Abstract
Abstract We report on the successive occurrence of 0.″5 wide photospheric vortices with strong transverse shear flows at the edge of a sunspot light bridge (LB), and the subsequent ejection of chromospheric surges observed using a Visible Inteferometry Spectrograph, a broadband TiO filter, and a Near InfRared Imaging Spectrograph of the Goode Solar Telescope operating at Big Bear Solar Observatory. The Hα surges ejected at the location of the vortices often appeared in a hollow cylindrical structure. We also observed quasi-periodic vortex-associated bright Hα plasma blobs moving upward with a speed of up to 4 km s−1. In view of the strong shear flow at the edge of the LB, it is likely that the vortices form under the Kelvin–Helmholtz instability. The surges may result from either the magnetic tension generated after magnetic reconnection or an acoustic impulse of a fast photospheric transverse flow. Otherwise, the surges could also be associated with Alfvénic waves, in which case their origin could be torsional magnetic fields generated in the process of the vortex formation.
Highlights
Hα surges, which are jets of cool chromospheric plasma, are protruding into the solar corona 10–100 Mm above the photosphere
After the observing run the two sunspots eventually merged so that the light bridge (LB) disappeared by the time the sunspots reached the west limb
It showed multiple lanes running parallel to the LB axis rather than a single central dark lane with short and narrow dark and bright lanes branching out of that central dark lane (Lites et al 2004; Guglielmino et al 2017). These multiple lanes inside the LB are similar to the filamentary structures of a sunspot penumbra or an orphan penumbra, which are likely to be formed by flux ropes trapped in the photosphere by strong overlying preexisting fields (Lim et al 2013; Zuccarello et al 2014)
Summary
Hα surges, which are jets of cool chromospheric plasma, are protruding into the solar corona 10–100 Mm above the photosphere. The untwisting motion of reconnected magnetic field lines that were originally twisted at the time of emergence, can accelerate plasma along open field lines as well (Shibata & Uchida 1986; Schmieder et al 1995; Pariat et al 2009, 2010, 2015, 2016; Moreno-Insertis & Galsgaard 2013). Another possible driver is the MHD waves from the photosphere.
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