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Topic Name: Voyager 2 spacecraft's Plasma Science instrument finds surprises at solar system's edge
Category: STAR (Space, Telecommunications & Radioscience)
Research persons: John Richardson
Location: Massachusetts Institute of Technology, United States
Details
The Voyager 2 spacecraft's Plasma Science instrument, developed at MIT
in the 1970s, has turned up surprising revelations about the boundary zone that
marks the edge of the sun's influence in space.
The unexpected findings emerged in the last few weeks as the spacecraft
traversed the termination shockwave formed when the flow of particles constantly
streaming out from the sun--the solar wind--slams into the surrounding thin gas
that fills the space between stars.
The first surprise is that there is an unexpectedly strong magnetic
field in that surrounding interstellar region, generated by currents in that
incredibly tenuous gas. This magnetic field is squashing the bubble of
outflowing gas from the sun, distorting it from the uniform spherical shape
space physicists had expected to find.
A second surprise also emerged from Voyager 2's passage through the solar
system's outer edge: Just outside that boundary the temperature, although
hotter than inside, was ten times cooler than expected. Theorists had to
scramble to come up with an explanation for the unanticipated chilling effect.
"It's a different kind of shockwave than we've seen anywhere else,"
says John Richardson, principal investigator for the Plasma Physics instrument
and a Principal Research Scientist at MIT's Kavli Institute for Astrophysics and
Space Science. The unexpected coolness, theorists now think, is caused by energy
going into particles that are hotter than those that can be measured by the MIT
plasma instrument.
Richardson will be taking part in a press conference reporting the new
findings on Monday, Dec. 10, at a meeting of the American Geophysical Union in
San Francisco.
The Voyager 1 and 2 spacecraft were designed primarily to study the planets
Jupiter and Saturn and their moons. After launch, Voyager 2's path was adjusted
to take it past Uranus and Neptune as well. Although the craft were only built
for a five-year mission, both are still working well three decades later.
"We were incredibly lucky to have it last 30 years," says John
Belcher, professor of physics at MIT and former principal investigator for the
Voyager Plasma Science instrument. The craft is now expected to keep working
until about 2020, and still has important scientific objectives ahead.
It is now passing through a boundary zone called the heliosheath, a region
where the solar wind interacts with the surrounding interstellar medium. But
sometime in the next decade, it will cross a final edge, called the heliopause,
where the sun's outflow of particles ends. At that point, it will be able to
measure characteristics of the interstellar medium, for the first time, in a
region unaffected by the solar wind and the sun's magnetism.
Although Voyager 1 had already crossed the termination shockwave three years
ago, the MIT Plasma Science instrument on that spacecraft had stopped working,
so the spacecraft could only indirectly detect the end of the sun's influence.
But with Voyager 2, the Plasma Science instrument not only detected the
boundary, making detailed measurements of the solar wind's temperature, speed
and density as the spacecraft crossed through it, but it actually encountered
the shockwave repeatedly. Because the outflow of the solar wind varies with
changes in the sun's activity level, building up during large solar flares and
quieting during lulls in sunspot activity, the boundary itself pulsates in and
out. These pulsations can wash across the craft multiple times, just as a boat
landing onshore may cross the ocean's edge multiple times as waves crash in and
then recede.
While Voyager 1 apparently made a single crossing, Voyager 2 apparently
crossed the boundary five times, producing a wealth of new data. It's even
possible that if there are large variations in that solar outflow, the shock
layer "could push past Voyager again," says Richardson. "That
would give us some idea of how elastic the shock is" -- that is, how far
out these pulsations may stretch. Until and unless such detections are made,
"we only have models" of how great such variations might be, he says.
Voyager 2 is now 7.879 billion miles from Earth, traveling away at almost
35,000 miles per hour. Voyager 1 is 9.797 billion miles away, going more than
38,000 mph.
The Plasma Science instrument was developed by the late Professor Herbert
Bridge and Alan Lazarus, a senior research scientist in the Department of
Physics and MIT's Kavli Institute for Astrophysics and Space Science. NASA has
sponsored the work.
Note for Solar wind
The solar wind is a stream of charged particles (i.e., a plasma) which are ejected from the upper atmosphere of the sun. It consists mostly of high-energy electrons and protons (about 1 keV) that are able to escape the sun's gravity in part because of the high temperature of the corona and the high kinetic energy particles gain through a process that is not well understood at this time.
