Recently, new progress on the rarity mystery of solar chromospheric Morton waves has been made by the group of solar eruptions and radio technologies of the Space Science Climbing Team of Shandong University. The results have been published in the top academic journal The Astrophysical Journal Letters with the title "Why 'solar tsunamis' rarely leave their imprints in the chromosphere". Professor Zheng Ruisheng is the first author and corresponding author. Collaborators include Professors Chen Yao et al. of the research group, Professor Chen Pengfei of Nanjing University, Professor Tian Hui of Peking University, and Professor Robertus Erdélyi of the University of Sheffield, UK.

Figure 1: "Coronal tsunami" on 2009 February 13, observed by dual perspectives of STEREO. (Credit: Patsourakos & Vourlidas, ApJL, 2009)

As the main source of the hazard space weather, solar eruptions can also disturb the solar atmosphere and trigger oscillations of coronal structures. Coronal disturbances can be well observed in the extreme ultraviolet (EUV) wavelengths, therefore are commonly referred to as EUV waves. Coronal EUV waves propagate as tsunamis on Earth and are always known as "solar tsunamis" (Figure 1). "Solar tsunamis" can yield estimates of the coronal magnetic field strengths that are hard to measure directly, and may also accelerate solar energetic particles that influence space weather. The research on "solar tsunamis" are important for understanding the physical mechanisms of solar eruptions, for developing solar magneto-seismology, and for improving the forecast level of space weather.

Figure 2: "Chromospheric tsunami" on 2006 December 6, observed by the Optical Solar Patrol Network (OSPAN) telescope of the National Solar Observatory (NSO) in the United States. (Credit: NSO and US Naval Research Laboratory)

Currently, hundreds of coronal EUV waves have been observed. In theory, coronal EUV waves can also travel toward the solar surface and compress the chromosphere, which produces chromospheric imprints of "solar tsunami" (Figure 2). The "chromospheric tsunami" is often called the "Moreton wave" after the finder of solar physicist Moreton. Since its discovery in 1960, only tens of Moreton wave events have been detected, comparing with the large number of EUV wave phenomena. What causes such a huge difference in quantity? In other words, what is the reason for the rarity of Moreton waves? This is an unresolved mystery in Solar Physics.

To unravel this mystery, all coronal EUV waves and corresponding chromospheric Moreton waves since 2010 were examined. All selected examples were simultaneously observed from the dual views of Solar Dynamics Observatory (SDO) and Solar Terrestrial Relations Observatory (STEREO). For each EUV wave that was accompanied by a Moreton wave, there was a sharp segment at the bottom of the coronal wavefront. Interestingly, beneath the sharp segment, an arc-shaped wavefront was detected in the spectral line (304 Å) of the transition region which is the interlayer between the corona and chromosphere. Intriguingly, these examples of coronal EUV waves are all associated with inclined eruptions.

Figure 3: Comparison between inclined eruptions and radial eruptions, to reveal the key factors for triggering successfully chromospheric Moreton waves.

By carefully analyzing the light curves, differential emission measurements, and inclination angles of the sharp segments for coronal waves, and responses in the lower atmosphere for both radial and inclined eruptions, it has been proposed that one of the most important factors for triggering successfully chromospheric Moreton waves is the highly inclined configuration of the eruption. The inclination angle should be as high as ~70 degrees. Highly inclined configuration of the coronal EUV wavefront can cause much more strong compression on the lower solar atmosphere, and therefore successfully trigger a Moreton wave (Figure 3). In other words, the key factors to solving the rarity mystery of chromospheric Moreton waves may hide in the highly inclined configuration of eruptions.

This research provides key clues to unravel the rarity mystery of chromospheric Moreton waves, which is very significant for understanding the triggering mechanisms and three-dimensional propagations of "solar tsunamis".

In recent years, the group of solar eruptions and radio technologies of the Space Science Climbing Team of Shandong University responded to the national series of "Space Exploration of the Sun" mission requirements, and focused on the scientific goals of the first domestic solar satellites - the China Hydrogen-Alpha Solar Explorer (Xihe) and the Advanced Space-based Solar Observatory (Kuafu-1), and has achieved a series of research results in the physical mechanisms of solar eruptions, solar energetic particle acceleration, and radio bursts. In the past decade, Professor Zheng Ruisheng has conducted in-depth systematic research on key scientific issues of coronal EUV waves, such as observational characteristics and driving mechanisms. He is the earliest international researcher to focus on the properties of small-scale coronal EUV waves and the relationships between coronal EUV waves and weak eruption activities. His team has published a series of academic papers in international astrophysical journals such as Astrophysical Journal Letters, Astrophysical Journal, and Astronomy & Astrophysics, making significant contributions to the research on solar eruptions and radio technologies.

This research is supported by the National Natural Science Foundation of China and the Qilu Young Scholars Program of Shandong University.

Link to the paper: https://iopscience.iop.org/article/10.3847/2041-8213/acd0ac