Photospheric horizontal magnetic field decrease preceding a major solar eruption
Lijuan Liu, Hanzhao Yang
TL;DR
The study investigates whether short-timescale pre-flare changes in photospheric magnetic fields occur and what mechanisms drive them. It analyzes a well-observed X1.8 flare (SOL2011-09-07T22:32) using SDO/HMI high-cadence vector magnetograms and SDO/AIA EUV data to track $B_h$, $B_z$, and $α_w$, and to measure filament kinematics. The authors find a pre-flare decrease in $B_h$ of about $100$ G over ~30 minutes along the flaring PIL, accompanied by a pre-flare decrease in $α_w$, with no significant flux emergence or cancellation, and a slow-rise filament that correlates with the timing and location of the $B_h$ decrease; post-flare behavior includes a $B_h$ increase near the PIL and broader decreases elsewhere. They interpret the pre-flare $B_h$ decrease as likely caused by loop straightening due to the slow-rise of the pre-eruptive filament, linking coronal dynamics to photospheric field changes and suggesting a common precursor mechanism with coronal dimmings and slow-rise activity. The work highlights the potential of pre-flare $B_h$ decreases as eruption precursors and calls for statistical studies across many events using high-cadence vector magnetograms to establish their prevalence and predictive value.
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 $α_w$, 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.