Preferential Appearance of H$\alpha$ Moreton Waves along Photospheric Magnetic Network Boundaries

Abstract

Moreton waves are rare chromospheric signatures of large-scale coronal disturbances, often associated with big flares and coronal mass ejections (CMEs). Using high-cadence, full-disk H$\alpha$ spectroscopic observations from CHASE, together with the EUV data from SDO/AIA and magnetograms from SDO/HMI, we analyzed a coronal EUV wave and an H$\alpha$ Moreton wave event associated with a filament eruption on 2024 July 29. The Moreton wave fronts are roughly cospatial with the fast-mode coronal EUV wave fronts, which propagate with a speed of $\sim$600 km s$^{-1}$. By comparing the Moreton wave fronts with photospheric features, we found that they preferentially appear along photospheric supergranule boundaries characterized by 1600 \AA\ bright ridges, concentrated magnetic fields, and convective downflows. It is shown that the H$\alpha$ line profiles at the Moreton wave fronts are systematically redshifted. Gaussian fit yields a systematic downward Doppler velocity of 1.73 km s$^{-1}$. Using the bisector method, we further derived height-dependent Doppler velocities in the chromosphere. While there is an expected tendency for the downward velocity to decrease from 4.12 km s$^{-1}$ in the upper chromosphere to 1.60 km s$^{-1}$ in the lower chromosphere, it is intriguing to see an unexpected velocity enhancement in the lower chromosphere. We conjecture that coronal fast-mode MHD waves experience mode-conversion to slow-mode waves, which propagate along magnetic field lines of the magnetic canopy, forming preferential appearance of Moreton waves at magnetic networks, where the convective downflow contributes to the velocity enhancement in the lower chromosphere.