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Title: Mercury's magnetic field in the MESSENGER era
Author: Johannes Wicht, Daniel Heyner

MESSENGER magnetometer data show that Mercury's magnetic field is not only exceptionally weak but also has a unique geometry. The internal field resembles an axial dipole that is offset to the North by 20% of the planetary radius. This implies that the axial quadrupol is particularly strong while the dipole tilt is likely below 0.8 degree. The close proximity to the sun in combination with the weak internal field results in a very small and highly dynamic Hermean magnetosphere. We review the current understanding of Mercury's internal and external magnetic field and discuss possible explanations. Classical convection driven core dynamos have a hard time to reproduce the observations. Strong quadrupol contributions can be promoted by different measures, but they always go along with a large dipole tilt and generally rather small scale fields. A stably stratified outer core region seems required to explain not only the particular geometry but also the weakness of the Hermean magnetic field. New interior models suggest that Mercury's core likely hosts an iron snow zone underneath the core-mantle boundary. The positive radial sulfur gradient likely to develop in such a zone would indeed promote stable stratification. However, even dynamo models that include the stable layer show Mercury-like magnetic fields only for a fraction of the total simulation time. Large scale variations in the core-mantle boundary heat flux promise to yield more persistent results but are not compatible with the current understanding of Mercury's lower mantle.

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Solar Wind May Explain Planet Mercury's Puny Magnetic Field

The mystery of why Mercury's magnetic field is so weak may just have been solved: It is being stifled by the solar wind, researchers think.
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Solar wind may explain Mercury's weak magnetic field

The mystery of why Mercury's magnetic field is so weak may just have been solved: It is being stifled by the solar wind, researchers think.
Mercury and Earth are the only rocky planets in the solar system to possess global magnetic fields, and for years scientists have puzzled over why Mercury's is so flimsy.
To study Mercury's magnetic field, researchers created 3-D computer simulations of the planet's interior and of the solar wind, the deluge of energetic particles from the sun that constantly bombards its nearest planet.
The computer models suggested that the churning of Mercury's molten iron core ordinarily would amplify the magnetic field up to Earth-like levels, in a so-called dynamo process like the one within our planet.
However, the onrushing solar wind likely prevents that from happening, researchers said.

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