May 18, 2023

Europeans are still trying to wrap their minds around what happened after sunset on April 23, 2023. Everyone knew that a CME was coming; photographers were already outside waiting for auroras. But when the auroras appeared, they were very strange.

"I had never seen anything quite like it," says Heiko Ulbricht of Saxony, Germany. "The auroras began to tear themselves apart, pulsating as they formed individual blobs that floated high in the sky."

auroral blobs_germany_strip.jpeg

"It literally took my breath away," he says. "My pulse was still racing hours later!" The same blobs were sighted in France and Poland, and in Denmark they were caught flashing like a disco strobe light.

Ordinary auroras don't act like this.

Indeed, "these were not ordinary auroras," confirms space physicist Toshi Nishimura of Boston University. "They are called 'proton auroras,' and they come from Earth's ring current system*."

Most people don't realize that Earth has rings. Unlike Saturn's rings, which are vast disks of glittering ice, Earth's rings are invisible to the naked eye. They are made of electricity--a donut-shaped circuit carrying millions of amps around our planet. The ring current skims the orbits of geosynchronous satellites and plays a huge role in determining the severity of geomagnetic storms.


Sometimes during strong geomagnetic storms, protons rain down from the ring system, causing a secondary shower of electrons, which strike the atmosphere and make auroras. Earth-orbiting satellites have actually seen these protons on their way down. Ordinary auroras, on the other hand, are caused by particles from more distant parts of Earth's magnetosphere and have nothing to do with Earth's ring current.

Mystery solved? Not entirely. "We still don't know why proton auroras seem to tear themselves apart in such a dramatic way," says Nishimura. "This is a question for future research."

"It was very exciting to watch," recalls Ulbricht. "I would definitely like to see these again."

Good, because they'll be back. Solar Cycle 25 ramping up to a potentially-strong Solar Maximum next year. Future storms will surely knock more protons loose from the ring current system.

Here's what to look for: (1) Proton auroras tend to appear around sunset. Why? Electric fields in Earth's magnetosphere push the protons toward the dusk not dawn side of our planet. (2) Proton auroras love to pulse--a sign of plasma wave activity in Earth's ring current. (3) Proton auroras are sometimes accompanied by deep red arcs of light (SARs), the glow of heat leaking from the ring current system. These red arcs were also seen on April 23rd.

Solar Max is coming. Let the proton rain begin!


* Earth's ring current system: A ring current is an electric current carried by charged particles trapped in a planet's magnetosphere. It is caused by the longitudinal drift of energetic particles. The ring current lies at a distance of approximately 6,200 to 37,000 miles (10,000 to 60,000 km) from Earth. The ring current was hypothesized in the early 20th century to explain observed global decreases in the Earth’s surface magnetic field, which can be measured by ground magnetometers. Such changes of the ground magnetic field are described by what's called the Sym-H index. “Previously, the state of the ring current had been inferred from the variations of the Sym-H index, but as it turns out, those variations represent the dynamics of only the low-energy protons,” said Matina Gkioulidou, a space physicist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. “When we looked at the high-energy proton data from the RBSPICE instrument, however, we saw that they were behaving in a very different way, and the two populations told very different stories about the ring current.”

The Van Allen Probes, launched in 2012, offer scientists the first chance in recent history to continuously monitor the ring current with instruments that can observe ions with an extremely wide range of energies. The RBSPICE instrument has captured detailed data of all types of these energetic ions for several years. “We needed to have an instrument that measures the broad energy range of the particles that carry the ring current, within the ring current itself, for a long period of time,” Gkioulidou said. A period of one year from one of the probes was used for the team’s research.

“After looking at one year of continuous ion data it became clear to us that there is a substantial, persistent ring current around the Earth even during non-storm times, which is carried by high-energy protons. During geomagnetic storms, the enhancement of the ring current is due to new, low-energy protons entering the near-Earth region. So trying to predict the storm-time ring current enhancement while ignoring the substantial pre-existing current is like trying to describe an elephant after seeing only its feet,” Gkioulidou said.



The flow of field lines to and from the magnetic tail and to the latter are attributed visual auroras, the geomagnetic ring current and some changing patterns of the ionospheric current system. Field lines in the tail reconnect across a ‘neutral sheet’, converting magnetic to particle kinetic energy ; the lines then contract elastically towards the Earth. Particles with small pitch angles are further accelerated mainly along the field lines and are precipitated into the ionosphere to cause auroras. Particles having large pitch angles are also accelerated but remain trapped and move deep into the magnetosphere to replenish the outer trapping zone and form a ring current. They cause electric polarization which changes the ionospheric current system and drift pattern and may explain low-latitude red arcs and the asymmetric pattern of ionospheric blackouts.