Stars are giant balls of ionized gas - a plasma of mostly free electrons and protons. Such electrically charged particles are forced to spiral around magnetic fields, and magnetic fields essentially become attached or "frozen in" to the plasma. When the pressure of the plasma is stronger than the magnetic field pressure, plasma motions drag the magnetic field with it. Stellar interiors are very dynamic, with regions of strong convective flows. Stars also do not spin uniformly with the same rotation period - the Sun's equator rotates faster than its pole by 50% or so. There is broad understanding that this differential rotation, combined with convective motions, generates and amplifies the magnetic field by winding and folding it - much like stretching and folding an elastic band - but the details of the process and exactly where in the Sun it happens has remained a subject of intense debate.
When current Earnest Rutherford Fellow Nick Wright of Keele University was a postdoc at Smithsonian, he and I examined the way stellar rotation influenced X-ray output from stars. Faster rotating stars generate more magnetic field and are brighter in X-rays. We were missing some key types of star from the sample though: cool, slowly rotating old M dwarfs - stars with less than half the mass of the Sun. We subsequently observed two of them using NASA's Chandra X-ray Observatory and found X-ray data for two more from older observations. To our surprise, the trend with stellar rotation for these M dwarfs was the same as for Sun-like stars. We were surprised because these stars do not have the central radiative zone that the Sun does, but are convective all the way to the centre. They have no tachocline like that in the Sun, but their magnetic behaviour with rotation is the same. The results indicate that the tachocline is not a significant factor in stellar magnetic dynamos: magnetic field must be generated elsewhere in the convection zone, likely by the differential rotation that also exists there. This study was published in the 2016 July 28 issue of Nature and also featured in a Chandra press release.
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