New observations of the second planet from the Sun made with ESA’s Venus Express spacecraft during a period of reduced solar wind pressure have provided evidence that the ionosphere of the planet is elongated downstream, rather like a long-tailed comet.
Comet-like ionosphere of Venus (ESA / Y. Wei et al)
The ionosphere is a region of weakly electrically charged gas high above the main body of a planet’s atmosphere. Its shape and density are partly controlled by the internal magnetic field of the planet.
The Earth’s ionosphere is relatively stable under a range of solar wind conditions. Venus does not have its own internal magnetic field and relies instead on interactions with the solar wind to shape its ionosphere. The extent to which this shaping depends on the strength of the solar wind has been controversial, but new results from Venus Express reveal for the first time the effect of a very low solar wind pressure on the ionosphere of an unmagnetized planet.
A rare opportunity to examine what happens when a tenuous solar wind arrives at Venus came on August 3-4, 2010, following a series of large coronal mass ejections on the Sun. The observations measured a drop in solar wind density to 0.1 particles per cubic cm, around 50 times lower than normally observed.
As this significantly reduced solar wind hit Venus, the spacecraft saw the Venus’s ionosphere balloon outwards on the planet’s ‘downwind’ night side, much like the shape of the ion tail seen streaming from a comet under similar conditions.
“The teardrop-shaped ionosphere began forming within 30–60 minutes after the normal high pressure solar wind diminished. Over two Earth days, it had stretched to at least two Venus radii into space,” said Dr Yong Wei of the Max Planck Institute for Solar System Research in Germany, lead author of a paper reporting the findings in the journal Planetary and Space Science.
The new observations settle a debate about how the strength of the solar wind affects the way in which ionospheric plasma is transported from the dayside to the night side of Venus. Usually, this material flows along a thin channel in the ionosphere, but scientists were unsure what happens under low solar wind conditions. Does the flow of plasma particles increase as the channel widens due to the reduced confining pressure, or does it decrease because less force is available to push plasma through the channel?
“We now finally know that the first effect outweighs the second, and that the ionosphere expands significantly during low solar wind density conditions,” explained co-author Dr Markus Fraenz, also of the Max Planck Institute.
A similar effect is also expected to occur around Mars, the other non-magnetized planet in our inner Solar System.
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Bibliographic information: Y. Weia et al. 2012. A teardrop-shaped ionosphere at Venus in tenuous solar wind. Planetary and Space Science, vol. 73, no. 1, pp. 254–261; doi: 10.1016/j.pss.2012.08.024
