Ze Zhong1,2, Yao Chen1,2, Yiwei Ni3, Pengfei Chen3, Ruisheng Zheng1,2, Xiangliang Kong1,2, & Chuan Li3,4
1. Center for Integrated Research on Space Science, Astronomy, and Physics, Institute of Frontier and Interdisciplinary Science, Shandong University, Qingdao 266237, China
2. Institute of Space Sciences, Shandong University, Weihai 264209, China
3. School of Astronomy and Space Science, Key Laboratory of Modern Astronomy and Astrophysics, Nanjing University, Nanjing 210023, China
4. Institute of Science and Technology for Deep Space Exploration, Suzhou Campus, Nanjing University, Suzhou 215163, China
Solar eruptions can drive various atmospheric wave phenomena, with Moreton waves being among the most striking. Discovered in 1960, these waves appear as bright Hα arcs in the line center and blue wing (or dark arcs in the red wing), propagating at speeds of 300–2000 km/s. The prevailing scenario suggests that Moreton waves are the chromospheric imprint of coronal fast-mode shock waves. As these coronal waves compress the chromosphere downward, they produce the observed wavefront motion in the Hα spectrum[1,2].
Since the launch of the Solar Dynamics Observatory (SDO) in 2010, hundreds of coronal fast-mode waves have been detected in EUV passbands[3]. However, their chromospheric counterparts – Moreton waves – remain exceedingly rare, with only 10 cases reported. This is partially due to the fact that the chromosphere is much denser, so the chromospheric imprints of coronal shocks are much weaker or even invisible. Zheng et al.[4] demonstrated that highly inclined eruptions are crucial for generating Moreton waves, as in such configurations, coronal shocks can compress the lower atmosphere more intensely. They have not considered the effect of the magnetic field configuration, even though it is critical in determining the eruption morphology. This study aims to fill in this research gap.
To do this, we conducted a systematic survey of all Moreton-wave events associated with M- and X-class flares observed during the SDO era (2010–2023). This gives the largest data sample ever used for the Moreton wave study[5]. We classified all events into three groups according to the mapped locations of flare ribbons on the line-of-sight HMI magnetograms (Fig. 1). The flare ribbons in 1600 Å exhibit complex patterns of motions, including perpendicular separation (e.g., Fig. 1(a)), elongation parallel to the polarity inversion line (e.g., Fig. 1(l)), and other irregular motions. Our results show that more than 80% of the events (Groups I and II) occur at the edges of active regions (ARs).
Figure 1| Mapping of flare ribbons superimposed on the HMI line-of-sight magnetograms (grayscale) for Groups I and II. (a)-(f) All magnetograms display one main magnetic polarity in the center of the AR, with opposite polarity at the periphery. The newly brightened flare ribbons are superimposed on the magnetogram, with the time lapse given by the rainbow color bar. (g)-(l) All magnetograms show multiple pairs of magnetic polarities in the center of the AR. Flare ribbons of all events are located at the edge of the ARs.
We reconstructed the corresponding coronal magnetic field to investigate the underlying magnetic configuration. For Group I, we took the Moreton wave on 2011 October 1 as an example. As shown in Figs. 2(a) and (b), a bundle of open lines is rooted to the north of the closed ones. This gives rise to a wall-like configuration within which the eruption tends to move toward the southern part with weaker fields. A similar configuration was observed for Group II events (see Figs. 2(c) and (d)). We concluded that such inclined magnetic configurations exist in most events of our sample, acting as a wall to force the eruption and the Moreton waves to propagate toward weaker fields.
Figure 2|Two main types of magnetic configurations that generate Moreton waves: open fields and magnetic loops. (a) The 3D magnetic field illustrates the relative position between the magnetic field lines and the source region that generates the Moreton wave. (b) The magnetic field with a zoomed-in view of the region corresponding to the orange dotted box in (a). The field lines are inclined toward the south. (c) and (d) Similar to (a) and (b), but illustrating the magnetic loop configuration that generated the Moreton wave. The magnetic field lines in (d) are inclined toward the north.
We further examined whether radial eruptions could generate Moreton waves (Fig. 3). We first analyzed an X1.4-class flare near the solar center associated with a fast halo CME. In this case, the EUV wave exhibits a circular front in the AIA 193 Å image, with no counterpart in the 304 Å image. We then investigated a limb radial event with an X8.2-class flare, which was the second-largest one during solar cycle 24. According to the time–distance diagram, we found that the wave was visible only in 304 Å, with no obvious Hα response. We concluded that it is difficult to generate Hα Moreton waves with radial eruptions, even for X-class flares.
Figure 3|Two X-class flares that did not produce observable Moreton waves. (a) The running-difference image of the AIA 193 Å data shows the EUV wave front depicted by cyan dots. (b) AIA 304 Å running-difference image, with no visible wave front. (c) A photospheric magnetogram, overlaid with the evolution of flare ribbons. (d) A snapshot of an eruption captured in the AIA 304 Å image. The cyan solid line points out the direction of the wave propagation. (e) Time–distance diagram of AIA 304 Å image displays the motion of the wave front along the cyan solid line in (d). (f) Similar to (e), but for the GONG Hα time–distance diagram.
Based on the above analyses, we highlight two primary points to explain the rarity of Moreton waves. One is the high density of chromospheric plasmas, which makes it difficult for the coronal shock to leave imprints. Another factor is the eruption geometry. Since many eruptions occur around the center of bipolar ARs, any imprints, if they exist, are relatively weak without an inclined magnetic configuration.
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