Many phenomena are directly related to the solar wind, including geomagnetic storms that can knock out power grids on Earth, aurorae (e.g., Northern Lights) and the plasma tail of a comet always pointing away from the sun. While early models of the solar wind used primarily thermal energy to accelerate the material, by the 1960s it was clear that thermal acceleration alone cannot account for the high speed solar wind. Some additional acceleration mechanism is required, but is not currently known, but most likely relates to magnetic fields in the solar atmosphere.
The solar wind is responsible for the overall shape of Earth's magnetosphere, and fluctuations in its speed, density, direction, and entrained magnetic field strongly affect Earth's local space environment. For example, the levels of ionizing radiation and radio interference can vary by factors of hundreds to thousands; and the shape and location of the magnetopause and bow shock wave upstream of it can change by several Earth radii, exposing geosynchronous satellites to the direct solar wind. These phenomena are collectively called space weather.
Note for Interstellar medium
In astronomy, the interstellar medium (or ISM) is the gas and dust that pervade interstellar space. The interstellar medium is the matter that exists between the stars within a galaxy. The energy, in the form of electromagnetic radiation, that occupies the same volume is the interstellar radiation field.
The interstellar medium consists of an extremely dilute (by terrestrial standards) mixture of ions, atoms, molecules, larger dust grains, cosmic rays, and (galactic) magnetic fields. The matter consists of about 99% gas and 1% dust by mass. It fills interstellar space, and blends smoothly into the surrounding intergalactic medium. The ISM is usually extremely tenuous, with densities ranging from a few thousand to a few hundred million particles per cubic meter, and an average value in the Milky Way Galaxy of a million particles per cubic meter. As a result of primordial nucleosynthesis, the gas is roughly 90% hydrogen and 10% helium by number of nuclei, with additional heavier elements ("metals" in astronomical parlance) present in trace amounts.
The ISM plays a crucial role in astrophysics precisely because of its intermediate role between stellar and galactic scales. Stars form within the densest regions of the ISM, molecular clouds, and replenish the ISM with matter and energy through planetary nebulae, stellar winds, and supernovae. This interplay between stars and the ISM helps determine the rate at which a galaxy depletes its gaseous content, and therefore its lifespan of active star formation.
About Voyager 2
Voyager 2 is an unmanned interplanetary spacecraft launched on August 20, 1977. Identical in form to its sister Voyager program craft, Voyager 1, Voyager 2 followed a slower trajectory that allowed it to be kept in the ecliptic (the plane of the Solar System) so that it could be sent to Uranus and Neptune by means of gravity assist during the 1981 encounter at Saturn. Because of this trajectory, Voyager 2 could not see the moon Titan up close as its twin had, but the probe did become the first and only spacecraft to travel to Uranus and Neptune, thus completing the Planetary Grand Tour, a rare geometric arrangement of the outer planets that only occurs once every 176
years.
Voyager 2 is perhaps the most productive space probe yet deployed, visiting four planets and their moons, including two primary visits to previously unexplored planets, with powerful cameras and a multitude of scientific instruments, at a fraction of the money later spent on specialized probes such as the Galileo spacecraft and the Cassini-Huygens
probe.
In figure 1, Voyager's passage through the termination shock. The solar wind is shown in yellow, and the heliosheath in brown. The image shows how Voyager crossed the boundary multiple times. The blue graph at top shows the plasma data readings from which this passage was reconstructed (the gap shows an interval when no data was received).
In figure 2, Voyager 2 (and its twin, Voyager 1) carried a wide array of instruments to take pictures, measure magnetism and a variety of other properties, and communicate with Earth. The Plasma Instrument, developed 30 years ago at MIT and run by MIT researchers ever since, is the small yellow triangle located on top of the boom at right.
Figure 3 illustrates how the constant outflow of particles from the sun, called the solar wind, interacts with the surrounding interstellar medium (ISM). Where the outflow first encounters the ISM, it forms a shockwave called the termination shock--the boundary that Voyager 2 just crossed. It is now in the intermediate zone, called the heliosheath, where the two regions interact. Within the next few years, it will cross another boundary, called the heliopause, where the sun's influence ends.
 
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