Voyager 2 data hid a secret about Uranus

In January 1986, the Voyager 2 probe flew over Uranus, recording its magnetic field in passing. New analyzes of the data reveal that the probe had passed through a plasma bubble called a plasmoid. This kind of bubble is involved in the erosion of planetary atmospheres.

After the discovery of Hot Jupiters in the world of exoplanets, the astronomers also discovered the equivalents, mass and size of planets Neptune and Uranus. To advance the understanding of the data which will flow more and more on these exoplanets, it is therefore necessary to return to visit Neptune and Uranus but more completely than the mission had done Voyager 2.

The nasa probe had met the ice giant Uranus for a few hours on January 24, 1986 when it was launched in 1977. It approached it about 81,000 kilometers away, gathering information on the magnetic field and the field of gravity of the planet, taking pictures of its surface at -214 ° C as well as its moons and its rings.

The data collected can still speak and they are still being studied as shown in a publication in Geophysical Research Letters. Physicist Gina DiBraccio from Goddard Space Flight Center of the Nasa explains, with his colleague Dan Gershman, that he realized that Voyager 2 had passed inside what the planetologists, specialists in plasmas in the magnetospheres and ionospheres of Solar system, call a plasmoid (plasmoid in English).

Plasma bubbles confined by a magnetic field

This name was created in 1956 by the physicist Winston Bostick (1916-1991) to initially designate a sort of torus of plasma confined by its magnetic field, similar to smoke rings. More generally today, it also designates kinds of plasma bubbles also confined by a magnetic field and which can detach from the tails of magnetospheres like that of the Earth.

We know that these bubbles therefore take with them a part of the ionosphere of a planet, which is filled with particles originally from its atmosphere. Plasmoids contribute to complex interactions, the solar wind and the atmospheres planets with a magnetosphere, which can lead to atmospheric erosion. Mars was the victim of erosion which led it to lose its atmosphere and the water of its oceans.

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DiBraccio and Gershman discovered this plasmoid in the Voyager 2 data because they are part of a group of researchers working on a future mission to Uranus and Neptune. They were particularly interested in an oddity of Uranus. In the case of the Earth, its axis of rotation is slightly inclined with respect to the axis of its dipole magnetic field. This is well understood because, within the framework of the theory of Earth geodynamo, there is a connection between its rotation, via the Coriolis force, and the generation of its magnetic field by turbulent and convective currents in the metallic part of its core liquid.

Uranus clashes in this regard, firstly because, unlike the other planets of the Solar System, its axis of rotation is not almost perpendicular but almost parallel to its orbital plane but especially, secondly, because this axis of rotation is inclined 60 ° from its magnetic axis.

A key to the erosion of planetary atmospheres

Review with look Voyager 2 data concerning Uranus’ magnetic field could therefore be fruitful, and this is how the two researchers re-analyzed the data from the magnetometer of the probe and discovered by zooming in on the measurement curves a magnetic field peak for 60 seconds which had remained unnoticed by their colleagues of the last century.

By comparing these data with those of the plasmoids already observed and studied in the case of Jupiter, Saturn and Mercury, they estimated that it had a cylindrical shape at least 204,000 kilometers long and up to about 400,000 kilometers in diameter. Like all planetary plasmoids (there are some with the Sun and the comets), it was full of charged particles, mainlyhydrogen ionized.

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The magnetometer data also seem to indicate that the internal structure of the plasmoid’s field lines is fairly regular with smooth, closed magnetic loops, exactly what you would expect if this object were ejected from the planet’s magnetosphere by her centrifugal force, taking away at the same time part of its atmosphere.

The two planetologists estimate that plasmoids like this one could represent between 15 and 55% of the loss of atmospheric mass of Uranus. But do not worry, as in the case of Earth, there are reserves for a very long time.

A presentation of Uranus and its many singularities. To obtain a fairly faithful translation into French, click on the white rectangle at the bottom right. English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Translate automatically”. Choose “French”. © National Geographic

What you must remember

  • In January 1986, the Voyager 2 probe flew over Uranus, recording its magnetic field in passing. New analyzes of the data reveal that the probe had passed through a plasma bubble called a plasmoid.
  • This kind of bubble is involved in the erosion of planetary atmospheres. In the case of Uranus, plasmoids like this could represent between 15 and 55% of the loss of its atmospheric mass.

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