Among the smallest and most fleeting occupants of the zoo of solar phenomena are bipolar ephemeral active regions (BEARs). In today’s article, researchers studied the evolution of these regions to understand how they generate miniature solar flares — and understand what this might mean for how massive solar flares are ejected.
This map of the Sun’s magnetic field from the Solar Dynamics Observatory shows large bipolar active regions, where field lines pointing in opposite directions are located close together. These regions are associated with sunspots, solar flares, and coronal mass ejections. [NASA/SDO]
The Life Cycle of an Active Region
The roiling plasma of the solar surface gives rise to an amazing variety of phenomena. Many solar phenomena are linked to active regions: places where loops of the solar magnetic field emerge from beneath the solar surface. Sunspots arise from active regions, and the evolution of the plasma and magnetic fields contained within large active regions is thought to power solar flares, coronal mass ejections, and other solar outbursts.
However, a large active region might take weeks or even months from the moment it emerges to the moment it unleashes a solar flare or a coronal mass ejection. During those weeks or months, active regions will rotate out of view, hiding parts of their evolution on the far side of the Sun. Is there a way to study the full life cycle of an active region?
A drawing of the magnetic field orientation around a BEAR situated in a region of open solar field lines that extend out into space. The top panel shows a top-down view and the bottom panel shows a side view. [Moore et al. 2022]
A Range of Scales
To answer that question, a research team led by Ronald Moore (University of Alabama in Huntsville and NASA Marshall Space Flight Center) turned to bipolar ephemeral active regions, or BEARs, which are among the smallest active regions. BEARs contain arching magnetic field lines that rise above the solar surface, span roughly 10,000 km, and aren’t associated with sunspots. As the “ephemeral” part of the name suggests, their lives are fleeting — BEARs emerge in just half a day and disperse roughly two days later, making it possible to study their entire lives in detail.
Moore and collaborators note that the structure of the magnetic field lines above BEARs is largely the same as it is above larger active regions, and both types of active regions are associated with solar explosions: massive solar flares arise from large active regions and microflares erupt from BEARs. This suggests that all active regions, regardless of size, are governed by the same processes and evolve in the same way. By that logic, we can gain the same understanding from studying the smallest active regions as we can from the largest.
Toward a Universal Mechanism
To study the life cycles of solar BEARs and determine what makes them eject microflares, Moore and collaborators used data from the Solar Dynamics Observatory to track the magnetic field strength and extreme-ultraviolet emission from 10 BEARs as they evolved.
Histogram showing the number of microflares produced by each BEAR in the team’s sample. [Moore et al. 2022]
Bonus
Check out this video from the authors’ article, which shows the evolution of the extreme-ultraviolet emission and magnetic field in one of the BEARs in the sample. The left panel shows the 21.1-nanometer extreme-ultraviolet emission, the middle panel shows the magnetic field (white indicates outward-directed magnetic flux and black indicates inward-directed magnetic flux), and the right panel superimposes magnetic field strength contours on an extreme-ultraviolet image. The movie shows 16 hours of the BEAR’s evolution.
Citation
“Bipolar Ephemeral Active Regions, Magnetic Flux Cancellation, and Solar Magnetic Explosions,” Ronald L. Moore et al 2022 ApJ 933 12. doi:10.3847/1538-4357/ac6181