Searching for Sunquakes

A sunquake might sound like something that would be pitted against a tsunami in a B-level disaster movie, but these ripples on the Sun’s surface aren’t likely to affect us here on Earth. Instead, astronomers study sunquakes to understand their connection to another important solar phenomenon: solar flares.

Disturbances on the Solar Surface

When the Sun unleashes a huge burst of electromagnetic radiation in the form of a solar flare, the flare sometimes generates a flurry of seismic waves in the Sun’s interior and on its surface, in the photosphere, called a sunquake. Though sunquakes are often described as “waves” or “ripples,” these terms belie the full force of these events: the first sunquake discovered to be connected to a solar flare was estimated to be equivalent to an 11.3-magnitude earthquake — roughly 50 times stronger than the strongest earthquake ever recorded.

Somehow, solar flares power these massive seismic events, but the exact mechanism isn’t known. So far, the majority of sunquakes have been observed in the photosphere, but recent observations suggest they also occur in the chromosphere, the region above the photosphere where the temperature begins to rise. One key to understanding these events is to pin down where they occur, leading Sean Quinn (Queen’s University Belfast, Ireland) and collaborators to seek them out.

Snapshots of the Sun

Quinn and coauthors began their search with a set of 62 sunquakes that had previously been discovered in pictures of the solar photosphere. Their goal was to find evidence of these same sunquakes in 160- and 170-nm ultraviolet images from the Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly, which probe higher temperatures — and therefore higher altitudes — than the photospheric images.

Using the locations of the 62 photospheric sunquakes to guide their search, Quinn and collaborators first examined the SDO ultraviolet images by eye. The team found that 25 of the photospheric sunquakes also produced some kind of motion in the ultraviolet images, and nine of these events were confirmed based on a secondary analysis of the motion of the sunquake wavefronts.

Because the SDO ultraviolet images capture photons from a range of altitudes, it’s challenging to discern the exact location of these sunquakes. However, previous analysis of SDO images found that flare-associated 160- and 170-nm emission arises from the chromosphere. This suggests that the nine sunquakes studied in this article also occurred in the chromosphere and can be used as a starting point for future studies.

The Investigation Continues

There’s still much we don’t know about sunquakes. Why are only some solar flares accompanied by sunquakes while others are seismically quiet? Similarly, why do some sunquakes only ripple through the photosphere, while others extend up into the chromosphere? The most widely accepted model of sunquake production suggests that nearly all solar flares should generate sunquakes, so alternative models may be necessary. With a little luck — and a lot of image analysis — these questions and more may be answered by further investigations of data from Solar Dynamics Observatory and other spacecraft.

Citation

“Flare-induced Sunquake Signatures in the Ultraviolet as Observed by the Atmospheric Imaging Assembly,” Sean Quinn et al 2021 ApJ 920 25. doi:10.3847/1538-4357/ac0139