Centrifugal forces due to the rotation of the Sun may cause the solar wind and the interplanetary magnetic field to vary away from the solar equatorial plane. Consequently, space probes that journey out of the ecliptic are important for filling in the details that cannot be provided by the usual interplanetary missions. The motion of the Sun through interstellar space probably, creates a similar, although much larger-scale, magnetospheric-shock wave-tail structure as occurs near the Earth.

The ZODIACAL LIGHT appears as a conical glow extending up¬wards along the ecliptic. It is visible on clear, moonless evenings and at dawn just at the end of twilight. Zodiacal light visible before morning twilight is popularly called the false dawn. It is easiest to see when the ecliptic is perpendicular to the horizon. For this reason it is most conspicuous in the tropics, and on spring evenings or autumn mornings in temperate zones. Polarization and other studies suggest that the zodiacal light is caused by sunlight reflected from interplanetary dust particles several microns in size. Interactions with the solar wind and radiations will charge these particles to several volts positive. Enhancements of the zodiacal light have been reported after intense solar flares. Related to the zodiacal light is the more feeble GEGENSCHEIN or COUNTER-GLOW. It is visible on exceedingly dark and clear nights as a luminous elliptical area exactly opposite the Sun’s position in the sky. This also is caused by the reflection of sunlight by interstellar dust.

The solar wind blows on all the planets and their satellites, and its effect on these depends upon their magnetic fields, atmospheres and electrical conductivities. A planet such as Jupiter, which has a relatively strong magnetic field, has a well-developed magneto-sphere. The rapid rotation of the body of the planet and its magnetic field leads to a large centrifugal force on the particles in the radiation belts, which thus acquire a flattened shape.

The nearby satellite, lo, strongly interacts with this magneto-sphere and trapped particles, producing decametric radio bursts.

Planetary magnetic fields are usually considered to be due to rotation of some fluid mass exciting a dynamo action. We there¬fore only expect magnetic fields for planets satisfying these conditions. Saturn provides an interesting possibility in which the planet’s rings may be of some influence.

A slowly-rotating planet such as Venus offers no magnetic field sufficient to develop radiation belts and a magnetosphere, although its ionosphere makes it appear conductive to the solar wind and a bowshock is observed. In the anti-Sun direction a plasma shadow with subsequent turbulent motions occurs. Mars has little magnetic field and so its interactions are similar to that of Venus. The lack of a lunar atmosphere allows the solar wind to impinge directly on to the Moon’s surface. The magnetic field of Mercury is capable of producing a magnetosphere, although the weakness of this field and the proximity to the Sun mean that the magnetopause is only about 2 000km from its sunward surface. The source or cause of Mercury’s magnetic field is not yet understood.

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