Extreme Climate Survey
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Since the dawn of the Space Age, when robotic probes first left the atmosphere, scientists have known that the solar wind — a stream of charged particles released from the sun’s atmosphere — accelerates as it blows out into the solar system (SN: 8/18/17). Theoretical calculations also indicate that the solar wind’s temperature should drop as it expands into space. This drop does occur, but measurements find that it happens slower than predicted.
Observations from Earth have previously spotted Alfvén waves swaying near the sun. Such waves are oscillations in the magnetic fields of the plasma emerging from the sun. They are sometimes so large they turn back on themselves in what have been called “switchbacks” (SN 1/15/21). The observed Alfvén waves had the right out amount of energy to explain the two longstanding head-scratchers about the solar wind’s speed and temperature, but direct evidence was still lacking.
Enter Parker Solar Probe and Solar Orbiter. In late February 2022, Parker was passing through a region approximately one-fifth the distance between the sun and Mercury, exactly where these switch-backing Alfvén waves flutter. By chance, Solar Orbiter flew through the same plasma stream a little under two days later at roughly the orbit of Venus.
“You have these two spacecraft intercepting the same solar wind, allowing us to quantify the energy of these waves,” says Yeimy Rivera, a heliophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.
Parker measured the plasma stream zipping by at roughly 1.4 million kilometers per hour, while Solar Orbiter found it to be tearing along at 1.8 million km/h. The plasma at Solar Orbiter was also a blazing 200,000 degrees Celsius,…
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