Unlike most planets, including Earth, Venus does not exhibit an intrinsic magnetic field. Sensitive measurements by orbiting spacecraft have shown that any dipole field originating from within Venus must be no more than 1/8,000 that of Earth's. The lack of a magnetic field may be related in part to the planet's slow rotation because, according to the dynamo theory that explains the origin of planetary magnetic fields, rotation helps to drive the fluid motions within the planet's interior that produce the field. It is also possible that Venus may lack a magnetic field because its core is fluid but does not circulate or simply because the core is solid and hence is incapable of supporting a dynamo.
As the solar wind bombards a planet at supersonic speeds, it generally forms a bow shock on the planet's sunward side— that is, a standing wave of plasma that slows down, heats, and deflects the flow around the planet. For some planets the bow shock lies at a considerable distance from the surface, held off by the planet's magnetic field. For example, because of Jupiter's enormous magnetic field, the bow shock exists about 3 million km (1,900,000 miles) from the planet; for Earth the distance is about 65,000 km (40,000 miles). Because Venus lacks a detectable field, however, its bow shock lies just a few thousand kilometres above the surface, held off only by the planet's ionosphere. This closeness of the bow shock to the surface leads to particularly intense interactions between the solar wind and Venus's atmosphere. In fact, the top of the ionosphere, known as the ionopause, lies at a much lower altitude on the dayside of Venus than on the night-side owing to the pressure exerted by the solar wind. The density of the ionosphere is also far greater on the dayside of the planet than on the nightside.
Venus's interaction with the solar wind results in a gradual, continuous loss to space of hydrogen and oxygen from the planet's upper ionosphere. This process is equivalent to a gradual loss of water from the planet. Over the course of Venus's history, the total amount of water lost via this mechanism could have been as much as a few percent of a world ocean the size of Earth's.
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