Charged particles escaping from the planet can be ionized planetary atmosphere or solar wind reflected by stars, both of which are important components of planetary space environment. Their interaction with supersonic background solar wind is an important content of planetary space physics research. In the literature, the term "solar wind pickup" is usually used to describe this process: solar wind farms accelerate these charged particles and eventually make them move with the solar wind. From the perspective of conservation of momentum and energy, it is generally believed that this process will make the local solar wind lose momentum/energy, reduce its speed, and squeeze the interplanetary magnetic field moving with the solar wind, thus leading to the enhancement of the local magnetic field.

Because the space environment and process of the moon background are extremely simple, the mutual scene between the moon and the solar wind provides a natural laboratory for testing the above traditional views. Zhang Hui and other research teams from Institute of Geology and Geophysics, Chinese Academy of Sciences have studied the interaction between solar wind ions reflected from the surface of the moon and the background solar wind in detail, and found that the main function of the solar wind pickup process is not to slow down the solar wind, but to make it swing. This progress provides a new idea for analyzing the specific physical process of "solar wind picking". Because Mars, Venus and Mercury all have the process of picking up ions reflected from ionized atmosphere or stars, the research results are of reference significance for studying the asymmetric structure of magnetic field and plasma in the magnetic tails of these stars.

The moon is a good absorber of plasma. From the point of view of magnetic fluid, when the solar wind hits the moon, it will be absorbed by the sun, thus forming a plasma cavity on the back of the moon. The moon is like an insulator, and the interplanetary magnetic field can penetrate the moon and enter the plasma cavity without hindrance. Later, due to the need of magnetic fluid pressure balance, the surrounding solar wind will gradually backfill into the plasma cavity. The backfilling process will dilute the surrounding interplanetary magnetic field and squeeze the magnetic field in the middle of the cavity to enhance it. These global-scale plasma and magnetic field disturbances constitute the "wake" of the moon. No matter from the plasma or magnetic field, the researchers did not expect that the lunar wake would present an asymmetric structure about the solar wind (see figure 1).

In this study, when a new coordinate system related to solar wind farm is established to display these data, it is found that the wake of the moon presents obvious asymmetric characteristics (Figure 2). The further analysis of magnetic field and plasma shows that all parameters show asymmetric distribution related to electric field. It is particularly important that these parameters will be modulated periodically every two lunar radii on the wake side (the right side of each figure in Figure 3) where the electric field points; The magnetic field will be periodically strengthened and weakened (fig. 3a); The plasma density shows jump enhancement (fig. 3b); The plasma velocity increases intermittently (fig. 3c); The plasma temperature rises periodically (fig. 3d). Because the spatial period of these parameters is about 2 months radius, which is equivalent to the electric field drift scale in the cyclotron period of reflected particles (see the trajectory of solar wind reflected ions shown by white/black lines in Figure 3), the causal relationship between them is explained.

The analysis shows that the solar wind ions reflected by the magnetic anomaly on the solar side of the moon can periodically bypass the moon star and enter the tail after electric drift and large radius rotation. The periodic increase of plasma temperature in the wake is the direct evidence that reflected ions periodically invade the wake: when reflected particles enter the wake, the temperature of local ions will increase due to the difference of cyclotron phase (Figure 3d). During the invasion, these reflected ions exchange momentum with the backfilled solar wind plasma in the direction of the electric field under the action of the electric field (Figure 4): When the reflected ions move along (against) the electric field, the ions are accelerated (decelerated) and the solar wind deviates from (along) the electric field. This process causes the backfilled solar wind speed to oscillate in the electric field direction (Figure 3c). The oscillation and deflection of the solar wind speed can periodically compress and relax the central region of the wake, thus causing periodic disturbance of the magnetic field (Figure 3a) and density (Figure 3b). These results show that the direction of electric field is the main direction of solar wind pickup process, while the traditional direction of solar wind is not important, which provides new clues for studying the specific physical mechanism of solar wind pickup.

The research results were published in international academic journals (Zhang Hui *, Zhong Jun, Jacqueline, Cao Jinbin, Fu Suiyan, Chen,. Meaningful lunar wake generated by reflecting solar wind [j]. Geophysical research letters, 202 1, 48 (24). DOI: 10. 1029/202 1gl 096039)。 This research was awarded by Class B pilot project of Chinese Academy of Sciences (XDB4/KOOC-0/000000) and National Natural Science Foundation project (4/KOOC-0/774/KOOC-0/75, 4/KOOC-0/94/KOOC-0/00/KOOC-0/4/KOOC)

Editor: Fu

Proofreading: Qin Huaqing