By Miles Hatfield and Lina Tran
NASA’s Goddard Space Flight Center
The solar wind — the hot gas streaming from the Sun — shapes the very space around us. It douses the solar system in a soup of energetic particles and magnetic fields. It sparks aurora on Earth and Jupiter. It has changed the very habitability of planets — four billion years ago, it blew away Mars’s atmosphere.
But there’s still much we don’t understand about the solar wind. As NASA plans to send more spacecraft and astronauts to space, understanding the solar wind is key to protecting them on their journey.
One of the biggest open questions about the solar wind is where, exactly, it comes from. By the time we first detect it with spacecraft close to Earth, the solar wind has already traveled 92 million miles along a winding and convoluted path. Mapping its full journey — from Sun to spacecraft — takes careful measurements and sophisticated computer models.
Here’s how Samantha Wallace, a Ph.D. candidate at the University of New Mexico, does it.
Start with a Magnetogram
The first step is to create a magnetic map of the Sun, since the solar wind travels along the Sun’s magnetic field lines as they spiral outwards from our star.
She starts at the solar surface, known as the photosphere, where the magnetic fields can be imaged with special cameras. But Wallace doesn’t want to image the entire photosphere: She only wants the part that faces the Earth. That’s the only part that blows solar wind towards our planet. (And towards NASA’s Advanced Composition Explorer, or ACE spacecraft, which detects the solar wind.)
But capturing a picture of the Sun’s Earth-facing side isn’t so simple, because the Sun won’t hold still. It rotates by about 13 degrees every day, completing one full revolution — known as a Carrington rotation — about every 27 days.
Scientists like Wallace overcome this challenge by taking snapshots of the Earth-facing side of the Sun as it rotates, day by day. Each snapshot reveals a slightly different portion of the Sun. A new part comes into view while an old part rotates past the horizon. Once the Sun completes a full Carrington rotation, they stitch together the images into a single rectangular plot. The result is a 2-dimensional map that contains information about the entire surface of the Sun at the moment it was facing Earth. It looks something like this:
This is a magnetic map of the Sun’s photosphere. The top and bottom of the graph are the north and south poles of the Sun, respectively. Along the left and right, the graph depicts the Sun’s Earth-facing surface as it rotated a full 360. Different shades of gray show the strength and direction of the magnetic field. Darker colors are magnetic fields that point in towards the Sun, lighter point away, and medium is a neutral magnetic field.
This map is a start, but it doesn’t tell us where the solar wind truly originates. After it leaves the surface, the hot gas imaged in this map weaves through tangled magnetic fields until it reaches the corona. There, at the Sun’s outer atmosphere, it can escape and become the solar wind.
So, next, Wallace needs to model that coronal magnetic field.
Model the Corona
We don’t have the capability to directly measure the magnetic fields in the corona yet. Instead, scientists use models to predict how the magnetic field at the solar surface transforms as it expands outwards.
Using a model, Wallace estimates the coronal magnetic field. She starts with the observed photospheric field. Then she extrapolates outwards, by a distance about two and a half times the diameter of the Sun, to estimate the coronal magnetic field. Here’s what it looks like:
The corona’s magnetic field looks much simpler and smoother that the photosphere. On the upper half, the uniform dark gray shows magnetic fields pointing in toward the Sun. On the bottom half, light gray shows magnetic fields pointing away. At the photosphere, depicted in the first graph, the Sun’s magnetic field is complex and rippled. But by the time we reach the corona, that magnetic field has smoothed out as it empties into the solar wind. North and south meet in the middle at the yellow wiggly line. This line marks the heliospheric current sheet, where the Sun’s magnetic field abruptly changes direction.
Connect It to the Spacecraft
Now, when Wallace looks at ACE’s solar wind measurements, she finally has what she needs to cite their sources on the Sun.
Once the solar wind exits the corona, it travels more or less in a straight line. Wallace uses a model that follows individual parcels of solar wind along those straight paths until they reach ACE. Once she connects all the dots, it looks something like this:
Red crosshairs mark which parts of the Sun were directly in front of ACE as it collected measurements. The red vertical lines also note the date when ACE measured a specific parcel of the solar wind.
The yellow lines connect the solar wind that ACE measured at that time to their origins on the surface. As you can see, they come from all over the Sun! Once those parcels of solar wind navigate through the corona, they have already been re-directed quite a bit.
Solve Solar Mysteries
With the 2018 launch of NASA’s Parker Solar Probe, scientists have entered a new era in the study of the solar wind. As Parker passes closer to the Sun than any spacecraft before it, it is observing the solar wind in its freshest state yet. These observations will be key to prying open new questions about the solar wind and the complicated processes on the Sun that produce it.
To prepare for whatever they’ll find in Parker’s data, Wallace and her coauthors used the techniques described here. But the applied it not to ACE, but rather to the second closest spacecraft to the Sun. The German-American Helios mission, launched in 1974, flew as close as 27 million miles from the solar surface. Using archival data, Wallace and her coauthors mapped Helios’s 45-years-old solar wind observations back to the Sun. It was the first time this had ever been done for Helios. The results have already shed light on the nature of the slow solar wind. . . And they also whet scientists’ appetite for the insights that lay ahead as Parker beams its data back to Earth.