Chromospheric Plasma Ejections in a Light Bridge of a Sunspot

Abstract

It is well-known that light bridges inside a sunspot produce small-scale plasma ejections and transient brightenings in the chromosphere, but the nature and origin of such phenomena are still unclear. Utilizing the high-spatial and hightemporal resolution spectral data taken with the Fast Imaging Solar Spectrograph and the TiO 7057Å broadband filter images installed at the 1.6 meter New Solar Telescope of Big Bear Solar Observatory, we report arcsecond-scale chromospheric plasma ejections (1. ′′ 7) inside a light bridge. Interestingly, the ejections are found to be a manifestation of upwardly propagating shock waves as evidenced by the sawtooth patterns seen in the temporal-spectral plots of the Ca ii 8542Å and Hα intensities. We also found a fine-scale photospheric pattern (1 ′′ ) diverging with a speed of about 2 km s −1 two minutes before the plasma ejections, which seems to be a manifestation of magnetic flux emergence. As a response to the plasma ejections, the corona displayed small-scale transient brightenings. Based on our findings, we suggest that the shock waves can be excited by the local disturbance caused by magnetic reconnection between the emerging flux inside the light bridge and the adjacent umbral magnetic field. The disturbance generates slow-mode waves, which soon develop into shock waves, and manifest themselves as the arcsecond-scale plasma ejections. It also appears that the dissipation of mechanical energy in the shock waves can heat the local corona.

Frequently Occurring Reconnection Jets from Sunspot Light Bridges

“…It is worth mentioning that magnetic reconnection occurring near the photosphere could also generate slow-mode waves. These waves can then propagate upward and develop into shock waves (Takasao et al 2013;Yang et al 2014;Song et al 2017). A similar idea was proposed by Shibata et al (1982) more than 30 years ago, though in their simulation the source of the waves is an arbitrary sudden pressure enhancement in the low atmosphere.…”

Sunspot Light Walls Suppressed by Nearby Brightenings

“…Analysing images obtained in the 1600Å ultraviolet (UV) channel of the Transition Region and Coronal Explorer, Berger & Berdyugina (2003) found persistent brightness enhancements over a light bridge. In some other studies, more dynamic brightenings and surges were observed in the lower atmosphere above sunspot light bridges (Asai et al 2001;Shimizu et al 2009;Tian et al 2014;Louis et al 2014;Toriumi et al 2015a, b;Robustini et al 2016;Song et al 2017).…”

The transition region above sunspots

“…It is worth mentioning that previously magnetic reconnection was usually suggested to trigger these surges (Asai et al 2001;Shimizu et al 2009;Louis et al 2014;Toriumi et al 2015). IRIS observations have also revealed occasionally occurring reconnection events at light bridges (e.g., Song et al 2017;Hou et al 2017). Reconnection can heat the local plasma to typical TR temperatures at light bridges.…”