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Voyager 2: Digging Deeper into the Data from Uranus

Voyager 2’s flyby of Uranus and its moons occurred on January 24, 1986, returning images that for many of us will always be associated with the outpouring of grief over the loss of Challenger, which occurred a scant four days later. But Voyager’s data were voluminous, its images striking, as we examined the ice giant and its unusual moons up close. The spacecraft closed to 81,500 kilometers of the cloud tops, examining the ring system and discovering 11 new moons.

Image: The planet Uranus, in an image taken by the spacecraft Voyager 2 in 1986. The Voyager project is managed for NASA by the Jet Propulsion Laboratory. Credit: NASA/JPL-Caltech.

Uranus was already known from early analysis of the Voyager data to have an odd magnetosphere, created where solar wind plasma interacts with the planet’s magnetic field. The planet spins on its side, and its magnetic field axis is tilted 60 degrees away from its spin axis, producing a magnetosphere that wobbles in ways that researchers liken to a poorly thrown American football. Modeling a magnetosphere like this is no easy task, but we’d like to know more because these interactions with the solar wind affect local space and the circulation of plasma, with a known effect on atmospheric plasma escape.

Now we learn from a new paper taking a deeper look at the Voyager magnetometer data that the spacecraft’s passage past Uranus took it through a type of magnetic ‘bubble’ called a plasmoid, one that needs to be factored into our understanding of the planet’s magnetic environment. The paper calls plasmoids ‘helical bundle[s] of magnetic flux’ that pinch off the end of a planet’s magnetotail, as the magnetosphere is shaped and pushed by the Sun.

To turn up the plasmoid, a phenomenon little studied at the time of the flyby, authors Gina DiBraccio and Dan Gershman, both at Goddard Space Flight Center, fine-tuned the analysis of the magnetometer data by plotting new datapoints every 1.92 seconds. What they found was a quick blip that occupied 60 seconds out of a total 45-hour flyby, but it revealed a plasmoid believed to consist mostly of ionized hydrogen some 200,000 kilometers long and 400,000 kilometers across.

Plasmoids are interesting here and elsewhere because by drawing ions out of a planet’s atmosphere, they alter its composition. They’ve shown up from Mercury to Saturn and have been observed at Earth, but this is the first time one has been identified at Uranus. The smooth, closed magnetic loops the scientists found are characteristic of plasmoids formed as a rotating planet loses atmosphere to space. Says Gershman: “Centrifugal forces take over, and the plasmoid pinches off.”

Image: An animated GIF showing Uranus’ magnetic field. The yellow arrow points to the Sun, the light blue arrow marks Uranus’ magnetic axis, and the dark blue arrow marks Uranus’ rotation axis. Credit: NASA/Scientific Visualization Studio/Tom Bridgman.

This is the first observation of a plasmoid in an ice giant magnetosphere. Should we expect the same thing to occur at Neptune? From the paper:

Although no relevant measurements are available for Neptune due to the 1989 Voyager 2 flyby trajectory [Stone and Miner, 1989], we suggest that its systematic mass loss may include a significant plasmoid contribution as well. Similar to Uranus, Neptune’s magnetosphere exhibits a large variance between the rotation and magnetic axes at an angle of ~47. However, in contrast to Uranus, Neptune’s rotation axis is not aligned with the solar wind (~30 inclination). This difference may allow for internal effects to play a larger role in mass loss and overall plasma convection. For this reason, unlike at other magnetospheres throughout the solar system, the planet’s rotation and solar wind forcing may have nearly equal contributions to the energy and plasma input at Uranus and Neptune.

It’s clear that circulation within the magnetosphere and the processes of atmospheric loss are major topics for both of our ice giants, leading the authors to note the importance of new in situ measurements “to definitively determine the relative contributions of planetary rotation and solar wind forcing in driving global plasma dynamics…” For now, the ice giants remain mysterious, revealing themselves only through our single Voyager flyby.

