Earth’s aurora origin mystery solved by ‘surfing’ electrons
Earth’s aurorae are caused when electrons emitted from the Sun hurtle towards Earth and are funnelled down Earth’s magnetic field lines, where they collide with oxygen and nitrogen molecules in the ionosphere – the upper atmosphere between 80 and 600 kilometres (50 and 370 miles). The absorption of energy by these ions causes them to move to an ‘excited’ high-energy state. To relax, the molecules reradiate the energy as light, producing impressive auroral displays.
Though scientists understand what causes aurorae, a mystery remains – just how do these electrons accelerate to speeds of up to 72 million kilometres (45 million miles) per hour on the last stretch of their journey into the ionosphere? Scientists have now discovered that the electrons catch a wave – specifically Alfvén waves – that travel along magnetic field lines above aurorae. Scientists simulated conditions in the Earth’s aurora magnetosphere, then launched Alfvén waves down a 20 metre (65 feet) chamber and recorded whether electrons within the chamber were affected by the Alfvén waves.
“Measurements revealed this small population of electrons undergoes ‘resonant acceleration’ by the Alfvén wave’s electric field, similar to a surfer catching a wave and being continually accelerated as the surfer moves along with the wave,” said Gregory Howes, an associate professor of physics at the University of Iowa. This provides the first direct evidence that electrons surfing on Alfvén waves create aurora.