Charged particles fleeing from a planet can be ionized planetary atmospheres or solar winds reflected by stars, which are important components of the planetary space environment. Their interaction processes and effects with the supersonic background solar wind are important contents of planetary space physics research. In the literature, the term "Solar Wind Pickup" is often used to describe this process: solar wind farms accelerate these charged particles and eventually cause them to move with the solar wind. From the perspective of momentum and energy conservation, it is generally believed that this process will cause the local solar wind to lose momentum/energy, reduce speed, and cause the interplanetary magnetic field moving with the solar wind to be squeezed, causing the local magnetic field to increase.
Because the lunar background space environment and processes are extremely simple, the interaction between the moon and the solar wind provides a natural laboratory for testing these traditional views. The research team of Zhang Hui and others at the Institute of Geology and Geophysics of the Chinese Academy of Sciences studied in detail the interaction between solar wind ions reflected on the lunar surface and the background solar wind, and found that the main effect of the solar wind pickup process is not to slow down the solar wind, but to make the solar wind swing. This progress provides new ideas for dissecting the specific physical processes of "solar wind pickup". Since Mars, Venus and Mercury all have a process of picking up ions reflected from the ionized atmosphere or stars, the results of this study are of reference significance for studying the asymmetric structure of magnetic fields and plasma in the magnetic tails of these stars.
The Moon is a good absorber of plasma, from the perspective of magnetic fluids, when the solar wind hits the Moon, it will be absorbed by the Moon to the sun, thus forming a plasma cavity on the far side of the Moon. The Moon, in turn, is approximately an insulator, and the interplanetary magnetic field can penetrate the Moon without hindrance into the plasma cavity. Thereafter, due to the need for magnetohydrohydrodynamic pressure balance, the surrounding solar wind will gradually backfill into the plasma cavity. The backfilling process thins out the surrounding interplanetary magnetic field and squeezes the magnetic field in the middle of the cavity to strengthen it. These global-scale plasma and magnetic perturbations form lunar "wakes." Neither the plasma nor the magnetic field was expected by the researchers to present an asymmetrical structure of the solar wind in the lunar wake (see Figure 1).
Figure 1 Plasma (top) and magnetic field strength (bottom) distribution in symmetrical wakes (Zhang et al., 2014)
This study, when presenting these data by establishing a new coordinate system associated with solar wind farms, found that the lunar wake exhibits a distinctly sinuous asymmetrical feature (Figure 2). Further magnetic field and plasma analysis showed that all parameters exhibited an asymmetrical distribution associated with the electric field, and it was particularly important that the length of every 2 lunar radii at the wake pointed at the electric field (on the right side of each figure in Figure 3) was periodically modulated: the magnetic field periodically increased and weakened (Figure 3a); the plasma density showed a jumping increase (Figure 3b); the plasma velocity increased intermittently (Figure 3c); and the plasma temperature periodically increased (Figure 3d). Since the space period of these parameters is about 2 lunar radii, which is comparable to the scale of electric field drift within the orbital cycle of the reflected particles (see the trajectory of the solar wind reflected ions as shown in the white/black line of Figure 3), indicating a causal relationship between them.
Figure 2 A meandering lunar wake structure shown by the strength of the magnetic field
Figure 3 Distribution of magnetic field strength, plasma density, vertical velocity, and ion temperature in the wake. The white/black curve is the drift trajectory of the reflected ions of the solar wind on the sunny side
The analysis showed that the solar wind ions reflected on the solar magnetic anomaly on the solar side undergo electrical drift and large radius rotation, which can bypass the lunar star and periodically enter the tail fin. The periodic increase in the temperature of the intermediate ions in the wake is direct evidence of the periodic invasion of the wake by the reflected ions: when the reflected particles enter the wake, an increase in the local ion temperature can be caused due to differences in the cyclotron phase (Figure 3d). During invasion, these reflected ions exchange momentum with the backfill solar wind plasma in the direction of the electric field by acting on the electric field (Figure 4): as the reflected ions move along the (inverse) electric field, the ions are accelerated (decelerated) and the solar wind is deflected back (along) the electric field. This process causes the backfill solar wind velocity to oscillate in the direction of the electric field (Figure 3c). The oscillating deflection of the solar wind speed periodically compresses and relaxes the central region of the wake, causing periodic disturbances in the magnetic field (Figure 3a) and density (Figure 3b). These results all show that the direction of the electric field is the main direction of the solar wind picking up process, and the traditionally believed direction of the solar wind is not important, which provides new clues for the study of the specific physical mechanism of solar wind pickup.
Figure 4 The momentum exchange process between the solar wind and the positive reflected ions under the action of the electric field in the wake
Editor: Fu Shixu
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