The Solar Wind Around Pluto (SWAP) instrument on NASA’s New Horizons spacecraft has recorded significant changes in how the solar wind behaves far from the Sun. The results are published today in the Astrophysical Journal Supplement (arXiv.org preprint).
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. Image credit: NASA’s Goddard Space Flight Center Scientific Visualization Studio / Space Weather Research Center / Community-Coordinated Modeling Center / Enlil and Dusan Odstrcil, GMU.
SWAP, one of seven instruments on the New Horizons probe, collected three years’ worth of measurements before the July 15, 2015 Pluto flyby.
“SWAP measures the solar wind and ions created as the neutral interstellar material becomes ionized and is ‘picked up’ by the solar wind,” said lead author Dr. Heather Elliott from the Southwest Research Institute in San Antonio, Texas.
Data showed that 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.
“Because the Sun is the source of the solar wind, events on the Sun are the primary force that shapes the space environment,” Dr. Elliott explained.
“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 or by the collision of two 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,” Dr. Elliott said.
“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,” Dr. Elliott said.
“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.”
This figure shows solar wind observations measured by New Horizons from January 1 to August 25, 2015. This measurement of seed particles for anomalous cosmic rays in the solar wind is completely new in this region of space and is key for interpreting Voyager data further out in the interstellar boundary region. Points closer to the top of the graph correspond to higher-energy particles, and red and yellow colors show a larger number of particles hitting the detector. The particle instruments were shut down during certain spacecraft operations and trajectory maneuvers, resulting in brief data gaps. Image credit: NASA / New Horizons / Southwest Research Institute.
The new observations also show what may be the starting seeds of the energetic particles that make up cosmic rays.
The seeds for these energetic, super-fast particles may also help shape the boundary where the solar wind meets interstellar space.
Anomalous cosmic rays have been observed by the two Voyager spacecraft out near these boundaries, but only in their final stages, leaving questions as to the exact location and mechanism of their origins.
“The Voyagers can’t measure these seed particles, only the outcome. So with New Horizons going into that region, this blank patch in the observations is being filled in with data,” said Dr. Eric Christian from NASA’s Goddard Space Flight Center.
Filling in such a blank patch will help scientists better understand the way such particles move and affect the space environment around them, helping to interpret what Voyager is seeing on its journey.
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H.A. Elliott et al. New Horizons Solar Wind Around Pluto (SWAP) Observations of the Solar Wind From 11-33 AU. ApJS, published online April 6, 2016; arXiv: 1601.07156
