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Keeping a lid on it

posted Oct 11, 2019, 12:43 PM by Jeremy Drake   [ updated Oct 12, 2019, 1:55 PM ]
In a post from late 2016, I discussed the magnetic energy build-up on the surfaces of stars like the Sun, as the magnetic fields are twisted and stretched by the turbulence and flows beneath the stellar surface where they are anchored. This energy is released from time to time as the fields interact and snap back into less stressed states, resulting in phenomena such as flares and coronal mass ejections (CMEs).

CMEs are extremely difficult to detect on stars - see the posting on a monster CME from the demon star (Algol) for a very rare exception. This is a problem because CME impacts on exoplanets can erode their atmospheres and it is important to know how severe this effect is. If the relationship between X-ray flares and CMEs on the Sun is extrapolated to the most magnetically active stars there is an energy problem: the CMEs would require about 10% of the total stellar energy budget from nuclear fusion, which is not physically possible.  So how do CMEs behave on active stars and do we avoid the energy catastrophe?

From the research described in the 2016 post, it appeared that CMEs might be suppressed in active stars by their strong overlying magnetic fields. Extensive supercomputer CME simulations lead by SAO postdoc Julian Alvarado-Gómez and published in the Astrophysical Journal found that this is indeed the case - the field on an active star is essentially a magnetic lid that keeps CMEs contained. Up to a point that is: if they were given enough potential energy at the beginning of the simulation CMEs could still break through the lid. The suppression then keeps the lid on run-of-the-mill CMEs with energies similar to just about all the events ever observed on the Sun. Monster CMEs, though, can still escape and potentially ravage any planets in their way.