[meteorite-list] When A Planet Behaves Like A Comet (Venus Express)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 29 Jan 2013 10:24:41 -0800 (PST)
Message-ID: <201301291824.r0TIOfAe007373_at_zagami.jpl.nasa.gov>

http://www.esa.int/Our_Activities/Space_Science/When_a_planet_behaves_like_a_comet

When a planet behaves like a comet
European Space Agency
29 January 2013

ESA's Venus Express has made unique observations of Venus during a
period of reduced solar wind pressure, discovering that the
planet's ionosphere balloons out like a comet's tail on its
nightside.

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.

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 unmagnetised planet.

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.

As this significantly reduced solar wind hit Venus, Venus Express
saw the planet's ionosphere balloon outwards on the planet's
"downwind" nightside, 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," says Yong Wei of the Max Planck Institute for Solar System
Research in Germany, lead author of the new findings.

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 nightside 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," says Markus Fraenz, also of the Max
Planck Institute and co-author on the paper.

A similar effect is also expected to occur around Mars, the other
non-magnetised planet in our inner Solar System.

"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 Hakan Svedhem,
ESA's Venus Express project scientist.



Notes for editors

"A teardrop-shaped ionosphere at Venus in tenuous solar wind" by
Y. Wei et al is published in Planetary and Space Science 73, 2012.

For further information, please contact:

Markus Bauer
ESA Science and Robotic Exploration Communication Officer
Tel: +31 71 565 6799
Mob: +31 61 594 3 954
Email: markus.bauer at esa.int

Yong Wei
Max Planck Institute for Solar System Research
E-mail: wei at mps.mpg.de

Markus Fraenz
Max Planck Institute for Solar System Research
E-mail: fraenz at mps.mpg.de
Tel: +49 555 6979 441

Hakan Svedhem
Venus Express Project Scientist
Email: H.Svedhem at esa.int
Tel: +31 71 565 3370
Received on Tue 29 Jan 2013 01:24:41 PM PST