Universe Today
It's well-understood that magnetic shields protect planets from stars. These shields, generated on planets like Earth by their rotating, convective cores, deflect damaging solar wind that can strip away atmospheres and expose the planetary surfaces. If Earth didn't have one, then the ozone layer would be destroyed, and UV radiation levels on the planet's surface would reach hazardous levels, damaging DNA and causing cancer in lifeforms.
Eventually, Earth's water would disappear into space, and Earth would look a lot more like Mars does today, after it lost its magnetosphere: cold, dry, and lifeless.
But new research shows that worlds without protective magnetospheres can still have some protection from the solar wind and radiation. The research is titled "Detection of Zwan-Wolf effect in the ionosphere of Mars](https://www.nature.com/articles/s41467-026-72251-9)," and it's published in Nature Communications. The lead author is Christopher Fowler, a research assistant professor at the WVU Eberly College of Arts and Sciences and the WVU Center for Kinetic Plasma Physics.
This finding comes from observations with NASA's MAVEN (Mars Atmosphere and Volatile Evolution) spacecraft, which just ended its mission after more than 11 years of operation. One of MAVEN's primary science goals was to measure Mars' upper atmosphere and ionosphere to determine the structure and composition, and the processes that shape it.
The research focuses on the Zwan-Wolf effect, which is how strong dipole magnetic fields on planets like Earth deflect charged particles in the solar wind. When these particles slam into the magnetic field lines, they're forced to flow around the planet and continue on their way. No harm done.
"While the effect has been most studied at Earth, candidate observations have also been made at the outer planets," the authors write. "Here we present observations of the Zwan-Wolf effect occurring at Mars, an unmagnetized planet that lacks a dipole magnetic field."
On unmagnetized planets, the ZW effect is created by a planet's ionosphere, which is like an electrified outer shell that's ionized by solar radiation. On unmagnetized planets, the effect is below MAVEN's detection threshold most of the time, and that's why it's not widely observed. MAVEN only detected it during a powerful coronal mass ejection (CME) that struck Mars in December 2023.
"Our analysis of observations made by NASA’s Mars Atmosphere and Volatile EvolutioN spacecraft suggest that while the Zwan-Wolf effect is likely continuously active within the Martian ionosphere, it operates below detection thresholds of typical plasma analyzers most of the time," the researchers explain in their article. "However, an interplanetary coronal mass ejection impact at Mars in December 2023 greatly enhanced the Zwan-Wolf effect within the ionosphere, allowing it to be observed, and highlighting the importance of space weather events for these unmagnetized planetary systems."
This video shows the Sun emitting several CMEs.
Study lead author Fowler explained in a press release how the flow of charged particles from the Sun is kind of like a stream of water flowing around rocks.
“However,” Fowler said, “because the water in that stream is relatively dense, physical collisions between water molecules bumping into each other and the rock determine how the water is diverted. In contrast, the environment in space is so tenuous that solar wind particles do not bump into each other. Instead, electromagnetic forces control how particles are deflected around these bodies."
On a planet that has a magnetosphere, the ZW effect plays a specific role. It adds to the magnetic effect, helping to force the solar wind's plasma through magnetic flux tubes. These are cylindrical, tube-like structures, with magnetic field lines parallel to the tube. In these tubes, the magnetic field is squeezed and contained.
*This simple schematic shows a flux tube. No magnetic can enter through the sides. The amount of magnetic flux that leaves the tube through S2 is the same amount that enters the tube through S1. Image Credit: By Chetvorno - Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=81086298*
But for planets without magnetospheres, like Mars, the ZW effect is different. These unmagnetized planets have different interactions with the solar wind. When they have atmospheres and ionospheres, however tenuous, interactions with the solar wind create their own magnetospheres. Though weaker, they can still protect planets like Mars and Venus from some solar wind.
"We present here comprehensive observations of the Zwan-Wolf effect within a planetary ionosphere, namely that of Mars," the researchers write in their article. "The observations were made during the aftermath of an interplanetary coronal mass ejection (ICME) impact at Mars in December 2023, when the magnetosphere was highly disturbed and recovering from the space weather event."
The induced magnetosphere in Mars's ionosphere creates magnetic field lines that drape around the planet's dayside. Their shape is analogous to the shape of Earth's magnetosphere. The ZW effect operates in this environment.
Scientists thought that the ZW effect only happened in a planet's magnetosphere, above its actual atmosphere. Seeing it in Mars' ionosphere is a new discovery.
“The squeezing helps move the solar wind plasma around the planet, and it makes the plasma less dense in front of the planet,” Fowler said. “By finding this effect in the atmosphere of Mars, we are discovering new ways in which our sun can interact with and affect planets in our solar system. It’s amazing to think that an eruption on the sun can disturb the atmosphere of Mars 142 million miles away.”
The ZW effect at Mars was only detected because of a Interplanetary Coronal Mass Ejection in 2023. Interactions between the solar wind plasma and the ionosphere created large amplitude magnetic structures that draped around the planet. "Deflections in the plasma flow were observed coincident at the leading edges of each magnetic structure: the ionospheric plasma was flowing downward and tailward at each leading edge," the authors write.
The effects are likely continuous but undetectable most of the time. They were amplified into MAVEN's detection range during the ICME.
“We think this effect could occur in the Martian atmosphere all the time, but it’s usually such a small effect that our instruments aren’t sensitive enough to detect it,” Fowler said.
“The solar storm really hit Mars hard and disturbed the entire space environment around the planet. This seems to have amplified the Zwan-Wolf effect so that we could observe it during this time period. We got lucky, being in the right place at the right time with MAVEN to see this.”
The same thing is probably happening on other unmagnetized bodies in the Solar System, like Venus, comets, and even Saturn's moon Titan. "Understanding the generation, propagation, and impact of these structures at Mars thus broadens our knowledge of how our Sun interacts with our solar system, and of the physical processes that facilitate this interaction," the authors explain in their research article.
Since this is the first detection of the ZW effect on Mars, there are questions waiting to be answered. One concerns the depth of the effect.
“We observed these signatures all the way down to the lowest altitudes that MAVEN sampled, suggesting that it impacted the atmosphere even below the spacecraft,” Fowler said. Since MAVEN's mission was to study Mars' atmosphere, it followed an elliptical orbit at different altitudes. During standard science orbits, it orbited at about 150 km above the Martian surface. During its deeper dip campaigns it came to within 125 km of the surface.
“Understanding how these space weather events impact our solar system is important, not only for keeping our robotic — and potentially, human — explorers safe in the future, but for protecting the space assets that we rely on for our everyday technology here on Earth.”
