Photospheric Horizontal Magnetic Field Decrease Preceding a Major Solar Eruption

Significant photospheric magnetic field changes during major solar eruptions—interpreted as coronal feedback from eruptions to the photosphere—are well-observed. However, analogous short-timescale field changes preceding eruptions are rarely reported. In this study, we present the first detailed analysis of a preflare decrease in the photospheric horizontal magnetic field ( B h ) associated with an X1.8-class flare, using high-cadence vector magnetic field data from Helioseismic and Magnetic Imager onboard Solar Dynamics Observatory (SDO). We identify a region of gradual, spatially coherent B h decrease of about 100 G along the flaring polarity inversion line (PIL) during 30 minutes preceding the flare. This decrease is accompanied by a decrease in the force-free parameter α w , with no significant flux emergence or cancellation observed. After the flare onset, B h exhibited contrasting behaviors in different subregions: a steplike increase near the PIL and a continued decrease in surrounding regions, suggesting that the preflare B h decrease may also have a coronal origin, like its postflare counterparts. Coronal imaging from the Atmospheric Imaging Assembly onboard SDO reveals that the associated erupting filament underwent a slow-rise phase before the flare, whose timing and location closely match the occurrence of the preflare B h decrease. We propose that the slow-rise of the preeruptive filament stretched overlying coronal loops, increasing their verticality and thereby reducing B h at their photospheric footpoints. The results present the first detailed analysis of a preflare B h decrease and suggest it as a precursor to solar eruptions, causally linked to early filament activation and its impact on the photosphere.