A new “beaded” rhythm in the sun's radio song

Solar radio waves are a kind of natural broadcast from the Sun, carrying clues about its magnetic fields, plasma, and energetic particles. Among these emissions, scientists pay special attention to bursts that flicker quickly in time or change sharply with frequency. These fine details—known as fine spectral structures—reveal fast and small-scale processes in the Sun's atmosphere. Over the years, astronomers have spotted several kinds of such patterns, with names like “zebra stripes,” “fibers,” and “spikes.” But even now, many of their physical origins remain puzzling.

A team from China using the Chashan Broadband Solar Radio Spectrometer (CBSm)—a new Chinese instrument that observes solar radio waves between 90 and 600 MHz—has now added an entirely new pattern to this collection. During a large solar flare on May 8, 2024, CBSm recorded something no one had seen before: periodic beaded stripes.

These stripes appeared as a series of bright, narrow radio bands (each less than 1 MHz wide) drifting slowly from higher to lower frequencies, like descending musical notes. Each band was followed by a faint absorption region—an area where the signal briefly dimmed. The whole chain of drifting stripes typically spanned less than 10 MHz in frequency. What made this event truly remarkable, however, was the discovery that many of the stripes were themselves composed of smaller “beads”—tiny, rhythmic dots repeating every tenth of a second.

This exquisite level of detail was only possible thanks to CBSm's exceptional time and frequency resolution. Imaging from the Daocheng Radio Telescope confirmed that these emissions came from above the flare loops in an active solar region.

To explain this unusual structure, the researchers proposed a two-step mechanism. The main stripes, they argue, are caused by a phenomenon called double plasma resonance—a process where trapped electrons emit radio waves most efficiently at certain magnetic and plasma frequency combinations. This effect is known to produce the famous “zebra pattern” in solar radio spectra, but in this case, it seems to have occurred only within a narrow range of heights in the solar atmosphere, giving rise to just one or a few drifting stripes at a time.

The tiny “beads” decorating each stripe, on the other hand, were likely created by magnetohydrodynamic (MHD) waves—gentle oscillations that ripple through the Sun's plasma and modulate the radio emission. From these observations, the scientists could even estimate the conditions at the source: the magnetic field was relatively weak—about one gauss—suggesting that the emission came from the intersection of the multi-polar loop system or a high altitude above the Sun's surface.

This discovery not only adds a new member to the zoo of solar radio fine structures but also showcases how next-generation instruments like CBSm can open new windows into the Sun's dynamic atmosphere.

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