Rapid penumbral decay associated with an X2.3 flare in NOAA active region 9026

We present observations of rapid penumbral decay associated with a major flare in solar NOAA Active Region 9026 on 2000 June 6. Within 1.5 hr, an X2.3 flare accompanied by an 11° long filament eruption and a full-halo coronal mass ejection (CME) originated near the neutral line of a large δ-spot region, which was associated with significant changes in white-light structure and magnetic field topology: an increase of moving magnetic features (MMFs), flux emergence and cancellation, and, in particular, the rapid disappearance of two penumbral segments located in opposite-polarity regions on the north and south sides of the δ-spot. The rapid penumbral decay is believed to be the result of magnetic field topology change that was caused by rapid magnetic reconnection during the flare, rather than part of overall long-term evolution. We present a possible explanation of this event, using a "magnetic breakout" model for solar flares, considering its complex multipolar δ-configuration and associated filament eruption and CME, i.e., previously closed magnetic field lines opened up and reconnected at a null point above the neutral line of this δ-spot. The magnetic breakout caused an energy release from a highly sheared magnetic field in the umbrae and a transition of the magnetic arcades from low lying to high lying, which led to an increase of the inclination angle of the magnetic field lines in the peripheral penumbrae; i.e., the magnetic field turned from more inclined to more vertical and toward the inner umbrae. Once the magnetic field in the penumbrae was vertical enough, the Evershed flow ceased, the manifestation of which in white-light structure is the disappearance of peripheral penumbrae. We also discuss other possible flare models for this event and compare them in several observational features. The present observations provide further evidence that highly energetic events have a distinct associated photospheric magnetic field signature and support the findings of recent analyses of photospheric line-of-sight magnetograms from the Big Bear Solar Observatory (BBSO) and the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory (SOHO) that show rapid and permanent changes of photospheric magnetic fields associated with flares.