A new study published in the Journal of Geophysical Research: Space Physics suggests that the magnetosphere of the ice giant Uranus gets flipped on and off like a light switch every day as it rotates along with the planet. It’s ‘open’ in one orientation, allowing solar wind to flow into the magnetosphere; it later closes, forming a shield against the solar wind and deflecting it away from the planet.
Earth’s magnetic field is nearly aligned with its spin axis, causing the entire magnetosphere to spin like a top along with the Earth’s rotation.
Since the same alignment of Earth’s magnetosphere is always facing toward the Sun, the magnetic field threaded in the ever-present solar wind must change direction in order to reconfigure Earth’s field from closed to open. This frequently occurs with strong solar storms.
But Uranus lies and rotates on its side, and its magnetic field is lopsided – it’s off-centered and tilted 60 degrees from its axis.
Those features cause the magnetic field to tumble asymmetrically relative to the solar wind direction as the planet completes its 17.24-hour full rotation.
Rather than the solar wind dictating a switch like here on Earth, Uranus’ rapid rotational change in field strength and orientation lead to a periodic open-close-open-close scenario as it tumbles through the solar wind.
“Uranus is a geometric nightmare. The magnetic field tumbles very fast, like a child cartwheeling down a hill head over heels. When the magnetized solar wind meets this tumbling field in the right way, it can reconnect and Uranus’ magnetosphere goes from open to closed to open on a daily basis,” said study co-author Dr. Carol Paty, associate professor in the School of Earth Atmospheric Sciences at Georgia Institute of Technology.
“This solar wind reconnection is predicted to occur upstream of Uranus’ magnetosphere over a range of latitudes, with magnetic flux closing in various parts of the planet’s twisted magnetotail.”
Reconnection of magnetic fields is a phenomenon throughout the Solar System.
It occurs when the direction of the interplanetary magnetic field — which comes from the Sun and is also known as the heliospheric magnetic field — is opposite a planet’s magnetospheric alignment.
Magnetic field lines are then spliced together and rearrange the local magnetic topology, allowing a surge of solar energy to enter the system.
Magnetic reconnection is one reason for Earth’s auroras.
Auroras could be possible at a range of latitudes on Uranus due to its off-kilter magnetic field, but the aurora is difficult to observe because the planet is nearly 2 billion miles from Earth.
Dr. Paty and her colleague, Georgia Tech Ph.D. candidate Xin Cao, used numerical models to simulate the planet’s global magnetosphere and to predict favorable reconnection locations.
They plugged in data collected by NASA’s Voyager 2 during its five-day flyby in 1986. It’s the only time a spacecraft has visited.
“Learning more about Uranus is one key to discovering more about planets beyond our Solar System,” the authors said.
“The majority of exoplanets that have been discovered appear to also be ice giants in size,” Cao added.
“Perhaps what we see on Uranus and Neptune is the norm for planets: very unique magnetospheres and less-aligned magnetic fields.”
“Understanding how these complex magnetospheres shield exoplanets from stellar radiation is of key importance for studying the habitability of these newly discovered worlds.”
Xin Cao Carol Paty. Diurnal and Seasonal Variability of Uranus’ Magnetosphere. Journal of Geophysical Research: Space Physics, published online June 7, 2017; doi: 10.1002/2017JA024063