Monthly Roundup: Solar Science on a Solar Eclipse Day!

Today’s the day! The Moon’s shadow will sweep across Mexico, the United States, and eastern Canada, plunging millions of eclipse viewers into a few minutes of mid-afternoon twilight. I’ll be enjoying a partial eclipse in partly cloudy Colorado, having bailed on my original plans for totality in Texas due to thunderstorms. See you all in Spain in August 2026, perhaps?

Whether your plans have been sidelined by weather or you’re enjoying the eclipse under sunny skies, take a moment to delve into three solar physics research articles with us today to learn how researchers are studying our home star.

Launching the Solar Wind with Tiny Jets

The solar wind is a hot, tenuous plasma that constantly streams out from the Sun, whipping past Earth at about 400 kilometers per second. How the solar wind is launched from the Sun’s atmosphere is a hot topic, with recent research suggesting that thousands of small jets powered by the release of pent-up magnetic energy could provide the boost the solar wind needs to get going.

Recently, Alphonse Sterling (NASA’s Marshall Space Flight Center,) and collaborators examined the physics behind this hypothesis. Coronal jets and their smaller counterparts, jetlets, seem to be associated with the eruption of loops of coronal plasma called filaments. Sterling’s team cataloged a wide range of solar phenomena, from tiny minifilaments that span just a few thousand kilometers (“tiny” is relative!) to huge coronal mass ejections that shrug off the Sun’s gravity and explode into space. The team found that the sizes and occurrence rates of these coronal structures fall along a power law, suggesting that they may all be triggered by the same mechanism: the eruption of a twisted tube of magnetic fields called a magnetic flux rope.

Looking more closely at the connection between jets and the solar wind, Sterling’s team proposed that when a flux rope erupts, the material ensnared within the rope is launched outward, and the intrinsic twist of the magnetic field in a flux rope could be transferred to field lines that extend out into the solar system. The movement of this magnetic field twist could produce magnetic zigzags called switchbacks, which have been observed in detail by the Parker Solar Probe and Solar Orbiter, connecting the beginnings of the solar wind to phenomena seen farther downwind.

A Coronal Mass Ejection Seen from Mercury, Earth, and Other Locations in Space

With spacecraft orbiting Earth, zooming around other planets, and even skimming the Sun’s atmosphere, we can study the Sun from more angles than ever before. In February 2022, three spacecraft made ultraviolet observations of a loop of solar plasma suspended within the Sun’s magnetic field. The loop of plasma, called a prominence, eventually erupted as a coronal mass ejection. The BepiColombo spacecraft and the Parker Solar Probe weathered the coronal mass ejection from their positions near Mercury’s orbit, just 0.35 au from the Sun.

As the fleet of spacecraft exploring our solar system grows, it has become more common for coronal mass ejections to interact with multiple spacecraft (although it’s still quite rare!). This event gave researchers an exceptional opportunity to study coronal mass ejections, as BepiColombo and the Parker Solar Probe were separated by only a few million kilometers and were only a small angular distance apart.

This allowed Erika Palmerio (Predictive Science Inc.) and collaborators to study how the structure of a coronal mass ejection varies on these scales. Despite the small separation of the spacecraft, many of the properties they measured were considerably different. For example, they measured different directions of the shock as it passed over them, and the time it took for the coronal mass ejection to pass over them was measured to be different by more than 9 hours. This reveals that coronal mass ejections have considerable variations over the scales examined in this study, and more work is needed to understand how and why these variations exist.

A Potential Solution to a Persistent Problem

Magnetic field lines that emerge from the Sun’s surface and loop back to re-enter the surface are called closed field lines, while those that extend out into the solar system are called open field lines. For decades, researchers modeling the solar corona have encountered a persistent issue: models predict far less open magnetic flux than spacecraft measure. The discrepancy can be a factor of two or more, and the mismatch tends to be greatest when modeling the Sun at its most active.

A research team led by C. Nick Arge (NASA’s Goddard Space Flight Center) has proposed that the solution to this problem lies at the boundaries between regions of open and closed field lines. Arge’s team showed that the traditional method of summing up regions of open magnetic flux at the outer boundary of the model — around 2.5 solar radii — undercounts the amount of open flux, while a method that relies on tracing the magnetic field from the outer boundary down to the solar surface and from the solar surface to the outer boundary nearly matches the observed values.

The mismatch between the two methods is greatest when the grid cells of the model fall upon an active region — an area of especially strong and complicated magnetic fields — that lies on the edge of a coronal hole, where a concentration of open magnetic field lines carries solar plasma into space. This fact hints at why previous methods were least successful in matching the observed open flux when the Sun was active; high solar activity means more active regions, and more opportunities to miss open flux.

While the new method hasn’t yet been perfected, tending to overestimate the open magnetic flux in some cases, the authors hope that it will finally bring the open flux mystery to a close.

Citation

“How Small-Scale Jetlike Solar Events from Miniature Flux Rope Eruptions Might Produce the Solar Wind,” Alphonse C. Sterling et al 2024 ApJ 963 4. doi:10.3847/1538-4357/ad1d5f

“On the Mesoscale Structure of Coronal Mass Ejections at Mercury’s Orbit: BepiColombo and Parker Solar Probe Observations,” Erika Palmerio et al 2024 ApJ 963 108. doi:10.3847/1538-4357/ad1ab4

“Proposed Resolution to the Solar Open Magnetic Flux Problem,” C. Nick Arge et al 2024 ApJ 964 115. doi:10.3847/1538-4357/ad20e2