ESA Venus Express: Venus's Tail in the Solar Wind

A comparison of the ionosphere of Venus under different solar wind conditions. 

Credit: ESA/Wei et al. (2012).

Measurements obtained with ESA's Venus Express spacecraft have shed new light on the interaction between the solar wind and the second planet from the Sun.

During a rare period of very low density solar outflow, the ionosphere of Venus was observed to become elongated downstream, rather like a long-tailed comet.

Scientists have long known about the existence of the solar wind, a continuous outflow of electrons and protons which flows at high speed across interplanetary space. However, this stream of charged particles is highly variable, both in speed and density.

Under normal conditions, the solar wind has a density of 5 - 10 particles per cubic cm at Earth's orbit, but occasionally the solar wind almost disappears, as happened in May 1999.

Although such unusual episodes have been studied near Earth, which is surrounded by a strong magnetic field, there have been very few opportunities to study what happens near planets with negligible magnetic fields, such as Venus.

A rare opportunity to examine what happens when a tenuous solar wind arrives at Venus came 3 - 4 August 2010, following a series of large coronal mass ejections on the Sun.

NASA's STEREO-B spacecraft, orbiting downstream from Venus, observed that the solar wind density at Earth's orbit dropped to the remarkably low figure of 0.1 particles per cubic cm and persisted at this value for an entire day.

Meanwhile, Venus Express, which is in an extremely elliptical, near-polar orbit, was able to study the interaction between this sparse solar wind and the planet's ionosphere - the electrically charged region of its upper atmosphere.

The ionosphere is created by incoming extreme ultraviolet light and X-rays from the Sun which splits the atoms in the upper atmosphere of Venus and creates a layer of electrons and ions.