Supersonic Downflows in a Sunspot Light Bridge

TL;DR

This paper reports the first observation of supersonic downflows in a sunspot light bridge, revealing velocities up to 10 km/s and suggesting magnetic reconnection as a possible cause.

Contribution

It provides the first measurement of such strong photospheric flows in light bridges and links them to magnetic field changes and chromospheric brightness enhancements.

Findings

Supersonic downflows of up to 10 km/s detected in a sunspot light bridge.

Downflows are associated with rapid magnetic field changes and brightness enhancements.

Magnetic reconnection is proposed as a possible mechanism for these flows.

Abstract

We report the discovery of supersonic downflows in a sunspot light bridge using measurements taken with the spectropolarimeter on board the Hinode satellite. The downflows occur in small patches close to regions where the vector magnetic field changes orientation rapidly, and are associated with anomalous circular polarization profiles. An inversion of the observed Stokes spectra reveals velocities of up to 10 km/s, making them the strongest photospheric flows ever measured in light bridges. Some (but not all) of the downflowing patches are cospatial and cotemporal with brightness enhancements in chromospheric Ca II H filtergrams. We suggest that these flows are due to magnetic reconnection in the upper photosphere/lower chromosphere, although other mechanisms cannot be ruled out.