Twisted Flux Tube Emergence Evidenced in Longitudinal Magnetograms: Magnetic Tongues

LUONI M. L.; DÉMOULIN P.; MANDRINI C. H.; VAN DRIEL-GESZTELYI L.

SOLAR PHYSICS

SPRINGER

Lugar: Netherlands; Año: 2010

0038-0938

Bipolar active regions (ARs) are thought to be formed by twisted flux tubes, as the presence of such twist is theoretically required for a cohesive rise through the whole convective zone. We use longitudinal magnetograms to demonstrate that a clear signature of a global magnetictwist is present, particularly, in the emergence phase when the AR is forming in a much weaker pre-existing magnetic field environment.   The twist is characterized by the presence of elongated polarities, called ``magnetic tongues'', which are due to the azimuthal magnetic field component. These magnetic tongues  first extend in size before retracting when the maximum magnetic flux is reached. The sign of the magnetic helicity can be deduced from the spatial distribution of the flux in the tongues.  Using a sample of 40 ARs, we verify that the helicity sign, determined from the magnetic polarity distribution pattern,is coherent with the one derived from the computation of the magnetic helicity injection from magnetogram time series, as well as from other proxiessuch as coronal sheared loops (observed or modeled), sigmoids, flare ribbons and/or the associated magnetic cloud observed in in situ data obtained at 1 AU.  The evolution of the tongues observed in emerging ARs is also closely similar to the evolution found in recent MHD numerical simulations. However, the observed evolution of tongues in ARs is generally more complex than found in the simulations due to multiple emergences and/or, probably, a  non uniform twist profile.