On Mother's Day weekend in May 2024, Earth experienced its most powerful geomagnetic storm in two decades, spawning auroras — the ribbony bands of light in the nighttime skies in the Northern and Southern hemispheres — that may have been among the strongest displays on record in the past 500 years .
In a new study, space physicists led by the University of Iowa report the most detailed account to date about a string of eruptions on the sun that produced the Mother's Day storms on Earth.
The researchers found that the geomagnetic storms' strength was caused by 10 coronal mass ejections — fiery eruptions from the sun's surface that fling charged particles into space — occurring over four days. During that time, several of the earliest solar outbursts merged, expanding the magnetic cloud as it journeyed toward Earth. Later solar explosions produced their own magnetic clouds that caught up with and pushed the original, consolidated cloud, further expanding its size and strength.
That oversized, supercharged cloud then collided with Earth's magnetic shield, or magnetosphere, spawning the intense storms and auroras that could be seen as far away as Mississippi, the Himalayas, northern Italy, and Queensland, Australia.
"We haven't seen any case at least with current observation and simulation techniques where we have 10 coronal mass ejections that erupted and merged together," says Shirsh Soni , postdoctoral research fellow in the Department of Physics and Astronomy at Iowa and the study's corresponding author. "This was unique."
The solar eruptions that led to the geomagnetic storms on Earth occurred during solar maximum, a natural stage of the roughly 11-year cycle in which the sun transitions between low and high magnetic activity. Scientists believe the sun is about to enter a declining activity phase.
Eventually, though, the sun will enter an active phase, and work from Soni and computational modeling from researchers at the University of Leuven in Belgium could help identify the effects of solar outbursts before they arrive at our planet.
"Our study is important because most of the space weather models we have are for individual eruptions," Soni explains. "Now we know about interacting coronal mass ejections and the effects these merged clouds can have on Earth, and that is very important to understand to create more accurate space weather forecasts."
The researchers unspooled the solar outbursts through plasma and magnetic field data captured by the Wind instrument, which measures the solar wind — the continual stream of charged particles from the sun — near where it impacts Earth's magnetic shield.
Data from Wind showed four magnetic clouds passing by, which is one reason why physicists originally had surmised there were fewer solar eruptions. But Soni's team saw in the data that the solar wind speed after the clouds had passed was more than twice the typical velocity. The team theorized this was because there had been more undocumented eruptions on the sun.
The researchers employed advanced modeling and simulations of the sun's surface activity to document that there were 10 coronal mass ejections.
"We wanted to produce the entire picture," Soni says.
That picture revealed some key insights:
- Magnetic clouds from the first seven coronal mass ejections merged after erupting from the sun.
- Two subsequent coronal mass ejections merged with each other and caught up with the original cloud.
- The final coronal mass ejection produced a cloud that traveled initially at an eye-popping speed of nearly 1,000 miles per second before colliding with the aggregated magnetic cloud and pushing that cloud toward Earth.
This unusual chain of events spawned the geomagnetic storm on Earth "that was super intense," Soni says.
The study, "Comprehensive MHD modelling of ten successive CMEs driving a historic geomagnetic storm — the 2024 Mother's Day event," was published online Aug. 5 in the Astrophysical Journal.
Contributing authors are Anwesha Maharana, from the Centre for Mathematical Plasma Astrophysics, in Leuven, Belgium; Sanchita Pal, from Indian Institute of Technology Roorkee, in Uttarakhand, India; and Stefaan Poedts, from the Centre for Mathematical Plasma Astrophysics and the University of Maria Curie-Sklodowska, in Lublin, Poland.
The European Union funded the research, through grants to Maharana and Poedts.
