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Spin doctor prescribes new dynamo treatment

posted Oct 12, 2019, 1:52 PM by Jeremy Drake
There are many subfields of astrophysics in which a rudimentary understanding of the physics involved has been established for many years, but for which the real details of what is really going on remains elusive or uncertain. The Coronal Heating Problem is one of these: how does the Sun heat its outer atmosphere to temperatures exceeding a million degrees? During the late '70s and following the first results of a survey of stars in X-rays by the Einstein Satellite, it became clear that the magnetic field was the key. But the details remain elusive: are magnetic waves responsible, or is it all through magnetic reconnection - the energy released when magnetic fields are stretched and stressed and snap back to a more relaxed state?  

The magnetic dynamo responsible for generating the magnetic field in the first place is another: where exactly is the dynamo operating and how exactly do stellar rotation and convection interplay to make it happen as we observe it? The dominant source of the solar magnetic field is thought to be the "tachocline" - the interface between the convective envelope and radiative core that are observed through helioseismology to be spinning at different rates and are then a source of magnetic shear.  

The size of the radiative core in stars like the Sun shrinks toward lower masses, and at masses of about 1/3 that of the Sun it disappears altogether.  These stars should then have magnetic dynamos that differ from that in more massive stars. One way of probing this is to use X-rays emitted by hot coronal gas as a proxy for the rate of generation of magnetic field. Since stellar spin is ultimately the driver of differential rotation and dynamo activity, how does X-ray emission behave in fully-convective stars vs those with a radiative core that are spinning at different rates? An earlier post detailed evidence from four fully-convective stars showing that X-ray emission vs rotation rate appears the same in the two groups of stars.  In a much more extensive survey lead by spin doctor Nick Wright from Keele University and published in the Monthly Notices of the Royal Astronomical Society, we found further evidence that the radiative core makes little difference: stars either side of the fully-convective boundary behave very similarly. The implication is that stellar dynamos are not dominated by the tachocline, but instead by the differential rotation distributed through the lower convection zone.