Image of coronal hole on the sun's surface. Courtesy of NASA.

When Solar Probes Align: Data Confirms the Sun’s Magnetic Field Accelerates Solar Wind

A new study led by University of Michigan researchers published in The Astrophysical Journal confirms the sun’s magnetic field accelerates solar wind as it relaxes.

By:

Climate and Space Sciences and Engineering

Results will improve models for forecasting solar wind and space weather

When two probes orbiting the Sun aligned with one another, researchers harnessed the opportunity to track the Sun’s magnetic field as it traveled into the solar system. They found that the sharply oscillating magnetic field smooths out to gentle waves while accelerating the surrounding solar wind, according to a University of Michigan-led study published in The Astrophysical Journal.

The sharp S-shaped bends of the magnetic fields streaming out of the Sun, called magnetic switchbacks, have long been of interest to solar scientists. Switchbacks impact the solar wind—the charged particles, or plasma, that stream from the Sun and influence space weather in ways that can disrupt Earth’s electrical grids, radio waves, radar and satellites. 

The new understanding of magnetic switchback changes over time will help improve solar wind forecasts to better predict space weather and its potential impacts on Earth. 

“This study marks the first direct observation of switchback magnetic energy reducing with distance from the Sun,” said Shirsh Soni, a research fellow of climate and space sciences engineering at the University of Michigan and corresponding author of the study.

A graphic of the Sun emitting a conical-shaped magnetic projection. Two solar probes are positioned 25 solar radii from the Sun (time 1) and 150 solar radii (time 2). Solid, red lines with sharp S-bends closer to the Sun turn into dotted blue lines with gentler S-bends. Below the graphic, three cut out plots. From left to right: 1) Southern hemisphere Solar Orbiter at 150 solar radii—blue background indicates high relative velocity, solid blue lines meaning low dynamic pressure have gentle S-bends and travel eastward. 2) Northern hemisphere virtual projection at 150 solar radii—checkered blue and white background indicates alternating high and low relative velocity. The same lines as the previous graph travel westward but the dotted lines indicate this is a model. 3) Northern hemisphere Parker Solar Probe at 25 solar radii—white background indicates low relative velocity, solid red lines meaning high dynamic pressure with sharp S-bends traveling westward.
Magnetic switchback changes from the Parker Solar Probe (t1) to the Solar Orbiter (t2). The color bars indicate dynamic pressure (Pdyn) and relative velocity (Vp/Vsw). The boxes with solid lines represent actual observations, while those with dotted lines depict hypothetical observations if viewed from the same side of the Heliospheric Current Sheet, meaning the same hemisphere. Credit: Soni et al., 2024

The researchers pinpointed twelve time windows when the Parker Solar Probe and Solar Orbiter aligned. The Parker Solar Probe was positioned closest to the Sun, less than 30 solar radii away (Rs)—a unit of distance based on the Sun’s radius. The Solar Orbiter was further away at 130 Rs from the Sun—nearing the orbit range of Venus which lies around 156 Rs.

Comparing magnetic field and plasma moment measurements collected from both spacecraft during these windows, the researchers traced the changes in magnetic switchbacks from one point to the next.

They found that switchback patches—bundles of sharp magnetic switchbacks—smoothed out into microstreams with 30% fewer magnetic reversals, while the background proton velocity increased by 10%, indicating acceleration of the surrounding solar wind. 

The research team points to magnetic relaxation as the driving force of these changes. Essentially, as magnetic switchbacks travel outwards, the highly energetic switchbacks “relax”, transferring magnetic energy into kinetic energy to accelerate the surrounding plasma. 

The next step is to track where and how the magnetic energy transfer occurs and whether it converts to thermal energy alongside kinetic energy. Magnetic switchbacks have been ruled out as a cause of the Sun’s curiously hot corona, but could help solve another standing mystery of how the solar wind heats up as it travels through space. 

“The collaboration between Parker Solar Probe and Solar Orbiter allows us to piece together this complex puzzle, marking a significant step forward in solar physics,” said Soni.

“Magnetic switchbacks are the fingerprints of the Sun’s dynamic energy processes, revealing how it shapes the solar wind and, in turn, the entire solar system,” said Mojtaba Akhavan-Tafti, a U-M associate research scientist of climate and space sciences and engineering and co-corresponding author of the study. 

Additional co-authors: Gabriel Ho Hin Suen and Christopher Owen of University College London; Justin Kasper of the University of Michigan; Marco Velli of the University of California, Los Angeles; and Rossana De Marco of the National Institute for Astrophysics and Institute for Space Astrophysics and Planetology in Rome, Italy.

This work was supported by NASA (NNN06AA01C, 80NSSC20K1847, 80NSSC20K1014, and 80NSSC21K1662).

Full citation: “Switchback patches evolve into microstreams via magnetic relaxation,” Shirsh Lata Soni, Mojtaba Akhavan-Tafi, Gabriel Ho Hin Suen, Justin Kasper, Marco Velli, Rossana De Marco, Christopher Owen, Astrophysical Journal (2024). DOI: 10.3847/1538-4357/ad94da

Written by Patricia Delacey

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