It turns out that lightning striking Earth can actually have an affect on space activity. (Micah Tindell/Unsplash)

BOULDER, Colo. — Lightning doesn’t just strike downward; sometimes its effects reach thousands of kilometers into space. In a discovery that connects the weather we experience on Earth with the physics of our planet’s radiation belts, researchers from the University of Colorado Boulder have found that lightning storms can dislodge streams of near-light-speed electrons from space, sending them raining down toward our atmosphere in what amounts to an energetic cosmic shower.

Earth’s Cosmic Shield: Understanding Radiation Belts

For decades, researchers have known about Earth’s radiation belts, doughnut-shaped regions of charged particles trapped by our planet’s magnetic field. These belts, discovered in the early days of the Space Age, have generally been understood as separate systems from Earth’s atmosphere and weather. But new research published in Nature Communications shows that lightning bolts can cause streams of ultra-fast electrons to rain down from the inner radiation belt.

Most people are familiar with lightning as the flash that precedes thunder during storms. What’s less known is that lightning generates electromagnetic waves that travel not just through air but also through space around Earth. These waves, called whistlers (because they make a distinctive sound when converted to audio), can interact with electrons trapped in the radiation belts.

Previous research had established that lightning could knock loose lower-energy electrons from these belts, in a phenomenon called Lightning-Induced Electron Precipitation (LEP). But this new study provides the first direct evidence that lightning can also dislodge much more energetic electrons, ones traveling at nearly the speed of light.

“These particles are the scary ones or what some people call ‘killer electrons,’” says lead study author Max Feinland, a CU Boulder graduate, in a statement. “They can penetrate metal on satellites, hit circuit boards and can be carcinogenic if they hit a person in space.”

Cosmic Pinball: How Lightning Triggers Electron Showers

The research team analyzed a decade of data from NASA’s Solar Anomalous and Magnetospheric Particle Explorer (SAMPEX) satellite. SAMPEX carried instruments that could detect high-energy electrons, allowing researchers to identify 45 distinct events where streams of electrons with energies of at least 1 million electron volts (1 MeV), about 94% the speed of light, were detected coming from the inner radiation belt.

Rather than a single burst, the electrons arrived in a sequence of pulses separated by about 200 milliseconds, exactly what would be expected from electrons bouncing back and forth along Earth’s magnetic field lines after being disturbed by a lightning-generated whistler wave.

“You have a big blob of electrons that bounces, and then returns and bounces again,” says study co-author Lauren Blum, an assistant professor in the Laboratory for Atmospheric and Space Physics at CU Boulder. “You’ll see this initial signal, and it will decay away.”

Following a lightning strike, radio waves from Earth kick off a kind of manic pinball game in space. They knock into electrons in the inner belt, which then begin to bounce between Earth’s northern and southern hemispheres—going back and forth in just 0.2 seconds. Each time the electrons bounce, some of them fall out of the belt and into our atmosphere.

The discovery happened almost by accident. Feinland was analyzing data from the SAMPEX satellite when he saw something odd: clumps of what seemed to be high-energy electrons moving through the inner belt, which usually contains much lower-energy electrons.

“I showed Lauren some of my events, and she said, ‘That’s not where these are supposed to be,’” says Feinland. “Some literature suggests that there aren’t any high-energy electrons in the inner belt at all.”

Unexpected Findings and Geographic Patterns

How did this happen? Geomagnetic storms are disturbances in Earth’s magnetic field caused by solar activity. During these storms, high-energy electrons can be injected into the inner belt, where they remain trapped until something, like a lightning-generated whistler wave, knocks them loose.

“Typically, the inner belt is thought to be kind of boring,” says Blum. “It’s stable. It’s always there.”

When the researchers compared these electron events to lightning data from the National Lightning Detection Network, they found direct links. For events occurring over the United States, high-power lightning strikes preceded the electron microbursts by about one second. This is just the time needed for the electromagnetic waves from lightning to reach the radiation belt and trigger the electron cascade.

Most of the observed events occurred in the southern hemisphere, particularly around the southern tip of South America and the eastern coast of South Africa. This geographical clustering is related to Earth’s peculiar magnetic field, which is weaker in the southern hemisphere due to the South Atlantic Anomaly, a region where the radiation belts dip closer to Earth’s surface.

Space Weather and Earth Weather: A Two-Way Connection

This research may reveal that ordinary thunderstorms, occurring thousands of times daily around the world, may be having continuous effects on the space environment around our planet. The high-energy electrons dislodged by lightning could potentially damage spacecraft electronics or pose radiation risks to astronauts. The team’s results could help satellites and even astronauts avoid dangerous radiation in space.

“This is one kind of downpour you don’t want to get caught in,” adds Feinland.

Lightning, among the most familiar and ancient of natural phenomena, still has secrets to reveal, including its surprising role as a cosmic trigger, connecting the weather we experience with the mysterious realm of radiation belts circling silently above us. Knowing when and where these high-energy electron showers might occur could become as important to spacecraft operators as weather forecasts are to pilots. That lightning bolt illuminating your neighborhood could be setting off alarms on satellites orbiting high above in space.