An aurora is one of nature’s most spectacular light shows: It looks like green or red curtains rippling across the night sky. When viewed from the ground, this colorful display might seem far away and disconnected from anything happening on Earth’s surface.
Authors
- Sanjay Kumar
Research Fellow in Climate and Space Sciences and Engineering, University of Michigan
- Mark Moldwin
Professor of Climate and Space Sciences and Engineering and Applied Physics, University of Michigan
But while these beautiful displays are part of a much larger process known as “space weather” – changes in Earth’s space environment caused by activity from the Sun – they’re closely connected to electric currents that can flow through the ground under your feet.
Space weather can create a spectacle in the sky, while also driving electric currents that can damage power grids and other technological systems on the ground. Space physicists like us want to understand how space weather can affect infrastructure on Earth.
Our research shows that a single space weather event called a substorm , during which energy is released tens of thousands of miles away from Earth, can affect the ground within minutes. The strength of the impact can vary from place to place depending on the latitude and the structure of the soil and rock below your feet.
From solar storm to substorm
Earth is constantly immersed in the solar wind : a stream of charged particles flowing outward from the Sun. Occasionally, the Sun releases a much larger eruption called a coronal mass ejection .
This cloud of charged particles and magnetic fields does not simply “hit” Earth and immediately cause power-grid failures. Instead, it can set off a chain of processes in space that can pose a threat to power grids .
First, when the coronal mass ejection reaches Earth, the charged particles and magnetic fields it carries interact with Earth’s magnetosphere . The magnetosphere is the region of space around Earth dominated by its magnetic field.
If the coronal mass ejection’s magnetic field is oriented in a certain way, it can connect with Earth’s magnetic field on the side facing the Sun. The solar wind then carries these connected field lines around Earth away from the Sun and stretches them far out into space behind Earth. This process forms a long region of stretched magnetic field lines called the magnetotail.
Magnetic energy builds up in the magnetotail as the solar wind continues to push the field lines farther into space – until they come close enough to reconnect . Then, the stored energy is released in an explosive process known as a substorm . This release occurs on the side of Earth facing away from the Sun, roughly 28,000 to 40,000 miles (45,000 to 65,000 kilometers) from Earth.
During the substorm, energetic particles shoot along the magnetic field lines toward Earth. When these particles collide with oxygen and nitrogen atoms in the upper atmosphere, about 56 to 93 miles (90 to 150 kilometers) overhead, they produce the glowing aurora.
These energetic particles also drive strong electrical currents in Earth’s upper atmosphere . Rapid changes in these currents can cause Earth’s magnetic field at the ground level to change sharply.
These changing magnetic fields can induce electric currents in the ground and in power lines, potentially damaging transformers and tripping circuit breakers.
From the magnetotail to Earth’s surface
Our team wanted to find out how closely the aurora is linked to disturbances in Earth’s magnetic field and how long it takes for a disturbance released in the magnetotail to reach Earth’s surface.
We examined two auroral events in detail by combining data from NASA spacecraft – including THEMIS, time history of events and macroscale interactions during substorms , and NOAA’s GOES, geostationary operational environmental satellites – with data from a dense network of magnetometers across Arctic Canada that measure changes in Earth’s magnetic field at the ground. These changes in the magnetic field can drive electric currents in the ground and power lines.
The observations revealed a clear sequence. When these stretched magnetic field lines on the nightside suddenly reconnected, the spacecraft recorded an abrupt change in the magnetic field and a burst of charged particles moving toward Earth. About seven minutes later, the magnetic field at the ground began to change rapidly. The effects of this energy release, tens of thousands of miles away, reached Earth’s surface within minutes.
The aurora and ground disturbance do not appear everywhere at once. In our observations, they began at lower latitudes and then spread toward the North Pole over the next 20 to 30 minutes.
Nearby geomagnetic stations, however, often recorded very different responses. Stations tens to hundreds of miles apart could see strong magnetic swings at one site and weak ones at another. The difference depended on the strength of currents in Earth’s upper atmosphere and local conditions – including how electrically conductive the upper atmosphere is overhead and how conductive the ground beneath the magnetic station is. This atmospheric conductivity can vary with sunlight and auroral activity, while ground conductivity depends on local geology and moisture levels.
Seeing a spectacular aurora does not automatically mean the ground beneath it is experiencing the strongest magnetic disturbances or electric currents.
When the aurora becomes a grid problem
History shows that space weather poses a real risk to space and ground infrastructure. During a March 1989 geomagnetic storm, people witnessed spectacular auroras as far south as Texas, while rapid changes in magnetic fields contributed to the collapse of the Hydro-Québec power system . More than 6 million people lost electricity for about nine hours on a cold winter night. With the city lights extinguished, the aurora appeared even more spectacular.
A similar combination occurred during the Halloween storms of October 2003. Auroras illuminated skies across Europe, while geomagnetically induced currents contributed to a blackout affecting about 50,000 people in Malmö, Sweden.
Recently, during the May 2024 geomagnetic storm – the strongest in over two decades – geomagnetically induced currents stressed the power grid in southern Sweden, forcing a transmission link to disconnect and temporarily interrupting the flow of electricity through that part of the grid.
Space weather forecasts can warn power grid operators hours before solar activity reaches Earth, giving them time to monitor grid conditions and prepare for possible disruptions. But the effects of substorms, including potential risks to power grids, develop after the solar wind interacts with Earth’s magnetic environment. Our research shows that these effects unfold on timescales of minutes.
The next time the aurora ripples across the night sky, remember that the colorful light shining overhead is only one part of a larger process. Tens of thousands of miles away, an explosive release of magnetic energy may have just taken place. Within minutes, its effects can begin to reach the ground.
The beauty of the aurora and the risk of power-grid disruption from invisible currents above and below are two faces of the same space weather event.
