Unexpected discovery: Data from the Mars probe MAVEN reveal an effect that has never been detected in a planetary atmosphere. The solar wind particles are compressed and redirected by magnetic field lines. The surprising thing about it: Until now, this Zwan-Wolf effect was only known from the outer edge of the earth’s magnetosphere – far beyond the atmosphere. In addition, Mars does not have its own dipole magnetic field, so it should actually lack the prerequisites for this.
The Earth’s magnetic field is an important protective shield for our planet: its magnetosphere keeps out the high-energy, charged particles of the solar wind and cosmic radiation. The cage of magnetic field lines deflects these particle streams at the outer edge of the magnetic field. One of the factors involved in this is the Zwan-Wolf effect. It ensures that the incoming particles are compressed at the magnetopause and then directs these squeezed currents away to the side.
Mars: Planet without a magnetic dynamo
The crucial thing is that the characteristic particle patterns of the Zwan-Wolf effect normally occur high above the atmosphere. “Nobody expected that this effect could also occur in an atmosphere,” explains lead author Christopher Fowler from West Virginia University. In addition, a pronounced dipole magnetic field was considered a prerequisite for this phenomenon. Therefore, the Zwan-Wolf effect probably occurs not only in the Earth’s magnetic field, but also in Jupiter and Saturn.
However, this is different for the planet Mars: Although it produced its own magnetic field in its early days, its magnetic dynamo stopped around 3.7 billion years ago. Since then, our neighboring planet no longer has an internally generated dipole magnetic field. Only in some regions of Mars have local remnants of the former magnetic field been preserved. Additionally, the interaction between strong solar wind and the Martian atmosphere can create a temporary weak magnetic field. According to the common assumption, Mars does not meet the requirements for the Zwan-Wolf effect.
Surprising structures in the Martian ionosphere
This makes the discovery that Fowler and his team made while evaluating data from the Mars orbiter probe MAVEN all the more astonishing. This measured the magnetic field values and particle densities in the Mars ionosphere during and shortly after a solar storm in December 2023. “As I was analyzing the data, I suddenly noticed some very interesting fluctuations,” reports Fowler.
The data showed five parallel structures in which the local magnetic field values first increased abruptly and then gradually decreased again. “At the same time, the probe detected significant fluctuations in the ionospheric plasma density: the ion density decreased by 30 to 40 percent at the leading edge of these magnetic structures and then slowly increased again,” report Fowler and his colleagues. At the same time, the energy of the detected particles at the front edge of the structures also increased.
First detection of a planet without an internal magnetic field
According to the researchers, these data and other measurement results suggest that the structures observed by MAVEN in the Martian atmosphere are due to a Zwan-Wolf effect. “We are demonstrating for the first time that the Zwan-Wolf effect can also occur in the ionospheres of actually unmagnetized planets,” state Fowler and his team.
Contrary to what was previously assumed, the magnetic field on Mars induced by the solar storm and only encompassing parts of the planet is sufficient to compress and redirect the solar wind particles. Because the Martian magnetic field induced by the solar wind is smaller and weaker than a true dipole magnetic field, this phenomenon occurs at a lower altitude than on Earth: It does not take place far above the Martian atmosphere, but in the ionosphere, less than 200 kilometers above the planet’s surface.
“Physical processes that we didn’t know about”
“That makes it really exciting because it reveals physical processes that we didn’t know before,” says Fowler. A self-generated dipole magnetic field is therefore not necessary for the Zwan-Wolf effect – a partial magnetic field caused by the solar wind, which is temporarily created on the dayside of Mars, is sufficient. This finding means that this phenomenon may also occur on other supposedly non-magnetized planets.
The observations also suggest that Mars’ ionospheric Zwan-Wolf effect is not a short-lived phenomenon: “It is probably constantly active in the Martian ionosphere, but most of the time it is too weak to be detected by typical plasma instruments,” explains the team.
Source: Christopher Fowler (West Virginia University, Morganstown, USA) et al., Nature Communications, 2026; doi: 10.1038/s41467-026-72251-9