IAFE   05512
INSTITUTO DE ASTRONOMIA Y FISICA DEL ESPACIO
Unidad Ejecutora - UE
artículos
Título:
Twisted Flux Tube Emergence Evidenced in Longitudinal Magnetograms: Magnetic Tongues
Autor/es:
LUONI, M.L.; DÉMOULIN, P.; MANDRINI, C.H.; VAN DRIEL-GESZTELYI, L.
Revista:
SOLAR PHYSICS
Editorial:
SPRINGER
Referencias:
Lugar: Berlín; Año: 2011 vol. 270 p. 45 - 74
ISSN:
0038-0938
Resumen:
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 magnetic
twist is present, particularly, during the emergence phase when the AR
is forming in a much weaker pre-existing magnetic field environment. The
twist is characterised by the presence of elongated polarities, called
"magnetic tongues", which originate from the azimuthal magnetic field
component. The tongues first extend in size before retracting when the
maximum magnetic flux is reached. This implies an apparent rotation of
the magnetic bipole. Using a simple half-torus model of an emerging
twisted flux tube having a uniform twist profile, we derive how the
direction of the polarity inversion line and the elongation of the
tongues depend on the global twist in the flux rope. Using a sample of
40 ARs, we verify that the helicity sign, determined from the magnetic
polarity distribution pattern, is consistent with the sign derived from
the photospheric helicity flux computed from magnetogram time series, as
well as from other proxies such as sheared coronal loops, sigmoids,
flare ribbons and/or the associated magnetic cloud observed in situ 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. We also found that the elongation of the tongue formed by
the leading magnetic polarity is significantly larger than that of the
following polarity. This newly discovered asymmetry is consistent with
an asymmetric Ù-loop emergence, trailing the solar rotation, which
was proposed earlier to explain other asymmetries in bipolar ARs.