Helicity in a jet provided by transfer of twist from a sigmoidal flux rop
Résumé
Jets often have a helical structure containing hot/cool ejected plasma denser than the corona. Jets can be sources of CMEs. Various mechanisms have been proposed to explain how jets are triggered, primarily attributed to a magnetic reconnection between the emergence of magnetic flux and environment or that of twisted photospheric motions that bring the system into a state of instability. The former solution is the one found in our SDO observations of jets. We followed the magnetic history of the active region based on the analysis of the HMI vector magnetic field computed with the UNNOFIT code. We found the evidence of the pattern of a long sigmoidal flux rope (FR) along the polarity inversion line in the active region. The helical nature of the jet is characterized by the IRIS spectra.
We conjecture that there is a transfer of twist to the jet during the extension of the FR to the reconnection site without FR eruption. The reconnection would start in the low atmosphere in the bald patch reconnection region and extend at an X-point along the current sheet formed above.