As the solar wind moves away from the Sun, it cools slower than expected and accelerates. Where the energy that powers this phenomenon comes from has long been unknown. NASA’s Parker Solar Probe and the European Space Agency’s (ESA) Solar Orbiter, when they coincidentally lined up to observe the same solar wind stream, provided scientists with the measurements needed to answer this question.
The solar wind is a stream of charged particles, or plasma, expelled from the Sun’s atmosphere. As these streams expand into space, they are expected to cool down in the same way expanding gasses decrease in temperature, but this cooling happens more slowly than expected. Moreover, the streams also increase in speed as they move away from the Sun.
Scientists must study the same stream at different distances from the sun to determine the energy source behind this heating and acceleration. However, these studies require the trajectories of multiple spacecraft to align perfectly with a single stream. These alignments are rare, so previous studies have been limited by a lack of data on individual streams over large distances.