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Alfvénic waves in polar spicules

Abstract : Context. For investigating spicules from the photosphere to coronal heights, the new Hinode/SOT long series of high-resolution observations from space taken in CaII H line emission offers an improved way to look at their remarkable dynamical behavior using images free of seeing effects. They should be put in the context of the huge amount of already accumulated material from ground-based instruments, including high- resolution spectra of off-limb spicules.
Aims: Both the origin of the phenomenon and the significance of dynamical spicules for the heating above the top of the photosphere and the fuelling of the chromospheric and the transition region need more investigation, including of the possible role of the associated magnetic waves for the corona higher up.
Methods: We analyze in great detail the proper transverse motions of mature and tall polar region spicules for different heights, assuming that there might be Helical-Kink waves or Alfvénic waves propagating inside their multicomponent substructure, by interpreting the quasi-coherent behavior of all visible components presumably confined by a surrounding magnetic envelop. We concentrate the analysis on the taller CaII spicules more relevant for coronal heights and easier to measure. Two-dimensional velocity maps of proper motion were computed for the first time using a correlation tracking technique based on FFTs and cross-correlation function with a 2nd-order-accuracy Taylor expansion. Highly processed images with the popular mad-max algorithm were first prepared to perform this analysis. The locations of the peak of the cross-correlation function were obtained with subpixel accuracy.
Results: The surge-like behavior of solar polar region spicules supports the untwisting multicomponent interpretation of spicules exhibiting helical dynamics. Several tall spicules are found with (i) upward and downward flows that are similar at lower and middle levels, the rate of upward motion being slightly higher at high levels; (ii) the left- and righthand velocities are also increasing with height; (iii) a large number of multicomponent spicules show shearing motion of both left- and righthanded senses occurring simultaneously, which might be understood as twisting (or untwisting) threads. The number of turns depends on the overall diameter of the structure made of components and changes from at least one turn for the smallest structure to at most two or three turns for surge-like broad structures. The curvature along the spicule corresponds to a low turn number similar to a transverse kink mode oscillation along the threads.

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E. Tavabi, S. Koutchmy, A. Ajabshirizadeh, A. R. Ahangarzadeh Maralani, S. Zeighami. Alfvénic waves in polar spicules. Astronomy & Astrophysics, 2015, 573, ⟨10.1051/0004-6361/201423385⟩. ⟨insu-03644794⟩



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