The ionosphere is a layer high above Earth where our atmosphere starts to blend into the edge of space. It’s an active region that can be affected by solar flares or powerful explosions from deep space. In 2022, astronomers detected an extremely powerful gamma ray burst from outside our galaxy—that hit Earth’s upper atmosphere and changed it in unusual ways.

Surprisingly, only some of the techniques that monitor the ionosphere picked up changes. One method, based on very low frequency (VLF) radio waves, showed clear signs of disturbance. But another method that listens for natural resonances in Earth’s atmosphere—called Schumann resonances—did not show anything unusual.

We tried something different: looking at how the burst affected lightning-generated radio waves. During the gamma ray burst, we found that lightning waves at extremely low frequencies traveling from Africa to Europe actually slowed down. This was unexpected, because solar flares usually make the waves travel faster.

This slowdown showed that the gamma rays increased the number of electrons in several layers of the ionosphere. That change in electron density explains why some detection methods worked and others didn’t.

This new approach—tracking lightning wave speed—could become a powerful tool to study effects on Earth’s atmosphere.