Revisiting Decades-Old Voyager 2 Data, Scientists Find One More Secret

This oddity drew DiBraccio and her coauthor Dan Gershman, a fellow Goddard space physicist, to the project. Both were part of a team working out plans for a new mission to the ‘ice giants’ Uranus and Neptune, and they were looking for mysteries to solve. Uranus’ strange magnetic field, last measured more than 30 years ago, seemed like a good place to start.

So they downloaded Voyager 2’s magnetometer readings, which monitored the strength and direction of the magnetic fields near Uranus as the spacecraft flew by. 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?” Gershman asked DiBraccio, catching sight of the squiggle.

Magnetometer data from Voyager 2’s 1986 flyby of Uranus. The red line shows the data averaged over 8-minute periods, a time cadence used by several previous Voyager 2 studies. In black, the same data is plotted at a higher time resolution of 1.92 seconds, revealing the zigzag signature of a plasmoid.

Credits: NASA/Dan Gershman

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, or electrified gas, 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.

DiBraccio ran the data through her processing pipeline and the results came back clean. “I think it definitely is,” she said.

The bubble escape

The plasmoid DiBraccio and Gershman 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,” Gershman said.

Comparing their results to plasmoids observed at Jupiter, Saturn and Mercury, they estimated a cylindrical shape at least 127,000 miles (204,000 kilometers) long, and up to roughly 250,000 miles (400,000 kilometers) across. Like all planetary plasmoids, it was full of charged particles — mostly ionized hydrogen, the authors 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, DiBraccio and 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,” 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,” DiBraccio said. “Obviously it’s not going to show you anything about what the Sahara or Antarctica is like.”

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,” DiBraccio said. “You’re always going somewhere you don’t really know.”

Contacts and sources:
Miles Hatfield
NASA’s Goddard Space Flight Center,