Space environment data collected by New Horizons over a billion miles of its journey to Pluto will play a key role in testing and improving models of the space environment throughout the solar system. This visualization is one example of such a model: It shows the simulated space environment out to Pluto a few months before New Horizons’ closest approach. Drawn over the model is the path of New Horizons up to 2015, as well as the current direction of the two Voyager spacecraft – which are currently at three or four times New Horizons’ distance from the sun. The solar wind that New Horizons encountered will reach the Voyager spacecraft about a year later. Credit: NASA’s Goddard Space Flight Center Scientific Visualization Studio, the Space Weather Research Center (SWRC) and the Community-Coordinated Modeling Center (CCMC), Enlil and Dusan Odstrcil (GMU)
The Solar Wind Around Pluto (SWAP) instrument, operated by Southwest Research Institute (SwRI), collected 3 yrs’ worth of measurements before the July 15 Pluto flyby. Data showed the tumultuous flow of solar particles, which in the inner solar system is structured by the interaction of fast and slow flows as well as eruptive events on the Sun, becomes more uniform by the time the solar wind has traversed the 3 billion miles to Pluto’s orbit.
SWAP measures the solar wind and ions created as the neutral interstellar material becomes ionized and is “picked up” by the solar wind. These interstellar pickup ions can have up to twice the speed and 4X the energy of the solar wind. Farther out in space, these ions may be the seeds of the extremely fast energetic particles: anomalous cosmic rays, which pose a radiation threat to astronauts closer to Earth. These ions also play an important role in shaping the boundary where the solar wind hits interstellar space. New Horizons is currently at about 35 astronomical units (~35 times farther than the Earth to the Sun). It is the only operating spacecraft in the outer solar system. Only Voyager 2 has measured the solar wind farther away from the Sun; however, SWAP on New Horizons will be the first to measure the interstellar pickup ions in the outer solar system.
SWAP instrument was busy even when the rest of New Horizon’s instruments were “hibernating” to save energy on the long, nine-year voyage to Pluto.
Shocks in the solar wind – which can produce space weather, such as auroras, on worlds with magnetic fields – are created either by fast, dense clouds of material called coronal mass ejections CMEs or by the collision of 2 different-speed solar wind streams. These individual features are easily observed in the inner solar system, but New Horizons didn’t see the same level of detail. “At this distance, the scale size of discernible solar wind structures increases, since smaller structures are worn down or merge together,” said Elliott. “It’s hard to predict if the interaction between smaller structures will create a bigger structure, or if they will flatten out completely.”
Subtler signs of the Sun’s influence are also harder to spot in the outer solar system. Characteristics of the solar wind – speed, density, and temperature – are shaped by the region of the Sun it flows from. As the Sun and its different wind-producing regions rotate, patterns form. New Horizons didn’t see patterns as defined as they are when closer to the Sun, but it nevertheless did spot some structure. “Differences in speed and density average together as the solar wind moves out,” said Elliott. “But the wind is still being heated as it travels and faster wind runs into slower wind, so you see evidence of the Sun’s rotation pattern in the temperatures even in the outer solar system.”
http://www.swri.org/9what/releases/2016/new-horizons-solar-wind-pluto.htm
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