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Zooming in on interplanetary shocks

28 August 2020

High-resolution observations of a passing solar wind event support some hypotheses but throw others into question.

MMS illustration.
An illustration of one of the four Magnetospheric Multiscale mission spacecraft. Credit: NASA GSFC

Interplanetary shocks occur when fast-moving solar wind overtakes slower-moving solar wind, spawning disruptions that ripple outward through our solar system. They can trigger geomagnetic storms when they touch Earth’s magnetosphere. Such shocks are fairly common, but they’re hard to measure. Observing one is often a matter of being in the right place at the right time.

In January 2018 the Magnetospheric Multiscale (MMS) mission—a quartet of spacecraft launched in 2015 that observe the solar wind from a highly elliptical orbit around Earth—happened to be in an ideal position when a shock blew over it. The result was the highest-resolution measurement of an interplanetary shock ever recorded. Since then, researchers have taken advantage of the rare opportunity to study the shock’s properties and effects.

Viewing the shock via MMS was like watching a water droplet through a high-speed camera rather than with the naked eye, according to Ian Cohen, a space physicist at the Johns Hopkins University Applied Physics Laboratory. “Instead of just seeing this large-scale shock going past a single spacecraft, we’re able to see lots of intermediate steps,” he says.

MMS enabled researchers to watch ions in several new ways. Whereas previous instruments could detect particles every 10 seconds, MMS does so every 120 milliseconds. Also unlike other probes that have dissected the solar wind, MMS can separate ions by mass.

Michael Starkey, a graduate student at the University of Texas at San Antonio, used the MMS data to analyze interstellar pickup ions—particles that are swept along and accelerated by solar wind, reaching speeds up to twice that of the solar wind. Scientists think that shocks are responsible for boosting the pickup ions but have had scant observational data to support that hypothesis.

Starkey and his colleagues used MMS’s high-resolution data to closely watch helium ions before and after the shock. As the helium ions encountered the shock, they were accelerated in a direction perpendicular to the shock’s magnetic field. The results were published in the Astrophysical Journal in June.

Cohen and coworkers used the MMS data to find specularly reflected ions—Sunward-moving particles that bounce 180 degrees off the advancing shock front. From theoretical models, scientists have suspected that such particles help the shock dissipate energy. By measuring ion velocities from MMS measurements, the researchers found a beam of ions that, according to Cohen, were reflected almost directly.

MMS also took high-resolution magnetic and electric field measurements of the shock, which allowed researchers to examine the shock’s microphysics. Cohen expected to see a steady, if rapid, increase in electric field strength at the shock front. Instead, he and his team found a nonlinear and far more chaotic ramp-up, hinting at never-before-seen processes at work. “There’s a lot more structure in [the shock] than we were ever able to resolve before,” says Cohen, whose work was published in the Journal of Geophysical Research last year.

The 2018 event is the only one MMS has viewed so far. Nonetheless, shock physicists remain optimistic—not least because MMS has a successor, the Interstellar Mapping and Acceleration Probe, which is slated to launch in 2024 with a more diverse suite of instruments for observing both ions and neutral atoms in solar wind.

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