At the heart of understanding our space environment is the knowledge that conditions throughout space — from the Sun to the atmospheres of planets to the radiation environment in deep space — are connected.
Studying this connection – a field of science called heliophysics — is a complex task: Researchers track sudden eruptions of material, radiation, and particles against the background of the ubiquitous outflow of solar material.
A confluence of events in early 2020 created a nearly ideal space-based laboratory, combining the alignment of some of humanity's best observatories — including Parker Solar Probe, during its fourth solar flyby — with a quiet period in the Sun's activity, when it's easiest to study those background conditions. These conditions provided a unique opportunity for scientists to study how the Sun influences conditions at points throughout space, with multiple angles of observation and at different distances from the Sun.
The Sun is an active star whose magnetic field is spread out through the solar system, carried within the Sun's constant outflow of material called the solar wind, which affects spacecraft and shapes the environments of worlds throughout the solar system. We've observed the Sun, space near Earth and other planets, and even the most distant edges of the Sun's sphere of influence for decades. And 2018 marked the launch of a new, game-changing observatory: Parker Solar Probe, with a plan to ultimately fly to about 3.83 million miles from the Sun's visible surface.
Parker has now had four close encounters of the Sun. (The data from Parker's first encounters with the Sun has already revealed a new picture of its atmosphere.) During its fourth solar encounter, spanning parts of January and February 2020, the spacecraft passed directly between the Sun and Earth. This gave scientists a unique opportunity: The solar wind that Parker Solar Probe measured when it was closest to the Sun would, days later, arrive at Earth, where the wind itself and its effects could be measured by both spacecraft and ground-based observatories. Furthermore, solar observatories on and near Earth would have a clear view of the locations on the Sun that produced the solar wind measured by Parker Solar Probe.
"We know from Parker data that there are certain structures originating at or near the solar surface. We need to look at the source regions of these structures to fully understand how they form, evolve, and contribute to the plasma dynamics in the solar wind," said Nour Raouafi, project scientist for the Parker Solar Probe mission at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland. "Ground-based observatories and other space missions provide supporting observations that can help draw the full picture of what Parker is observing."
This celestial alignment would be of interest to scientists under any circumstances, but it also coincided with another astronomical period of interest to scientists: solar minimum. This is the point during the Sun's regular, approximately 11-year cycles of activity when solar activity is at its lowest level — so sudden eruptions on the Sun such as solar flares, coronal mass ejections and energetic particle events are less likely. And that means that studying the Sun near solar minimum is a boon for scientists who can watch a simpler system and thus untangle which events cause which effects.
"This period provides perfect conditions to trace the solar wind from the Sun to Earth and the planets," said Giuliana de Toma, a solar scientist at the High Altitude Observatory in Boulder, Colorado, who led coordination among observatories for this observation campaign. "It is a time when we can follow the solar wind more easily, since we don't have disturbances from the Sun."
For decades, scientists have pulled together observations during these periods of solar minimum, an effort co-led by Sarah Gibson, a solar scientist at the High Altitude Observatory, and other scientists. For each of the past three solar minimum periods, scientists pooled observations from an ever-expanding list of observatories in space and on the ground, hoping the wealth of data on the undisturbed solar wind would unveil new information about how it forms and evolves. For this solar minimum period, scientists began gathering coordinated observations starting in early 2019 under the umbrella Whole Heliosphere and Planetary Interactions, or WHPI for short.
This particular WHPI campaign comprised a broader-than-ever swath of observations: covering not only the Sun and effects on Earth, but also data gathered at Mars and the nature of space throughout the solar system — all in concert with Parker Solar Probe's fourth and closest-yet flyby of the Sun.
The WHPI organizers brought together observers from all over the world — and beyond. Combining data from dozens of observatories on Earth and in space gives scientists a chance to paint what might be the most comprehensive picture ever of the solar wind: from images of its birth with solar telescopes, to samples shortly after it leaves the Sun with Parker Solar Probe, to multi-point observations of its changing state throughout space.