MessageToEagle.com - Planet Venus' ionosphere balloons out like a comet's tail on its nightside, according to unique observations
of the planet made by the ESA's. Venus Express.
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.
For Earth, which has a strong magnetic field, the ionosphere is relatively stable under a range of solar wind conditions.
By comparison, Venus does not have its own internal magnetic field and relies instead on interactions with the solar
wind to shape its ionosphere.
Click on image to enlarge
Comet-like ionosphere at Venus
The change in ionosphere of Venus during normal solar wind conditions (left) and reduced solar wind activity
(right), as observed by ESA’s Venus Express spacecraft in August 2010. The yellow lines show a projection of the
solar magnetic field lines as they interact with the ionosphere.
Venus Express follows an elliptical orbit around the planet once every 24 hours, passing within 250 km of the north
pole and 66 000 km over the south pole. The observations were made on the nightside of the planet, when Venus Express
was within 15 000 km of the centre of the planet. Although the spacecraft only took measurements within
two Venus radii, the findings suggest that the ionosphere likely extends to even greater distances during
periods of reduced solar wind intensity. Credits: ESA/Wei et al. (2012
The observations were made in August 2010 when NASA's Stereo-B spacecraft measured a drop in solar wind density to 0.1
particles per cubic centimetre, around 50 times lower than normally observed; this persisted for about 18 hours.
"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," says Yong Wei of the Max Planck Institute
for Solar System Research in Germany, lead author of the new findings.
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," says Markus Fraenz, also of the Max Planck Institute and co-author
on the paper.
"We often talk about the effects of solar wind interaction with planetary atmospheres during periods of intense
solar activity, but Venus Express has shown us that even when there is a reduced solar wind, the Sun can still
significantly influence the environment of our planetary neighbours," adds Håkan Svedhem, ESA's Venus Express
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