earth and other planets, but no one had detected plasmoids at Uranus — yet.
Dr. Gina DiBraccio, a space physicist at NASA’s Goddard Space Flight Center and project scientist for the Mars Atmosphere and Volatile Evolution (MAVEN) mission, and her colleague, Dr. Dan Gershman, analyzed data from Voyager 2’s magnetometer.
With no idea what they’d find, they zoomed in closer than previous studies, plotting a new datapoint every 1.92 seconds.
Smooth lines gave way to jagged spikes and dips. And that’s when they saw it: a tiny zigzag with a big story.
“Do you think that could be … a plasmoid?” Dr. Gershman asked Dr. DiBraccio, catching sight of the squiggle.
The plasmoid they found occupied a mere 60 seconds of Voyager 2’s 45-hour-long flight by Uranus. It appeared as a quick up-down blip in the magnetometer data.
“But if you plotted it in 3D, it would look like a cylinder,” Dr. Gershman said.
Comparing their results to plasmoids observed at Jupiter, Saturn and Mercury, they estimated a cylindrical shape at least 204,000 km (127,000 miles) long, and up to roughly 400,000 km (250,000 miles) across.
Like all planetary plasmoids, it was full of charged particles — mostly ionized hydrogen, the researchers believe.
Readings from inside the plasmoid — as Voyager 2 flew through it — hinted at its origins.
Whereas some plasmoids have a twisted internal magnetic field, Dr. DiBraccio and Dr. Gershman observed smooth, closed magnetic loops. Such loop-like plasmoids are typically formed as a spinning planet flings bits of its atmosphere to space.
“Centrifugal forces take over, and the plasmoid pinches off,” Dr. Gershman said.
According to their estimates, plasmoids like that one could account for between 15 and 55% of atmospheric mass loss at Uranus, a greater proportion than either Jupiter or Saturn. It may well be the dominant way Uranus sheds its atmosphere to space.
How has plasmoid escape changed Uranus over time? With only one set of observations, it’s hard to say.
“Imagine if one spacecraft just flew through this room and tried to characterize the entire Earth. Obviously it’s not going to show you anything about what the Sahara or Antarctica is like,” Dr. DiBraccio said.
But the findings help focus new questions about the planet. The remaining mystery is part of the draw.
“It’s why I love planetary science. You’re always going somewhere you don’t really know,” Dr. DiBraccio said.
The findings were published in the journal Geophysical Research Letters.
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Gina A. DiBraccio & Daniel J. Gershman. Voyager 2 constraints on plasmoid-based transport at Uranus. Geophysical Research Letters, published online August 9, 2019; doi: 10.1029/2019GL083909
