Research progress on magnetospheric soft X-ray radiation during interplanetary coronal mass ejection

Solar Wind Charge Exchange (SWCX) refers to the collision of high-valence ions (C, N, O, etc.) and neutral components (mainly neutral H in Earth’s space) in the solar wind to obtain an electron into the excited state, and then release photons in the soft X-ray band in the process of returning to the ground state. The SWCX soft X-ray radiation of the Earth’s magnetosphere mainly occurs in the magnetic sheath and extreme tip region on the solar side, so the soft X-ray observation technology can be used to deeply understand the characteristics of magnetospheric X-ray radiation and the coupling characteristics of solar wind-magnetosphere.

The Coronal Mass Ejection (CME) is the largest and most intense energy release activity in the solar system. The release of CME from the Sun into interplanetary space is the Interplanetary Coronal Mass Ejection (ICME). The propagation of ICME into near-Earth space will cause geomagnetic storms, auroras and other phenomena, which can have a significant impact on space exploration, satellite operation, communications, power facilities and oil pipelines.

With ICME, SWCX soft X-ray radiation from Earth’s magnetosphere also responds. However, it was uncertain whether magnetospheric X-ray radiation increased or decreased under the action of ICME ontology. Because, on the one hand, the valence state of heavy ions in ICME is usually higher and the types of ions are richer, and the SWCX radiation intensity tends to increase; On the other hand, due to the expansion of ICME, the plasma number density inside it is generally lower than that of the surrounding solar wind, and the soft X-ray emission of SWCX tends to be weakened. The geophysical effects of ICME generally occur in the period when the ICME body acts on the magnetosphere (non-ICME-driven sheath region), so measuring the intensity of SWCX soft X-ray radiation and the response characteristics of spectral lines is of great significance for the detection of solar wind-magnetospheric coupled X-ray imaging.

Recently, Sun Tianjie, a researcher and doctoral student Zhang Yingjie from the research group of Academician Wang Chi of the Key Laboratory of Solar Activity and Space Weather of the National Space Science Center of the Chinese Academy of Sciences, cooperated with the University of Leicester, the Purple Mountain Observatory of the Chinese Academy of Sciences, the National Astronomical Observatory and other units to use the XMM-Newton astronomical X-ray satellite to study the magnetospheric SWCX soft X-ray radiation during ICME. This event occurred on February 18, 2011, and the Earth’s magnetosphere was influenced first by the ICME-driven sheath region (formed by the solar wind around CME compression, which is characterized by high plasma density) and then by the ICME body (characterized by low plasma density).

Figure 1 Schematic diagram of magnetospheric soft X-ray radiation during continuous arrival of the ICME-driven sheath region (top) and ICME (bottom) to Earth.

The results showed that while solar wind proton fluxes in ICME declined to low levels, SWCX soft X-ray emissions increased. This is because the ions in ICME have a higher ionization state, effectively enhancing the intensity of line emission from heavy ions such as Ne, Mg, and Al. Thus, despite the low proton flux, the high abundance of high-valence ions in ICME favors magnetospheric soft X-ray observation. The Earth’s magnetic sheath under the action of ICME radiates richer spectral lines. In addition, the SWCX ion emission line intensity observed by the XMM-Newton satellite is in good agreement with the change of ion abundance measured in situ by ACE, which proves that SWCX radiation spectroscopy diagnosis can be used to remotely analyze the distribution of high-valence ions in the solar wind.

Fig. 2 Solar wind ion abundance measured in situ by ACE (left), SWCX efficiency factor calculated based on ACE observation (middle), SWCX ion emission intensity observed by XMM-Newton (right)

The results were published in The Astrophysical Journal Letters (APJL). The research was supported by the Key Projects of the National Foundation Committee, the Key Research Program of Frontier Science of the Chinese Academy of Sciences, the Strategic Pilot Program of Space Science, the Research Fund of the Chinese Academy of Sciences and the Special Research Fund of the State Key Laboratory, and the Excellent Member Funding Program of the Youth Promotion Association. (Source: National Space Science Center, Chinese Academy of Sciences)

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