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Planetary magnetic fields are generated by flows in the hot, liquid iron cores of the planets. Measurements made by Mariner 10 in 1974/75 showed that Mercury also has a magnetic field. According to the standard models, the dynamo effect in its metal core should generate similar field strengths to those on Earth.

Mercury's magnetic field is 150 times weaker than that of our planet, however. This has recently been confirmed by the NASA space probe Messenger. How can the large discrepancy in the field strength be explained?

This question has now been answered by a group headed by Karl-Heinz Glassmeier at the Technische Universität Braunschweig. Scientists have now presented a new explanation: the solar wind counteracts Mercury's internal dynamo and thus weakens its magnetic field.

The solar wind - a constant stream of charged particles - plays a significant role. At an average distance from the Sun of only 58 million kilometers - around one third of the distance of the Earth - Mercury is much more exposed to these particles.

"We must keep in mind that Mercury strongly interacts with the surrounding solar wind," says Daniel Heyner, lead author of the article published in Science and doctoral student at the International Max Planck Research School (IMPRS) in Katlenburg-Lindau.

This interaction drives strong electrical currents in the magnetosphere of the planet, whose magnetic fields counteract the internal dynamo effect. The team's new computer models show that a dynamo with this type of feedback is actually possible.

"These types of simulation of the dynamo process are the only possibility to sort of look into the iron core and to predict the strength and structure of the magnetic field," says Johannes Wicht from the Max Planck Institute for Solar System Research, whose model made a significant contribution to the study.

The results show unambiguously that the feedback ultimately causes the weak magnetic field. "The dynamo process in Mercury's interior is almost nipped in the bud by the interaction," explains Glassmeier.

Source: PhysOrg