Catching the Weird, Pulsing Auroras of Uranus

By the end of this article, you will understand why Uranus has the strangest auroras in the solar system, and how scientists caught them flashing in deep space.

In 2012 and 2014, scientists pointed the Hubble Space Telescope at Uranus, waiting for solar wind shocks to hit the ice giant. They were hunting for something incredibly rare: Uranian auroras. Because Uranus is so far away, observing its space weather is a massive challenge. But when the data came back, the team found a Surprise: six new detections of ultraviolet auroras, including the most intense ones ever recorded. This wasn’t a steady glow. The images revealed localized, glowing spots rotating with the planet. The active regions lit up for a few tens of minutes, but the core emissions were made of brief, intense pulses lasting less than two seconds. They had successfully mapped the hidden magnetic footprint of a deeply alien world.

Original Paper: ‘Uranus’ aurorae past equinox’ by L. Lamy et al., Journal of Geophysical Research (2017)

The detected emissions occur close to the expected arrival of interplanetary shocks.
— L. Lamy et al.

This is NOT like the auroras on Earth. On Earth, magnetic fields guide solar particles to the north and south poles, creating a continuous, glowing ring of light. Uranus is fundamentally different. The planet’s magnetic field is a mess. It is tilted 59 degrees away from the axis the planet spins on, and the magnetic center does not even line up with the center of the planet. The Salient Idea here is ‘pulsed cusp emissions.’ Because of this weird geometry, the solar wind hits the magnetic field at bizarre angles, causing localized, explosive reconnections. Instead of a steady halo, particles get dumped into the atmosphere in sudden, intense bursts, creating glowing spots that flash like a cosmic strobe light.

Even two billion miles away, the sun dictates the weather. Just like on Earth, auroras on Uranus are driven by the solar wind—a stream of charged particles blasted out by our star. By tracking when massive bursts of solar wind were predicted to reach Uranus, scientists knew exactly when to look. When these high-pressure fronts slam into Uranus’ lopsided magnetic shield, the shield compresses. This sudden squeeze forces trapped plasma down into the planet’s hydrogen-rich atmosphere. It proves that no matter where you are in the solar system, if you have an atmosphere and a magnetic field, the sun’s invisible storms will find a way to light up your skies.

Possible Uranian cusp aurorae discussed above might thus be similarly triggered by solar wind compressions.
— The Research Team

How do you see a flash of light that lasts one second, two billion miles away? It requires incredible Knowledge and Tools. The team used Hubble’s Space Telescope Imaging Spectrograph (STIS) in ‘time-tag mode.’ This instrument does not just take a long-exposure photograph; it records the exact arrival time of every single photon of ultraviolet light at a 125-microsecond resolution. By filtering out background noise and graphing the photon hits over time, they found clear spikes—auroral bursts well above the background level. They were literally counting individual packets of light to prove these alien auroras were flashing.

The time-tag mode enables us to process the data at the desired time resolution and to build time series of the counts.
— L. Lamy et al.

Q: Why didn’t Voyager 2 see these flashing spots when it flew by in 1986?
A: Uranus has extreme seasons that last decades. When Voyager 2 flew by, it was Solstice, and the magnetic geometry was totally different. By 2014, it was past Equinox, changing how the solar wind interacted with the planet.

Q: Are Uranus’ auroras visible to the naked eye?
A: No. Aside from being incredibly far away, these auroras radiate mostly in the far-ultraviolet spectrum, which is invisible to human eyes but perfectly visible to the Hubble Space Telescope.