Stolen from the Nazi Bar -
A new study reports a clear detection of the Zwan-Wolf effect inside the Martian ionosphere, using in-situ measurements from NASA's MAVEN @NASAMars spacecraft during the aftermath of an interplanetary coronal mass ejection that hit Mars in December 2023.
The event strongly disturbed and compressed the Martian plasma environment, making a normally subtle process large enough to detect.
The Zwan-Wolf effect is a plasma process first studied mainly at Earth. In simple terms, when magnetic field lines carried by the solar wind are compressed near a planetary obstacle, the resulting magnetic pressure gradients can squeeze plasma along those field lines and away from the region where the flow is being slowed or blocked.
At Earth, this happens near the magnetopause, where the solar wind is forced to move around the planet’s magnetic shield. The compression can produce a local drop in plasma density because the plasma is being pushed out along the magnetic field.
In the paper, the same kind of effect is observed at Mars, but in a different setting: not around a strong planetary dipole, but within the ionosphere of an unmagnetized planet where solar-wind magnetic fields are draped around the dayside of Mars.
MAVEN observed several large magnetic structures near its closest approach to Mars, at an altitude of about 185 km. These structures had steep leading edges, with magnetic field enhancements of around 50 nanotesla, roughly 40 percent above the background field.
As these compressed magnetic fronts passed through the ionosphere, MAVEN measured clear decreases in ionospheric plasma density, typically around 30 to 40 percent, together with a tailward flow of plasma. The interpretation is that the increased magnetic pressure at the front of each structure squeezed ionospheric plasma along the draped magnetic field, producing the density depletion and flow pattern expected from the Zwan-Wolf effect.
This is important because the Zwan-Wolf effect had usually been associated with planets that have strong magnetic fields, especially Earth. Mars shows that the same physical mechanism can also operate in induced magnetospheres, where the obstacle to the solar wind is not a dipole field but an atmosphere and ionosphere conducting electrical currents.
The researchers suggest that the effect may actually be active at Mars most of the time, but usually too weak to be detected by plasma instruments. During quiet conditions, the magnetic pressure changes are small, so the resulting plasma deflections are probably below instrumental resolution. During the December 2023 coronal mass ejection, however, the system was so disturbed that the effect became amplified and observable.
The study also shows how space weather can temporarily reshape the plasma environment of planets without global magnetic fields. A strong solar eruption did not simply “hit” Mars from outside; it launched disturbances that propagated into the induced magnetosphere and down into the ionosphere, creating compressed magnetic structures capable of moving plasma around.
The observations do not suggest that these structures caused major direct atmospheric escape during this event, because the plasma was not accelerated enough to exceed escape velocity. But they do show that extreme solar-wind conditions can drive real, measurable motion and heating in the upper atmosphere of Mars.
In broader terms, this result adds a new piece to the picture of how Mars interacts with the Sun. Even without an Earth-like magnetic shield, Mars is not passive. Its ionosphere, the solar wind, and the draped interplanetary magnetic field form a dynamic system where pressure, magnetic fields, and plasma flows constantly respond to one another.
The detection of the Zwan-Wolf effect at Mars shows that processes once thought mainly in the context of magnetized planets can also appear in the thinner, more exposed plasma environments of unmagnetized worlds.
