Objective: To determine the possibility of the Kelvin-Helmholtz instability in coronal loops.
Scientific Background: High resolution observations of the solar corona in coronal or chromospheric lines (through coronal rain or prominence observations) indicate a tendency of the field to organise itself in fine strand-like structure of a few hundred km widths (Lin et al. 2005, Reale, 2010; Antolin & Roupe van der Voort 2012; Brooks et al., 2013; Peter et al., 2013). Furthermore, small amplitude transverse MHD waves have been shown to permeate the solar corona. These are waves with a few km/s amplitude and periods of a few minutes, propagating or standing in magnetic field structures in the corona, observed in a wide range of wavelengths (De Moortel & Nakariakov, 2012; Lin, 2011; McIntosh et al., 2011; Antolin & Verwichte, 2011; Hillier et al., 2013).
A very recent numerical study suggests a link between the ubiquitous small amplitude transverse MHD oscillations and the small-scale strand-like structure in the corona (Antolin et al., 2014a). This link is based on the fact that a loop subject to transverse oscillations suffers from resonant absorption and from relatively strong shear motions toward the edges, which can lead to Kelvin-Helmholtz instabilities (Heyvaerts & Priest, 1983; Ofman et al., 1994; Ziegler & Ulmschneider, 1997; Terradas et al., 2008). In the case of the corona, this instability has so far been directly observed on large scales in CMEs (Foullon et al., 2011; Ofman & Thompson, 2011) and on much smaller scales in quiescent prominences (Berger et al., 2010). The KHI generates small scale vortices and current sheets, which, due to line-of-sight effects, appear as strand-like structure in intensity images. The spectral signatures of this model include specific Doppler shifts and line broadening across the loops, with characteristic phase relations with the transverse motion in the plane of the sky (Okamoto et al. 2014, in preparation).
The imaging and spectral signatures predicted by this model can be tested in the context of coronal loops with coronal rain. The high spatial resolution windows offered by the rain allows to detect both strand-like structure and small amplitude transverse MHD waves in the corona (Antolin & Verwichte, 2011). Coordinated observations between IRIS and Hinode allows an excellent temperature coverage of the chromosphere and transition region at high spatial, temporal and spectral resolutions (Antolin et al. 2014b, in preparation). Such combination represents a unique opportunity to determine the possibility of the Kelvin-Helmholtz instability in coronal loops. |
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