The paper is DiBraccio and Gershman, “Voyager 2 constraints on plasmoid‐based transport at Uranus,” Geophysical Research Letters 9 August 2020 (abstract).

{ 4 comments… add one }
  • Geoffrey Hillend March 27, 2020, 23:04

    I thought the plasmoids from Earth’s magnetotail were made from the ionized hydrogen protons of the solar wind. The plasmoids are made by the reconnection of Earth’s magnetic field lines which severs the plasma sheet and a piece of the sheet, the plasmoid is carried away by the solar wind. Edward W. Hones, Jr. 1985, CSIRO Australia, Magnetic Reconnection in Earth’s Magnetotail.

    My scientific intuition tells me the same process that makes Earth’s plasmoids can explain the how the plasmoids are made which break off from Uranus magnetotail.

  • Geoffrey Hillend March 28, 2020, 14:27

    The Earth’s magnetosphere contains trapped ionized and charged particles from the solar wind so a plasmoid has to be made from the same particles. The same thing is true about Uranus.

    Some of the particles of Uranus atmosphere could be knocked free by solar wind collisions when the poles are facing the solar wind when the magnetic field is open. They could be accelerated and carried up through the poles by the magnetic field lines leaving and entering the poles, but I think that the main source of the plasmoids hydrogen would have to be the solar wind since they are more abundant.

  • ljk April 6, 2020, 13:02

    APRIL 6, 2020

    Origins of Uranus’ oddities explained by Japanese astronomers

    by Tokyo Institute of Technology

    The origins of Uranus’ unusual set of properties has now been explained by a research team led by Professor Shigeru Ida from the Earth-Life Science Institute (ELSI) at Tokyo Institute of Technology. Their study suggests that early in the history of our solar system, Uranus was struck by a small, icy planet roughly one to three times the mass of the Earth, which tipped the young planet over and left behind its idiosyncratic moon and ring system as a smoking gun.

    The team came to this conclusion while constructing a novel computer simulation of moon formation around icy planets. Most of the planets in the solar system have moons of different sizes, orbits, compositions and other properties, which scientists believe can help explain how they formed. There is strong evidence that Earth’s own single moon formed when a rocky Mars-sized body hit the early Earth almost 4.5 billion years ago. This idea explains a great deal about the Earth and the moon’s composition, and the way the moon orbits Earth.

    Full article and link to paper here:


  • ljk June 16, 2020, 17:05


    Lessons learned from (and since) the Voyager 2 flybys of Uranus and Neptune

    Heidi B. Hammel

    More than 30 years have passed since the Voyager 2 fly-bys of Uranus and Neptune. I discuss a range of lessons learned from Voyager, broadly grouped into process, planning, and people. In terms of process, we must be open to new concepts: reliance on existing instrument technologies, propulsion systems, and operational modes is inherently limiting.

    I cite examples during recent decades that could open new vistas in exploration of the deep outer Solar System. Planning is crucial: mission gaps that last over three decades leave much scope for evolution both in mission development and in the targets themselves.

    I touch only briefly on planetary science, as that is covered in other papers in this issue, but the role of the decadal surveys will be examined in this section. I also sketch out how coordination of distinct and divergent international planning timelines yields both challenges and opportunity.

    Finally, I turn to people: with generational-length gaps between missions, continuity in knowledge and skills requires careful attention to people.

    The youngest participants in the Voyager missions (myself included) now approach retirement. We share here ideas for preparing the next generation of voyagers.

    Comments: 14 pages, 10 figures, 1 table

    Subjects: Popular Physics (physics.pop-ph); Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM)

    Cite as: arXiv:2006.08340 [physics.pop-ph]
    (or arXiv:2006.08340v1 [physics.pop-ph] for this version)

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    From: H. B. Hammel [view email]

    [v1] Fri, 29 May 2020 15:38:19 UTC (7,031 KB)


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