If a strong burst of radiation occurs on our sun, this can have consequences for terrestrial satellites and power grids. So far, however, it has not been possible to reliably predict such so-called flares. But now a research team has identified a possible warning sign. Accordingly, the corona above an active region shows a kind of flickering in the UV range shortly before the radiation bursts. Short, locally narrowly limited increases in brightness can be seen in this area. These mini-bursts, thought to be associated with enhanced magnetic field line rearrangements, could herald a major solar burst in the area, the team reports.
Although our sun is a comparatively quiet star, violent bursts of radiation and coronary mass ejections occur again and again on its surface. In such plasma eruptions, raging streams of particles and high-energy radiation are catapulted far into space. If such a solar storm hits the earth, in extreme cases it can paralyze satellites, GPS, telecommunications and the power supply - with correspondingly serious consequences. But there is often little time for an advance warning: Although the stream of charged particles from a plasma ejection takes several days to reach Earth, the "flare" of high-energy bursts of radiation races through space at the speed of light and reaches Earth after just eight minutes. It is therefore important to be able to predict such events as early as possible.
When is a solar flare imminent?
It is known that solar activity is closely related to the number and size of visible sunspots. In these areas of the sun's surface, which appear darker, the plasma is around 2000 degrees cooler than in the surrounding area. The reason for this is a zone of high magnetic field intensity: At this point, the field lines of the solar magnetic field form arcs that reach far up and twisted nodes that inhibit the rise of hot plasma. As a result, sunspots also form the most active zones on the sun's surface: If there is contact and recombination of the magnetic field lines, this can trigger a plasma eruption and a "flare" of high-energy radiation. But when such an outbreak will happen and which sunspots will be particularly active in this regard cannot be predicted with certainty.
This is where the study by researchers led by KD Leka from NorthWest Research Associates in Colorado comes in. They have compiled and evaluated the most comprehensive database of images from the Solar Dynamics Observatory (SDO) to date. This data catalog contains eight years of UV and extreme UV images of active regions on the Sun by this NASA space observatory. "It is the first time that a database like this is accessible to the scientific community," explains Leka's colleague Karin Dissauer. "She will be useful for many questions." For their study, Leka and colleagues used this data to search for features that distinguish active, flare-producing regions from non-flare-producing regions in advance. To do this, they compared the parameters detected by the solar observatory with data from an orbiting satellite specializing in the detection of solar X-ray bursts.
During their analysis, the research team actually discovered a difference: in the case of the sunspots, which caused a burst of radiation shortly afterwards, there were often noticeable fluctuations in the overlying solar corona beforehand. "The images indicate that such regions show a tendency towards short-lived, small-scale increases in brightness," report the scientists. As a result, the active zones show a large number of small "sparks" in the ultraviolet and extreme ultraviolet range. As the team explains, these small, brief bursts of brightness could be caused by small-scale reconnection events in these areas. According to current theory, such reconnections of magnetic field lines contribute to further entangling and charging the local magnetic field of the active regions. The accumulated energy is then discharged in one or more large bursts of radiation.
"Our results could thus provide us with a new marker that we can use to identify which active regions on the sun are likely to produce a burst of radiation in the near future and which will remain quiet for a longer period of time," says Leka. If this is confirmed, this finding could help to be able to better predict solar flares in the future.
Source: KD Leka (NorthWest Research Associates, Boulder) et al., The Astrophysical Journal, doi: 10.3847/1538-4357/ac9c04