Photospheric Horizontal Magnetic Field Decrease Preceding a Major Solar Eruption

Lijuan Liu

doi:10.3847/1538-4357/ae305f

2026-01-30

Lijuan Liu

Hanzhao Yang

DOI：10.3847/1538-4357/ae305f

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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 (Bh) 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 Bh 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 αw, with no significant flux emergence or cancellation observed. After the flare onset, Bh exhibited contrasting behaviors in different subregions: a steplike increase near the PIL and a continued decrease in surrounding regions, suggesting that the preflare Bh 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 Bh decrease. We propose that the slow-rise of the preeruptive filament stretched overlying coronal loops, increasing their verticality and thereby reducing Bh at their photospheric footpoints. The results present the first detailed analysis of a preflare Bh decrease and suggest it as a precursor to solar eruptions, causally linked to early filament activation and its impact on the photosphere.