Observations of Twist, Current Helicity, and Writhe in the Magnetic Knots of δ -sunspots Consistent with the Kink Instability of a Highly Twisted Flux Rope

The Astrophysical Journal. Letters

article

Peter J. Levens¹  Aimee A. Norton¹  Mark G. Linton²  Kalman J. Knizhnik²  Yang Liu¹ 

[1] HEPL Solar Physics, Stanford University;Naval Research Laboratory

DOI  :  10.3847/2041-8213/acf0c6

来源: IOP Publishing Limited

【 摘 要 】

We measure current helicity ( H _r ^c ) as well as proxies for twist ( α _r ) and writhe ( W ) in the isolated magnetic knots of three delta ( δ )-sunspots and report that the observations are consistent with a kink instability acting on a highly twisted flux tube.δ -spots are active regions (ARs) in which positive and negative umbrae share a penumbra. We identify and isolate "magnetic knots," i.e., opposite polarity umbrae that are in close proximity and forming theδ -configuration, in ARs NOAA 11158, 11267, and 11476 as observed with data from the Solar Dynamic Observatory Helioseismic and Magnetic Imager. We find thatH _r ^c ,α _r , andWhave the same sign for each magnetic knot, as predicted in simulations of a kink instability acting on highly twisted flux tubes. The deformed flux tube causing theδ -formation, the magnetic knot, is only a portion of the entire AR and demonstrates the potential for the kink instability to act on a smaller spatial scale within the AR. Each magnetic footpoint contains a single sign of the radial current,J _r , which suggests that we are observing the core of the flux rope without return currents. As a counterexample, we analyze oneβ -spot that showsH _r ^c andα _r have the opposite signs ofW . While our observations support the formation mechanism of the magnetic knots inδ -spots being the kink instability, a much larger sample is needed to determine confidently the prevalence of the kink instability as the cause of flux tube deformation.

【 授权许可】

Unknown