Coronal mass ejection

A coronal mass ejection in 2015, seen here by NASA’s Solar Dynamics Observatory, ended up weakening Earth’s magnetic field.

Solar Dynamics Observatory, NASA

Solar storms can weaken Earth’s magnetic field

The sun’s warm glow can sometimes turn menacing. Solar storms can shoot plasma wrapped in bits of the sun’s magnetic field into space, sweeping past Earth and disabling satellites, causing widespread blackouts, and disrupting GPS-based navigation. Now, a new study suggests that one such “coronal mass ejection” in 2015 temporarily weakened Earth’s protective magnetic field, allowing solar plasma and radiation from the same storm to more easily reach the atmosphere, potentially posing a danger to astronauts. The study also suggests a potential way to predict such storms in the future.

On 21 June 2015, a NASA spacecraft called the Solar and Heliospheric Observatory recorded a coronal mass ejection blasting off the sun at roughly 1300 kilometers per second. When the burst reached Earth roughly 40 hours later, its magnetic field was oriented opposite to Earth’s own magnetic field, which caused the fields to be attracted to each other and to interact strongly. “It is like bringing two magnets close together,” says physicist Sunil Gupta of the Tata Institute of Fundamental Research in Mumbai, India, and lead author of the new study.

The resulting interaction converted magnetic energy into kinetic energy and sent charged particles such as cosmic rays raining down on Earth’s magnetosphere, the region around Earth where its own magnetic field is stronger than other magnetic fields in space. The National Oceanic and Atmospheric Administration (NOAA) rated the geomagnetic storm 4 out of 5 on its scale of storm severity. Radio blackouts were reported, and the aurora borealis was spotted as far south as Texas.

Gupta and his team collected data from a telescope in India that measures the number of charged particles called muons that are created as byproducts when cosmic rays hit Earth’s atmosphere. Looking at data from 22 June 2015, they found a statistically significant spike in the number of muons that day. This result was consistent with a weakening of Earth’s magnetic field that allowed cosmic rays to stream more freely through Earth’s magnetnosphere and into the atmosphere without being deflected. “The weakening of Earth's magnetic field opens up floodgates for low-energy solar plasma to pour into the atmosphere,” says Gupta, whose team reports its findings this month in Physical Review Letters.

Overall, the team showed that Earth’s magnetic field is susceptible to temporary damage, rendering our planet’s atmosphere the last line of defense against energetic particles from space. Without Earth’s magnetic field, astronauts above the atmosphere are exposed to particles that can rip through human bodies and damage DNA, potentially causing cancer.    

The new results also suggest a possible method to detect impending geomagnetic storms. A successful early warning system is key to reducing the economic impact of such storms, which has been estimated by the National Academy of Sciences to be several trillion dollars in the most severe cases. Even with only a few hours of advance warning, power grids could redistribute currents to reduce their vulnerability to currents traveling through Earth and airplanes flying polar routes could be rerouted to avoid losing radio contact with controllers, for example.

Gupta and his colleagues propose using muons as early detectors of geomagnetic storms. The scientists begin by assuming that particles with lower energies take longer to travel through turbulent magnetic fields, much like a lazy moth takes longer to cross a windy valley than a quick bee. They accordingly reasoned that the highly energetic cosmic rays creating muons would reach Earth’s atmosphere ahead of the solar plasma and lower-energy cosmic rays that can be the brunt of a geomagnetic storm. “The muon burst could in principle serve as an early warning system before a storm,” Gupta says. “But a lot of research needs to be done to make it a practical proposition.”

James Chen, a plasma physicist at the Naval Research Laboratory in Washington, D.C., says that predicting the future might not be so simple. “[The muon burst] is part of an ongoing storm so it may have little forecasting value,” he says.

The results of Gupta and his team are timely: NOAA issued an alert last week warning of an impending “strong” geomagnetic storm. However, even when spotted by spacecraft, the predicted arrival times of storms are uncertain because they are based on simulations of how coronal mass ejections propagate through space. An Earth-based early alert system, based on particle data, might give less warning but be significantly more accurate.

Earlier this month, U.S. President Barack Obama signed an executive order mandating that the U.S. government “mitigate the effects of geomagnetic disturbances on the electrical power grid” and “ensure the timely redistribution of space weather alerts.” Our technological society, for all of its advances, is still susceptible to the whims of our closest star.