Even Without A Magnetosphere, Mars Can Still Deflect Some Solar Wind

It is well understood that magnetic shields protect planets from the harsh environment of their host stars. On Earth, these shields are generated by a rotating, convective core deep within the planet. This magnetic field deflects damaging solar wind, a stream of charged particles emitted by the Sun. Without this protection, the solar wind would strip away the atmosphere and expose the planetary surface to dangerous levels of radiation. Specifically, the ozone layer would be destroyed, and ultraviolet radiation levels would reach hazardous heights. This increased radiation would damage DNA and cause cancer in lifeforms. Eventually, Earth’s water would disappear into space. The planet would look very different, resembling Mars today: cold, dry, and lifeless.

However, new research shows that worlds without protective magnetospheres can still have some protection from solar wind and radiation. The study, titled "Detection of Zwan-Wolf effect in the ionosphere of Mars," was published in the journal Nature Communications. The lead author is Christopher Fowler, a research assistant professor at the West Virginia University Eberly College of Arts and Sciences and the WVU Center for Kinetic Plasma Physics. This finding comes from observations made by NASA’s MAVEN spacecraft. MAVEN, which stands for Mars Atmosphere and Volatile Evolution, ended its mission after more than eleven years of operation in 2025. One of MAVEN’s primary science goals was to measure Mars’ upper atmosphere and ionosphere. Scientists wanted to determine the structure and composition of these layers and understand the processes that shape them.

The research focuses on a phenomenon known as the Zwan-Wolf effect. On planets like Earth, strong dipole magnetic fields create this effect by deflecting charged particles in the solar wind. When these particles slam into the magnetic field lines, they are forced to flow around the planet and continue on their way. In this scenario, no harm is done to the planet’s surface. While the effect has been most studied at Earth, candidate observations have also been made at the outer planets. The researchers present observations of the Zwan-Wolf effect occurring at Mars. Mars is an unmagnetized planet that lacks a global dipole magnetic field.

On unmagnetized planets, the Zwan-Wolf effect is created by the planet’s ionosphere. The ionosphere is like an electrified outer shell that is ionized by solar radiation. On such planets, the effect is usually below MAVEN’s detection threshold. This is why it is not widely observed. MAVEN only detected the effect during a powerful coronal mass ejection, or CME, that struck Mars in December 2023. The researchers explain in their article that while the Zwan-Wolf effect is likely continuously active within the Martian ionosphere, it operates below the detection thresholds of typical plasma analyzers most of the time. However, an interplanetary coronal mass ejection impact at Mars in December 2023 greatly enhanced the effect within the ionosphere. This allowed it to be observed and highlighted the importance of space weather events for these unmagnetized planetary systems.

This video shows the Sun emitting several coronal mass ejections.

Study lead author Fowler explained how the flow of charged particles from the Sun is similar to a stream of water flowing around rocks. In that analogy, the water molecules collide with each other and the rock to 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 with a magnetosphere, the Zwan-Wolf 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. Inside these tubes, the magnetic field is squeezed and contained. The squeezing helps move the solar wind plasma around the planet. It also makes the plasma less dense in front of the planet.

For planets without magnetospheres, like Mars, the Zwan-Wolf effect is different. These unmagnetized planets have different interactions with the solar wind. However, if they have atmospheres and ionospheres, even if they are thin, interactions with the solar wind create their own magnetospheres. Though weaker than Earth’s, these induced magnetospheres can still protect planets like Mars and Venus from some solar wind. The induced magnetosphere in Mars’ ionosphere creates magnetic field lines that drape around the planet’s dayside. Their shape is analogous to the shape of Earth’s magnetosphere. The Zwan-Wolf effect operates in this environment.

Scientists previously thought that the Zwan-Wolf effect only happened in a planet’s magnetosphere, above its actual atmosphere. Seeing it in Mars’ ionosphere is a new discovery. Fowler noted that finding this effect in the atmosphere of Mars allows us to discover new ways in which our Sun interacts with and affects planets in our solar system. It is amazing to think that an eruption on the Sun can disturb the atmosphere of Mars 142 million miles away.

The Zwan-Wolf effect at Mars was only detected because of an 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 effects are likely continuous but undetectable most of the time. They were amplified into MAVEN’s detection range during the ICME.

Fowler stated that they think this effect could occur in the Martian atmosphere all the time. Usually, it is such a small effect that instruments are not sensitive enough to detect it. 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 they could observe it during this time period. They got lucky, being in the right place at the right time with MAVEN to see this.

The same phenomenon is probably happening on other unmagnetized bodies in the Solar System. These include Venus, comets, and even Saturn’s moon Titan. Understanding the generation, propagation, and impact of these structures at Mars broadens our knowledge of how our Sun interacts with our solar system. It also helps explain the physical processes that facilitate this interaction.

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. Fowler observed these signatures all the way down to the lowest altitudes that MAVEN sampled. This suggests that the effect impacted the atmosphere even below the spacecraft. 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 kilometers above the Martian surface. During its deeper dip campaigns, it came to within 125 kilometers of the surface.

Understanding how these space weather events impact our solar system is important. This knowledge is essential for keeping our robotic and potentially human explorers safe in the future. It is also crucial for protecting the space assets that we rely on for our everyday technology here on Earth.