Scientists discovered the upper reaches of the Mercury bow shock wave to station the whistle wave

When the solar wind interacts with the planet’s magnetosphere, it forms a collision-free shock wave upstream – bow shock wave. One of the important parameters affecting collision-free shock waves is the Alphen Mach number MA (the ratio of solar wind velocity to Alphen velocity). Under different MAs, collision-free shock waves have different structural characteristics. The solar wind MA is different near the orbit of different planets. Since it is closest to the Sun, the solar wind MA at the orbit of Mercury is the lowest among all planets (about 4~6), especially during the interplanetary coronal mass ejection events (ICMEs) passing by Mercury, the MA will be lower (about 2). Therefore, studying the Mercury bow shock wave is an important way to explore low Mach number collisionless shock waves. 

Whistle waves are widely present upstream of planetary bow shocks, participating in the formation of shock waves and interacting with particles. At present, two types of whistle waves have been identified from shock ramps – propagating whistle waves and phased standing whistle. The propagation direction of the whistle wave is small relative to the magnetic field and can travel far upstream. They are widely observed upstream of Earth’s bow shock and are often referred to as “1Hz” waves. In addition, they are often observed in bow shocks on other planets such as Mercury, Venus, Mars, and Saturn. Conversely, stationed whistle waves are less common. 

The standing whistle sound wave has a constant phase spatially relative to the shock wave, propagates in the direction of the shock normal, and decays rapidly over several wave cycles (Figure 1). As the satellite moves from upstream to downstream, the standing whistle wave appears right-hand polarized relative to the direction of the magnetic field (the direction of the wave’s polarization in the green box). Conversely, when the satellite moves in the opposite direction, it manifests as left-handed polarization. Standing whistle sound waves are rarely observed upstream of Earth’s bow shock waves because they typically occur under low MA conditions.

Figure 1. Schematic diagram of the upstream stationed whistle sound wave of the Mercury bow shock

Wang Yang, doctoral student at the Key Laboratory of Earth and Planets, Institute of Geology and Geophysics, Chinese Academy of Sciences, and Zhong Jun, associate researcher, and Pan Yongxin, a researcher, used MESSENGER’s observation of Mercury to explore the characteristics of bow shock waves during ICMEs’ passage through Mercury. The study found that stationing whistle waves were ubiquitous upstream of Mercury’s bow shock during ICMEs. 

A typical standing whistle sound wave observation is shown in Figure 2. Within minutes, the motion of the bow shock caused MESSENGER to cross the bow shock several times and observe elliptical polarization waves with rapid amplitude decay upstream of the shock. In this event, when the satellite enters the magnetic sheath (downstream) from the solar wind (upstream), the observed wave is right-handed polarization relative to the direction of the magnetic field (B3), and vice versa, left-hand polarization is observed. This characteristic indicates that the phase of these waves is spatially fixed relative to the shock wave.

Figure 2. Typical observation of the stationed whistle sound wave upstream of the Mercury bow shock

Studies have shown that 36 stationing whistle-wave events were found in all ICMEs passing through Mercury, with a incidence of about 20%. Relative to the less reported from Earth, Mercury is characterized by a high incidence. These elliptic polarization statistical characteristics include propagation along the shock normal (Figure 3a), rapid attenuation of amplitude over several wavelengths (Figure 3c), radial spatial scale within 50 km (Figure 3b), and an average frequency of approximately 1.67 Hz in a satellite coordinate system.

Figure 3. Statistical characteristics of the upper standing whistle sound wave of the Mercury bow shock. (a) the angle between the wave vector and the magnetic field (kB) and the shock normal (kn), (b) the attenuation distance of the wave in the satellite coordinate system, (c) the ratio of amplitude attenuation time to wave period, (d) wave frequency distribution, (e) the ratio of wave period to shock ramp, (f) correlation between amplitude and shock angle cosine.

Previous studies have proposed that shock waves are the last half of the periodic wave of the stationed whistle wave. In this study, the ratio of wave period to shock crossing time observed by satellite (Figure 3e) was used to test this hypothesis. The observations show that the width of the shock wave is close to one wavelength of the standing whistle wave, so this conjecture is currently unreliable. Some scholars have theoretically proposed the mechanism by which the current in the shock wave produces the standing whistle sound wave. Based on this theory, the relative amplitude of the wave is positively correlated with the cosine of the shock angle (magnetic field and shock normal angle). This observation, consistent with this theory (Figure 3f), suggests that Mercury’s standing whistle waves are most likely generated by current in a shock wave. 

This study further shows that the upstream standing whistle sound wave of the shock wave is the inherent structure of the collision-free shock wave at low Mach number. The Mercury weak bow shock is a natural laboratory for studying stationed whistle. Launched in 2018 and expected to be in orbit by the end of 2025, the European Space Agency’s joint binary satellite BepiColombo is expected to bring high-quality, multi-instrument joint detection data to help scientists further dissect these fluctuations. 

The findings were published in Geophysical Research Letters (GRL). (Source: Institute of Geology and Geophysics, Chinese Academy of Sciences)

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