MOCIÓ QUE PRESENTA EL GRUP MUNICIPAL EU-L'ENTESA

The main conclusions from the study, presented in the chapter, are as follows:

1. For the first time, the resultant displacement (δ ξr) estimated from both the POS

and LOS velocity components were analyzed with the cross-sectional width (δ ξW), photometric (δ I) and azimuthal shear/torsion (δ ξt) variations for off-limb

spicules. Time-distance (TD) plots show the displacement of the oscillatory axis of the spicular structure in both the perpendicular planes of motion, indicating the presence of the kink wave-mode, with periodic longitudinal (field-aligned) flows (Vf). Observations of spicular waveguide and TD analysis further reflected

the ‘pulse-like’ behavior of the wave mode with the dominant period of the order of the flux-tube’s lifetime.

2. Concurrent temporal and spectral (kz, f ) analysis showed profound coupling

between the observables (δ ξr, δ ξW, δ I, δ ξt) over the tube/period-scale, sup-

ported by the mutual phase relationships. The frequency ( f ) analysis showed a period-doubling and -tripling aspect for these quantities which could indicate the nonlinear behavior of the system. An important aspect to note here is, that the peak-frequency of the azimuthal shear/torsion component was located exactly

4.4 Conclusions 85

where other observables had lost their power densities, with the frequency (pe- riod) twice (half) as that of the primary peaks of the transverse displacement and cross-sectional width variations. The observed behavior could also be explained in terms of the linear MHD theory, with independent wave-modes coupled in the presence of the magnetic twist (internal and/or external) and/or irregular shape in MFT waveguides (Giagkiozis et al., 2015; Terradas and Goossens, 2012). Due to the presence of the twist, a single pulse-like driver can result in the coupling observed in wavenumber and frequency domains. However, in the absence of the twist, a fine-tuning would be essential for all the drivers of the kinematic parameters, associated with the observable wave-modes, which is highly unlikely.

3. The investigation opens a way to obtain more accurate information about the actual complex 3D spicular motion, whose components were earlier identified ubiquitously/independently as transverse, field-aligned, rotational and cross- sectional motions by the numerous studies listed in the Introduction section. This study also provides much needed insight into the coupled behavior of the different dynamical components of the spicule motion.

Chapter 5

Coupled evolution of transverse

dynamics with height: Helical vs.

Non-helical motions

5.1

Background

Kinematic wave-modes, that include the transverse oscillations, along with the as- sociated cross-sectional width and azimuthal shear/torsion variations were found omnipresent, in both, on-disk and off-limb MFT structures, and are key to the con- duct of the energy across the interface-region. These motions reflect the dominant linear/nonlinear MHD wave-mode behavior, confined in the observed thin MFT waveg- uides, and were primarily analyzed, for their independent evolution in time and also for the estimation of associated wave energy flux. A few, theoretical (e.g., James et al., 2003; Shibata and Suematsu, 1982), as well as, observational (e.g., Khutsishvili et al., 2017; Morton, 2014; Stangalini et al., 2017; Zaqarashvili and Skhirtladze, 2008) studies, however, did examined the evolution of the longitudinal wave motion, to understand the origin and dynamics of the transverse motions.

De Pontieu et al. (2007d) observed the ubiquitous transverse motions in the spicule structures, in the Hinode/SOT data, and reported the presence of the Alfvén wave-mode in chromospheric MFT structures, though, this interpretation was later questioned in many consequent studies (e.g., Erdélyi and Fedun, 2007; Van Doorsselaere et al., 2008; Zaqarashvili and Erdélyi, 2009). However, Zaqarashvili and Skhirtladze (2008) suggested that the helical kink motions could be responsible for the observed transverse oscillations in the observed spicule structures. They proposed that a MFT waveguide emanating from the solar photosphere, could experience continuous buffeting from

the granular cells at its foot-point. This photospheric buffeting action can inject (single/multiple) kink pulse in the observed feature, which while propagating up, through the stratified solar atmosphere, appears like transverse oscillation, as reported by De Pontieu et al. (2007d). They further highlighted the helical behavior of the polarized dynamic components of the injected kink pulse, in different planes of motion (towards and perpendicular to the observer’s LOS).

Though, kink waves can displace the flux tube axis, the degree of polarization and the associated plane of dominant motion will depend strongly on the foot-point driver. In case, multiple kink waves are injected into the waveguide, the resulting transverse motion would be more complex and the linear superposition of the injected waves. Zaqarashvili and Skhirtladze (2008), for two linear kink waves, polarized in perpendicular planes, suggested that the overall helical transverse motion would be circularly polarized, for equal amplitudes of the two kink waves, and elliptically polarized, for unequal kink amplitudes, with dominant motion in the plane of the kink wave with the higher amplitude. Stangalini et al. (2017) traced the photospheric foot-point motions and concluded the observed foot-point motions to evolve into the helical kink behavior, in the chromospheric flux tube structures.

However, it must be noted that, these studies only considered the superposition of the linear kink waves in spicular structures, without any possible contribution from the other observed kinematic components (e.g., cross-sectional width and azimuthal shear/torsions). This chapter investigates the complex motions of the spicule structures across the interface-region, through spatial-, temporal- and spectral-analysis. Observed dynamic parameters, with associated multiple/nonlinear wave-modes are examined, to understand the overall transverse motions, for the structures in complex chromospheric environment.