The solar wind is a constant feature of the solar system - except on Christmas Day 2022. On this day the solar wind suddenly disappeared completely. A NASA spacecraft orbiting Mars detected the strange event, which caused the planet's magnetosphere to triple in size.
As the sun works as a giant nuclear fusion reactor, it spits out a constant stream of charged particles we call the solar wind. The solar wind has a buffering effect on all planets. Without the protection of the magnetosphere, their atmospheres would be carried away by the solar wind.
The solar wind comes and goes, but last December it suddenly stopped completely for a few days, surprising astronomers. MAVEN, NASA's orbiter that studies the Martian atmosphere, discovered the anomaly, with its instruments recording a 100-fold drop in solar wind density. Without the pressure of the solar wind, the Red Planet's magnetosphere and ionosphere expand to three times their normal size.
Jasper Halekas, lead author of the study, said: "When we first looked at these data, the magnitude of the decline in the solar wind was almost unbelievable. We set up a working group to study this event, and we found a lot of incredible discoveries during this period."
MAVEN found that the ionosphere changed from a magnetized state to a non-magnetized state, while the electromagnetic activity in the boundary region was much less than usual. About two days later, the solar wind returned, giving the NASA team a rare window into the unusual situation.
"We're really seeing how Mars reacts when the solar wind is effectively removed," Harlekas said. "This is a good outlier study of what Mars would look like if it orbited a less 'windy' star."
The team said the drop was caused by rare interactions between solar wind waves. A fast-moving wave overtakes a slow-moving wave, causing them to compress into a single wave, leaving a less dense area behind. This phenomenon is known to occur on rare occasions, including a flyby of Earth in 1998, but there is usually no well-positioned spacecraft like MAVEN to study the phenomenon.
The event occurred as a result of increased solar activity, as the sun is at the peak of its 11-year cycle.
The research will be presented at the American Geophysical Union fall meeting. The research team describes the work in the video below.