Download Jupiter

Encountering Jupiter
Courtesy NASA
Nearly eight years from its Pluto target, the
New Horizons SWAP instrument observes
solar wind interactions near Jupiter
A
s it rounded Jupiter for a gravity-assist that will speed its
journey to the edge of the solar system, the New Horizons
spacecraft began testing its science payload and making
scientific observations. The Solar Wind Around Pluto (SWAP)
instrument, built by Southwest Research Institute, generated
data that will help resolve puzzling questions about the interactions between the solar wind, the million-mile-per-hour stream
of ionized gas flowing out from the Sun, and Jupiter’s magnetosphere, the magnetic bubble that surrounds the planet and
encloses ionized gas.
From a distance of about 0.4 astronomical unit (one AU is
the distance from the Earth to the Sun, or about 100 million
miles) from Jupiter, SWAP observed an immense structure of
compressed, dense, hot ionized gas that forms in the solar
wind, called a co-rotating interaction region. These structures
form when solar wind streams that are both fast and slow come
out of the Sun, and flow out in different directions in response to
the rotation of the Sun. The fast layers try to overtake the slow
layers yet are unable to flow through them, instead compressing
the slow material like a snow plow and bunching up solar wind
to create the co-rotating interaction region. These regions contain significantly higher densities and pressures that eventually
expand and form discontinuities, or shocks, in the solar wind,
which spread out and away from the high pressure regions.
“These solar wind structures collide with the magnetospheres of planets and cause major variations in their size and
structure,” said Dr. David McComas, SWAP principal investigator and senior executive director of the SwRI Space Science
and Engineering Division. “Because it has the largest magnetosphere in the solar system, understanding the effects of the
solar wind at Jupiter could have significant implications for all
the planets.”
Studies of these interactions at Jupiter could help determine
how much of the Jovian magnetosphere and aurora are driven by
external processes, such as the solar wind, versus internal
processes, such as planetary rotation.
“There’s an active debate about how much solar wind variability affects what magnetospheric responses we will see at
Jupiter,” McComas continued. “We’ve never had the opportunity
to simultaneously measure the interactions upstream of Jupiter as
we’re observing its aurora, but the New Horizons encounter is
changing that.”
The team is making collaborative studies that combine SWAP
data with imaging and spectroscopic observations of Jupiter’s
aurora using the Hubble Space Telescope. The fusion of these data
provides the first simultaneous upstream observations of solar wind
interactions at Jupiter as the aurora builds and subsides.
The Jupiter encounter is also enabling SWAP to take measurements inside Jupiter’s magnetosphere, on an orbit that has never
before been traveled. That orbit is carrying the spacecraft deep
down the magnetotail, the portion of the magnetosphere that is
pushed away from the Sun by the flowing solar wind. This route
provides the first close look at Jupiter’s more distant magnetotail.
Built to evaluate the solar wind’s interaction with Pluto at about
30 AU from the Sun, SWAP’s sensitivity was successfully
decreased at Jupiter’s relatively close distance of about 5 AU from
the Sun to generate these early results.
New Horizons is the first mission in NASA’s New Frontiers program. The Johns Hopkins University Applied Physics Laboratory
manages the mission and will operate the spacecraft for the NASA
Science Mission Directorate.v
Comments about this article? Contact McComas at (210) 522-5983
or [email protected].
Technology Today • Spring 2007
7