The magnetic origin of the mystery of rare H$\alpha$ Moreton waves

TL;DR

This study reveals that the magnetic field configuration at the edges of active regions influences the generation and propagation of Moreton waves, explaining their rarity and association with specific flare and CME conditions.

Contribution

It demonstrates that inclined magnetic configurations are crucial for Moreton wave generation, providing a magnetic origin explanation for their occurrence.

Findings

Over 80% of Moreton waves occur at active region edges.

Moreton waves propagate along directions with rapidly decreasing horizontal fast-mode wave speed.

Inclined magnetic fields are key to generating Moreton waves, even in X-class flares.

Abstract

Over the past three decades, a lot of coronal fast-mode waves were detected by space missions, but their counterparts in the chromosphere, called the Moreton waves, were rarely captured. How this happens remains a mystery. Here, to shed light on this problem, we investigate the photospheric vector magnetograms of the Moreton wave events associated with M- and X-class solar flares in 2010--2023. The H$\alpha$ data are taken with the Global Oscillation Network Group (GONG) and the Chinese H$\alpha$ Solar Explorer (CHASE). Our statistical results show that more than 80\% of the events occur at the edge of active regions and propagate non-radially due to asymmetric magnetic fields above the flares. According to the reconstructed magnetic field and atmospheric model, Moreton waves propagate in the direction along which the horizontal fast-mode wave speed drops the fastest. The result supports that the inclined magnetic configuration of the eruption is crucial to generate Moreton waves, even for X-class flares. It may explain the low occurrence rate of Moreton waves and why some X-class flares accompanied with coronal mass ejections (CMEs) do not generate Moreton waves.