The Magnetic Field And Magnetosphere

Neptune, like most of the other planets in the solar system, possesses an internally generated magnetic field, first detected in 1989 by Voyager 2. Like Earth's magnetic field, Neptune's field can be represented approximately by that of a dipole (similar to a bar magnet), but its polarity is essentially opposite to that of Earth's present field. A magnetic compass on Neptune would point toward south instead of north. Earth's field is thought to be generated by electric currents flowing in its liquid iron core, and electric currents flowing within the outer cores of liquid metallic hydrogen in Jupiter and Saturn may similarly be the source of their magnetic fields. The magnetic fields of Earth, Jupiter, and Saturn are relatively well centred within the respective planets and aligned within about 12° of the planetary rotation axes. Uranus and Neptune, by contrast, have magnetic fields that are tilted from their rotation axes by almost 59° and 47°, respectively. Furthermore, the fields are not internally well centred. Uranus's field is offset by 31 percent of the planet's radius. Neptune's field, having an offset of 55 percent of the radius, is centred in a portion of the interior that is actually closer to the cloud tops than to the planetary centre. The unusual configurations of the magnetic fields of Uranus and Neptune have led scientists to speculate that these fields may be generated in processes occurring in the upper layers of the planetary interiors.

The magnetic field of Neptune (and of the other planets) is approximately apple-shaped, with the stem end and the opposite end oriented in the directions of the magnetic poles. The solar wind, a stream of electrically charged particles that flows outward from the Sun, distorts that regular shape, compressing it on the sunward side of the planet and stretching it into a long tail in the direction away from the Sun. Trapped within the magnetic field are charged particles, predominantly protons and electrons.

The region of space dominated by Neptune's magnetic field and charged particles is called its magnetosphere. Because of the high tilt of Neptune's magnetic field, the particles trapped in the magnetosphere are repeatedly swept past the orbits of the moons and rings. Many of these particles may be absorbed by the moons and ring material, effectively emptying from the magnetosphere a large fraction of its charged particle content. Neptune's magnetosphere is populated with fewer protons and electrons per unit volume than that of any other giant planet. Near the magnetic poles, the charged particles in the magnetosphere can travel along magnetic field lines into the atmosphere. As they collide with gases there, they cause those gases to fluoresce, resulting in classical, albeit weak, auroras.

interior structure

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