[meteorite-list] MESSENGER Measures Waves at the Boundary of Mercury's Magnetosphere

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 22 May 2012 11:45:15 -0700 (PDT)
Message-ID: <201205221845.q4MIjFqs019659_at_zagami.jpl.nasa.gov>

http://messenger.jhuapl.edu/news_room/details.php?id=223

MESSENGER Mission News
May 22, 2012

MESSENGER Measures Waves at the Boundary of Mercury's Magnetosphere

MESSENGER scientists have concluded that waves driven by the
Kelvin-Helmholtz (KH) instability play a key role in driving Mercury's
magnetosphere. In a paper <
http://www.agu.org/journals/ja/ja1204/2011JA017268/> published recently
in the /Journal of Geophysical Research/, the team reports on frequent
detections of such waves at the outer edge of the innermost planet's
magnetosphere.

The paper was selected as an Editor's Highlight <
http://www.agu.org/cgi-bin/highlights/highlights.cgi?action=show&doi=10.1029/2011JA017268&jc=ja>
by the journal editor, and its findings are scheduled to be featured as
a Research Highlight in /Eos/, the weekly newspaper of the American
Geophysical Union.

KH waves can develop at boundaries between two media in relative motion.
"In principle, they are similar to the surface waves on water when
there's a strong wind blowing," explains Torbjorn Sundberg, a
postdoctoral fellow at NASA Goddard Space Flight Center and the lead
author of the paper, entitled "MESSENGER orbital observations of
large-amplitude Kelvin-Helmholtz waves at Mercury's magnetopause."

"You can, for example, sometimes see these waves develop on the upper
edge of clouds, where they manifest as rolled-up vortices.
Kelvin-Helmholtz waves can also be seen in Saturn's atmosphere, where
they can develop at the boundary between the counter-streaming gas in
the belts and the zones, the dark and bright bands seen across the
planet," says Sundberg.

In space plasmas, such waves are of interest because they can transfer
mass and energy across the boundary between two otherwise separated
regions. At Mercury, the boundary is between the relatively dense and
fast-streaming solar wind and the more rarified magnetosphere.

The existence of KH-waves at Mercury was confirmed
<http://www.agu.org/pubs/crossref/2010/2009JD013300.shtml> during
MESSENGER's third Mercury flyby. However, the observations left several
questions unanswered. "These waves had been seen at Earth and Saturn, so
we expected that we might see them at Mercury," Sundberg says. "But to
fully understand these waves, how they develop, and how they differ from
the ones found at Earth and Saturn, we needed more measurements."

The most recent observations were taken during the first 88 days of
MESSENGER's time in orbit. In six different sets of magnetic field
measurements made by the orbiter as it passed through Mercury's
magnetopause, the boundary that separates the planet's magnetosphere
from the shocked solar wind plasma in the surrounding magnetosheath,
Sundberg and his colleagues were able to observe the magnetic field
oscillations characteristic of fully developed KH waves.

"The waves are more frequent, are larger in amplitude, and first appear
much closer to the dayside subsolar point than expected, but only on the
post-noon or dusk side of the magnetopause," explains Sundberg.
"Moreover, magnetic disturbances arising from the KH waves are seen over
large parts of the dayside magnetosphere."

"These characteristics mean that the KH waves develop more readily and
are much more important for mass and energy transfer from the solar wind
into the magnetosphere than we had imagined, and there is also a
dawn-dusk asymmetry in how they develop," Sundberg says. "MESSENGER's
extended mission will allow us to look into remaining questions, such as
why the waves are seen on the planet's dusk side but not the dawn side."
------------------------------------------------------------------------
MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and
Ranging) is a NASA-sponsored scientific investigation of the planet
Mercury and the first space mission designed to orbit the planet closest
to the Sun. The MESSENGER spacecraft launched on August 3, 2004, and
after flybys of Earth, Venus, and Mercury will start a yearlong study of
its target planet in March 2011. Dr. Sean C. Solomon, of the Carnegie
Institution of Washington, leads the mission as Principal Investigator.
The Johns Hopkins University Applied Physics Laboratory built and operates
the MESSENGER spacecraft and manages this Discovery-class mission for NASA.
Received on Tue 22 May 2012 02:45:15 PM PDT