Photospheric horizontal magnetic field decrease preceding a major solar eruption

TL;DR

This study reports the first detailed observation of a pre-flare decrease in photospheric horizontal magnetic field ($B_h$) that precedes a major solar eruption, linking it to early filament activation and coronal loop stretching.

Contribution

It provides the first detailed analysis of a pre-flare $B_h$ decrease as a potential eruption precursor, connecting photospheric changes to filament dynamics and coronal processes.

Findings

Pre-flare $B_h$ decreases by about 100 G along the PIL.

The decrease correlates with filament slow-rise phase.

Post-flare $B_h$ shows contrasting regional behaviors.

Abstract

Significant photospheric magnetic field changes during major solar eruptions -- interpreted as coronal feedback from eruptions to the photosphere -- are well-observed. However, analogous short-time scale field changes preceding eruptions are rarely reported. In this study, we present the first detailed analysis of a pre-flare 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 $\alpha_w$, with no significant flux emergence or cancellation observed. After the flare onset, $B_h$ exhibited contrasting behaviors in different sub-regions: a step-like increase near the PIL and a continued decrease in surrounding regions, suggesting that the pre-flare $B_h$ decrease may also have a coronal origin, like its post-flare counterparts. Coronal imaging from Atmospheric Imaging Assembly onboard SDO reveals that the associated erupting filament underwent a slow-rise phase before the flare, whose timing and location closely matches the occurrence of the pre-flare $B_h$ decrease. We propose that the slow-rise of the pre-eruptive 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 pre-flare $B_h$ decrease and suggest it as a precursor to solar eruptions, causally linked to early filament activation and its impact on the photosphere.