Solar orbiter detects ion sorting at the solar wind’s magnetic boundary

A Southwest Research Institute (SwRI) study of data from the European Space Agency’s (ESA) Solar Orbiter gives the most detailed view of the heliospheric current sheet (HCS) to date. The HCS is a sprawling, undulating surface emanating from the sun to beyond the solar system that serves as the boundary between the sun’s north and south magnetic field hemispheres.

Anchored deep in the solar surface, the HCS acts as a high-speed pipeline, carrying crucial data and information from the solar corona straight into space. “For decades, scientists have faced a cosmic paradox: How does the sun blast a continuous supersonic stream of charged particles into space at more than 1 million miles per hour (1.6 million kilometers per hour)?” asks SwRI’s Dr. Keiichi Ogasawara, lead author of the study.

“Known as the solar wind, this invisible torrent shapes space weather, powers auroras on Earth, and can disrupt modern satellite technology,” Ogasawara said. “The key to solving this puzzle lies in magnetic connectivity. By tracing magnetic field lines to the solar surface, scientists can link local surface activity directly to solar wind gusts. The HCS offers a prime connection point thanks to its distinct, oppositely directed magnetic signatures.”

The new study, published in The Astrophysical Journal, provides scientists with a better understanding of the origin and composition of the HCS and will help define its relationship with the solar wind.

A closer look at the sheet

As the sun rotates, the HCS is twisted like a huge “ballerina skirt,” spiraling outward through the solar system and dividing the heliosphere into separate hemispheres where the sun’s magnetic field points in opposite directions. In one hemisphere, the field pushes away from the sun, while in the opposite hemisphere, the field pulls toward the sun.

The heliosphere is a vast bubble of plasma created by the solar wind that streams out in all directions from the sun. It surrounds the entire solar system and shields it from much of the high-energy galactic radiation found in interstellar space.

The Solar Orbiter recently passed through a fold of the HCS at about 26 million miles (42 million kilometers) from the sun, closer than the innermost planet, Mercury, allowing researchers to study the youngest version of the solar wind ever observed. Solar Orbiter used its high-quality field, plasma and composition instruments to study how particle populations behave in this mysterious region.

SwRI researchers studying the Solar Orbiter’s observations found a clear and distinct change in the composition and makeup of ions within the HCS. An ion is an atom that has either lost electrons and become positively charged or gained electrons and become negatively charged.

Ions shift at the boundary

“Within the HCS region, we identify a decrease in the ratio of iron and oxygen ions that lines up closely with the magnetic sector boundary itself,” said Ogasawara. “While the overall plasma on both sides is similar, we find a clear compositional change that is tightly aligned with where the magnetic polarity flips. This suggests that the HCS is not just a magnetic feature; it is also linked to how the sun sorts ions in the corona and releases them into the solar wind.”

While this new data does not define HCS origins, the researchers believe it helps set boundaries for future models and theories.

“We have provided a detailed, multifaceted view of an HCS crossing close to the sun, and we showed that it includes organized, measurable variations of the types and amounts of particles within the plasma. It’s not just a simple flip of the magnetic field,” Ogasawara said. “This offers clear constraints that future models of current sheet formation, solar wind heating and magnetic connection must satisfy. Our work is less about proving any one theory and more about defining what any successful theory has to explain.”

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Robert Egan

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