Twisted Flux Tube Emergence Evidenced in Longitudinal Magnetograms: Magnetic Tongues |
| |
| Authors: | M L Luoni P Démoulin C H Mandrini L van Driel-Gesztelyi |
| |
| Institution: | 1.Instituto de Astronomía y Física del Espacio,CONICET-UBA,Buenos Aires,Argentina;2.Facultad de Ciencias Exactas y Naturales,FCEN-UBA,Buenos Aires,Argentina;3.Observatoire de Paris, LESIA,UMR8109 (CNRS),Meudon Principal Cedex,France;4.UCL – Mullard Space Science Laboratory,Dorking,UK;5.Konkoly Observatory of the Hungarian Academy of Sciences,Budapest,Hungary |
| |
| Abstract: | 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 Ω-loop emergence,
trailing the solar rotation, which was proposed earlier to explain other asymmetries in bipolar ARs. |
| |
| Keywords: | |
| 本文献已被 SpringerLink 等数据库收录！ |
|
