Unexpected Local Enhancement of Chromospheric Moreton Waves on the Sun

Abstract

Solar eruptions often drive shock waves. When propagating in the solar corona, strong shock waves would leave imprints on the solar surface in the chromosphere, which are called Moreton waves. Moreton waves generally decay as they move away. However, to our knowledge, for the first time we report that a Moreton wave is significantly enhanced midway during propagation, as revealed by the spectroscopic observations from China's first solar satellite, the Chinese H$α$ Solar Explorer. Combining magnetic measurements from the Solar Dynamics Observatory, we found that the location of the Moreton wave enhancement corresponds to a magnetic null point at an altitude of $\sim$70 Mm in the corona. It is demonstrated that the unexpected local enhancement of the Moreton wave is due to mode conversion around the magnetic null point, i.e., fast-mode waves are converted to slow-mode waves which are trapped locally. It is further revealed that, even for ordinary Moreton wave fronts, the H$α$ Doppler redshift velocity is positively correlated with the height of widespread low-altitude magnetic null points, implying that these low-lying null points, $\sim$1--7 Mm above the solar surface, facilitate mode conversion with different efficiency, thereby accounting for the spatially inhomogeneous Dopplergram of Moreton wave fronts.