In a recent study published in the Monthly Notices of the Royal Astronomical Society, XIE Xiaoyan from Yunnan observatories of the Chinese academy of sciences, and her collaborators, investigated the disturbance to the solar atmosphere caused by the solar eruption via numerical experiments. Their results showed some interesting structures that have not been reported in previous simulation results.

Solar eruptions cause various types of disturbance in the solar atmosphere, including the Moreton wave, which is the wave observed in the chromosphere, and extreme ultraviolet (EUV) wave, which appears in the corona. The study of the disturbance could help understand physical properties of the solar eruption and perform the plasma diagnostics for the region of interest. Comparisons of numerical observational results would further help constrain the theory on the eruption and improve the numerical model.

In this study, the researchers explored the disturbance caused by solar eruptions via numerical experiments with special attention on large-scale waves, and they also saw a new structure known as the plasma pile-up in addition to the phenomena that have been showed by previous numerical experiments.

As the disrupting magnetic structure moves outwards, a fast-mode shock was invoked in front of it. The fast-mode shock expands sideward when propagating forward, and evolves to a crescent shape, eventually the two ends of the crescent touch the bottom boundary and cause various types of disturbances to the near region.

By comparing the simulation with the observation, the researchers first proposed the method of confirming the echo, and further showed that the echo can be observed, confirming the "true wave" nature of the EUV waves. A plasma pile-up region produced by plasma accumulation was behind the echo. This is a brand new phenomenon that has not been reported.

Two features of the pile-up region drew the attention, i.e., its height from the bottom boundary is similar to that of some EUV waves, and its velocity is about 1/3 the velocity of the fast-mode shock along the low layer of the atmosphere, which is believed to be the location of the Moreton wave front. The results suggested that the pile-up may be a source of the EUV wave as well.

Besides, the researchers obtain "observed" Solar Dynamic Observatory/ Atmospheric Imaging Assembly (SDO/AIA) images in different wave bands. The results showed that the characteristics of the EUV waves "observed" in different bands are indeed different, which is consistent with the true observational results regarding the EUV waves.

Figure: Panels (a)–(f) show the evolution of the system during eruption, in which the color shading indicates the distribution of the density and the black solid curves are the magnetic field lines. Panel (g) is a zoom-in of the region in the white box in panel (e) to make the enlarged features of the echo, shock and pile-up clearer. (Image by XIE Xiaoyan)