Listening to the Aurora's Electric Current

Scientists used a powerful radar to tune into faint ‘plasma lines’—tiny ripples in the upper atmosphere—to measure the invisible electric currents that power the Northern Lights. This groundbreaking technique provides a new window into the energetic heart of the aurora.

In the winter of 1999, a team of Swedish and Japanese scientists pointed the powerful EISCAT radar towards the sky, but they weren’t just looking for the Northern Lights—they were trying to listen to them. Their goal was to measure the invisible river of electricity, known as field-aligned currents, that flows between space and Earth’s upper atmosphere, causing the aurora to glow. To do this, they hunted for an incredibly faint and elusive signal called the plasma line. These signals are like tiny, high-frequency ripples in the ionosphere, created by the same energetic electrons that paint the sky with light. By capturing and analyzing these weak echoes, the team was able to map the direction and behavior of the auroral currents with unprecedented detail, revealing the hidden electrical engine behind the celestial display.

Read the original research paper: ‘Auroral field-aligned currents by incoherent scatter plasma line observations’

We’ve moved from just seeing the aurora to directly measuring the currents that bring it to life.
— Dr. Ingemar Häggström, Lead Researcher

Imagine the ionosphere—the electrically charged upper layer of our atmosphere—is a calm pond. When a radar sends a pulse into it, the main reflection is like a big, slow wave bouncing back. This is called the ‘ion line’. But there are also much smaller, faster ripples on the pond’s surface called Langmuir waves. The radar echoes from these tiny ripples are the ‘plasma lines’. Normally, these ripples are too small to detect. However, when the aurora is active, a stream of energetic suprathermal electrons rains down from space. This stream is like constantly skipping thousands of tiny pebbles across the pond, making the ripples much stronger and easier for the radar to ‘hear’. Crucially, these plasma line echoes are split into two types: upshifted and downshifted. By measuring which type is stronger, scientists can tell which way the current of electrons is flowing.

The currents measured in this study are the final link in a gigantic electrical circuit that starts at the Sun. The solar wind, a stream of charged particles, flows past Earth and interacts with our planet’s magnetic field (magnetosphere), acting like a massive generator. This process creates enormous currents that travel through space along magnetic field lines. When these currents are funneled down into our atmosphere near the poles, they’re called field-aligned currents. They deposit huge amounts of energy, exciting atmospheric atoms and molecules and causing them to emit light—the aurora. This research provides a direct measurement of this energy deposition in action. It’s like putting a multimeter on the final wire of the circuit to see exactly how much power is being delivered to light up the sky.

These measurements give us a ground-truth look at the power lines of space weather.
— NorthernLightsIceland.com Science Team

Measuring auroral plasma lines is incredibly difficult. The signals are extremely weak and can change in fractions of a second as an auroral arc sweeps across the sky. The research team used a highly optimized experiment with a special transmission technique called an alternating code to boost sensitivity. Even then, the raw data required careful analysis. To determine the altitude and strength of the echoes, they had to fit theoretical signal shapes to the noisy measurements. The team went even further by creating a new theoretical model of the incoherent scatter spectrum that included both the normal, warm ‘thermal’ electrons of the ionosphere and the hot, fast ‘suprathermal’ electrons from the aurora. In one breakthrough case, they successfully performed a full 7-parameter fit to their data, simultaneously measuring the temperatures, densities, and—most importantly—the drift speeds of both electron populations, and thus the electric current.

The highly optimised measurements enabled investigation of the properties of the plasma lines, in spite of the rather active environment.
— Häggström et al., 1999

Q: What is an ‘incoherent scatter radar’?
A: It’s a very powerful type of radar that can probe the ionosphere. It works by bouncing radio waves off individual electrons, and the faint, ‘incoherent’ echoes carry a wealth of information about the plasma’s temperature, density, composition, and velocity.

Q: What’s the difference between diffuse aurora and an auroral arc?
A: Diffuse aurora is a faint, widespread glow that can cover large parts of the sky, looking like a dim cloud. An auroral arc is a much brighter, more structured, and dynamic feature, often appearing as a sharp ribbon or curtain of light that moves and changes shape rapidly.

Q: What is a ‘suprathermal’ electron?
A: It’s an electron that has significantly more energy than the surrounding ‘thermal’ electrons in the ionosphere. In the context of the aurora, these are the high-energy electrons that have been accelerated in space and are precipitating down into the atmosphere.

Q: Why is it important to measure these currents?
A: These currents are a key component of ‘space weather’. They can heat the upper atmosphere, interfere with satellite orbits, disrupt radio communications, and even induce currents in power grids on the ground. Understanding them helps us predict and mitigate these effects.