Rapid Enhancement of Sheared Evershed Flow Along the Neutral Line Associated with an X6.5 Flare Observed by Hinode

TL;DR

This study documents the rapid enhancement of sheared Evershed flow along the neutral line following an X6.5 flare, revealing magnetic restructuring in the photosphere using high-resolution Hinode observations.

Contribution

First direct detection of increased Evershed flow speed along the neutral line post-flare, linking surface flow changes to magnetic field restructuring.

Findings

Evershed flow speed increased from 330 to 403 m/s after flare

Penumbrae decayed and magnetic fields became more horizontal

Field lines connected and restructured post-flare

Abstract

We present G-band and Ca II H observations of NOAA AR 10930 obtained by Hinode/SOT on 2006 December 6 covering an X6.5 flare. Local Correlation Tracking (LCT) technique was applied to the foreshortening-corrected G-band image series to acquire horizontal proper motions in this complex beta-gamma-delta active region. With the continuous high quality, spatial and temporal resolution G-band data, we not only confirm the rapid decay of outer penumbrae and darkening of the central structure near the flaring neutral line, but also unambiguously detect for the first time the enhancement of the sheared Evershed flow (average horizontal flow speed increased from 330+-3.1 to 403+-4.6 m/s) along the neutral line right after the eruptive white-light flare. Post-flare Ca II H images indicate that the originally fanning out field lines at the two sides of the neutral line get connected. Since penumbral structure and Evershed flow are closely related to photospheric magnetic inclination or horizontal field strength, we interpret the rapid changes of sunspot structure and surface flow as the result of flare-induced magnetic restructuring down to the photosphere. The magnetic fields turn from fanning out to inward connection causing outer penumbrae decay, meanwhile those near the flaring neutral line become more horizontal leading to stronger Evershed flow there. The inferred enhancement of horizontal magnetic field near the neutral line is consistent with recent magnetic observations and theoretical predictions of flare-invoked photospheric magnetic field change.