Sometimes you just need to look at things a different way. That’s what Atul Mohan, an astrophysicist and research fellow with The Catholic University of America who is assigned to the NASA-PHaSER project, found.
Researchers have long sought to understand the flare behavior of young “red dwarf” coronas. The massive, highly magnetised plasma eruptions, called coronal mass ejections (CMEs) form a major space weather hazard because they can erode planetary atmospheres on impact or trigger harmful chemical reactions that can destabilize bio-molecules.
Red dwarfs host most of the known Earth-like exoplanets at distances much closer to the star than the Earth-sun distance. This exposes them more to these violent eruptions than the inner solar system planets.
Understanding the CME-productivity of major red-dwarf flares is an important step in identifying plausible star-planet systems that could host life.
Decades of solar observations have shown that major CME events are closely associated with three distinct types of radio bursts: types II, III, and IV. For more than a decade, researchers engaged in day-long monitoring of active young red dwarfs to identify these burst types. Even though these red dwarfs had a very high rate of strong flares — flares that almost always produce a CME in the sun — no CME-associated radio burst types had ever been found.
Mohan notes, “These stars are super active, producing extremely energetic superflares like the once-a-century solar flare of 1859. The flares are associated with massive magnetic field reconfigurations on the surface. That is what produces the CME. So, with these highly magnetized stars, we had a mystery: Why aren’t we seeing any radio signatures of CMEs - something we should see?”
Mohan and his team approached the research from a new perspective.
“We wanted to explore this by using multi-spacecraft data of simultaneous observations. We did this by compiling a catalog of solar CME-associated radio bursts observed simultaneously by NASA’s Wind and STEREO-A and STEREO-B missions,” Mohan said.
Owing to their different orbits around the Sun, at any given date and time, these missions provided radio observations of the same event from different vantage points. Using this information, the researchers explored the effect of line of sight to the activity region in the detection of these bursts. Radio emission has an inherent beaming effect, similar to a laser beam.
They discovered that active regions must be within a +-60 deg viewing angle of the mission or there was major degradation in the observed signal, and the event often was undetectable with misaligned spacecraft.
Mohan targeted a specific star — AD Leo — because its active region belt is aligned well with our line of sight from the Earth. This strategy removed any effect from the emission beaming from contributing to non-detection.
The research team discovered on AD Leo the elusive radio burst indicators of massive eruptions that researchers expected in a young active star (Type-IV and long-duration Type-III radio burst signatures commonly associated with very strong CMEs in the sun).
In fact, AD Leo was very active, emitting superflares — flares that are stronger than the strongest ever reported solar flare (the Carrington flare of 1859) — multiple times per week. The CMEs associated with the Carrington event interrupted telegraph transmissions worldwide and led to auroras near the equator. This signified intense injections of energetic particles into the Earth’s magnetosphere, driving strong electric currents.
This is just the beginning of new research on the stars, providing an essential criterion of line-of-sight to the stellar active-region belt when choosing study targets in addition to the simple criterion of flaring rate.
Mohan said, “This way we can get stronger constraints on the CME rates which are important to assess the threats to habitability in these exo-worlds.”
These findings were published in Astronomy & Astrophysics and Astrophysical Journal, and presented (abstract | slides | video) during the annual American Astronomical Society (AAS) meeting held June 8-12, in Anchorage, Alaska.
