A comparison of the ionosphere of Venus under different solar wind conditions. The yellow lines indicate the solar magnetic field lines as they interact with the ionosphere.
When the solar wind's dynamic pressure is normal (left image) the ionosphere is confined to a region 150-300 km above the planet's day side. Positively charged particles (ions) travel quite quickly across the day-night terminator to create a similar ionosphere on the planet's night side.
When the solar wind pressure drops to a very low level (right image), the ionosphere expands to a higher altitude above the day side of Venus and weaker magnetic fields are created above the terminator. As a result, the region across which ions are able to travel from the day side to the night side becomes larger. This makes it easier for ions to flow across the terminator. Although the weaker pressure of the solar wind reduces the speed at which the ions travel toward the night side, the first effect outweighs the second, enabling the ionosphere to expand in the planet's wake. Observations from Venus Express show that the night-side ionosphere moved outward to at least 15,000 km from Venus' center over a period of only a few hours, creating a long, comet-like tail.
Illustration credit: ESA/Wei et al (2012)
Note: For more information, see The Tail of Venus and the Weak Solar Wind and Comet-Like Ionosphere at Venus.