【惑星科学】木星の独特な磁場

A map of Jupiter’s magnetic field at a range of depths is reported this week in Nature. Analysis of this map suggests that Jupiter’s magnetic field may be unlike all known planetary magnetic fields.

The Juno spacecraft, which is in a polar orbit about Jupiter, is providing the first direct measurements of the planet’s magnetic field close to its surface. A recent analysis of magnetic field observations from eight (of the first nine) Juno orbits has yielded a new reference model of Jupiter’s magnetic field. That model provides an unprecedented description of Jupiter’s magnetic field outside the planet, which is of particular interest for understanding processes in Jupiter’s magnetosphere. However, maps of the interior magnetic field, which will help understand how it is generated, have yet to be described.

Kimberly Moore and colleagues mapped and analysed Jupiter’s magnetic field at a range of depths and found that it is profoundly different from other known magnetic fields. They find that, within Jupiter, most of the magnetic flux (a measurement of the total magnetic field passing through a given area) leaves the electrically charged dynamo region through a narrow band in the northern hemisphere, and re-enters the planet near the equator at the Great Blue Spot. Elsewhere, the field is much weaker. The authors show that the non-dipolar part of the field is confined almost entirely to the northern hemisphere; the field is predominantly dipolar in the southern hemisphere. They suggest that Jupiter’s dynamo, unlike Earth’s, does not operate within a uniform, thick shell.