The MAVEN Mars orbiter witnessed a brief restoration of the long-lost Martian magnetosphere and a temporary tripling of its ionosphere, where the Sun ionizes the upper atmosphere.

The stream of charged particles known as the solar wind waxes and wanes. Mostly these are close enough to an average figure that the effects on planets are small, but occasionally the cocooning magnetic field is significantly affected. We now know its distance from the Sun doesn’t prevent proportionally similar effects on Mars.

The Mars Atmosphere and Volatile EvolutioN (MAVEN) orbiter’s prime task is to discover how and when Mars lost its atmosphere. The solar wind is the chief suspect here, its constant bombardment thought to have swept most of the once-abundant air away. Consequently, watching the way the solar wind affects the paltry remnants of the atmosphere and its former magnetic shield is a core activity.

The solar wind varies in speed, and sometimes a fast-moving region overtakes a slower patch. The result is a denser area, followed by a relative void. In late December 2022 a void worth screaming into emerged and MAVEN got to watch its effects on Mars, arriving on Christmas Day.

A year later the results are being released in the form of a conference presentation. They reveal both the magnetosphere and atmosphere expanded, having briefly escaped the solar wind’s pressure. While that was inevitable, the extent to which each occurred was remarkable.

"When we first saw the data, and how dramatic the drop in the solar wind was, it was almost unbelievable," Professor Jasper Halekas of the University of Iowa said in a statement. "We formed a working group to study the event, and we have found this time period to be rich with incredible findings."

The density of solar wind particles in the void was just 1 percent of the normal, equivalent to going from Earth’s atmosphere to that of Mars. Both the Martian magnetosphere and ionosphere tripled in size, stretching by thousands of kilometers. The atmosphere as a whole also spread out, although of course there was no increase in gas quantity.

The Martian ionosphere became demagnetized and activity faded at the boundary between the solar wind and the magnetosphere.

"We are really getting to see how Mars responds when the solar wind is effectively removed," Halekas said. "It makes for a great outlier study on what Mars would be like if it were orbiting a less ‘windy’ star."

We have discovered the existence of thousands of rocky planets, but can only study four closely, a poor sample size in anyone’s terms. Extreme variations like this offer a glimpse of how things could be for one of those four if one major forces shaping it were different. Halekas is co-author of a preprint arguing the study of the effects of the Sun has been impoverished by NASA’s Heliophysics Division’s focus on Earth’s magnetosphere and atmosphere at the expense of other planets.

This is the first time the wind has dropped to anything like this extent since MAVEN arrived at Mars nine years ago. Indeed, scientists studying the results had to go back to 1999 to find a time when Earth experienced anything similar.

The observations and initial analysis have been presented at the American Geophysical Union Fall conference, with several related talks about the event and its cause.