2020 ARTEMIS SCIENCE NUGGETS

Coupling of Ion and Electron Waves in the Distant Magnetotail

by Suiyan Fu and Duo Zhao
Peking University


Introduction

The magnetosphere is formed by the interaction of solar wind, ionized particles released from sun, with Earth’s intrinsic magnetic field. The solar wind can enter into the magnetosphere on the dayside and its energy is finally released on the elongated nightside, a region called magnetotail. Waves of different temporal and spatial scales are often generated to release the stored energy during magnetotail active times. Due to large mass differences, the characteristic scales of wave activities associated with ions and electrons are very much different, therefore, they are often regarded as independent processes of each other. When the ARTEMIS spacecraft was travelling in the tail around 54 RE, which is close to the moon's orbit, we found high frequency electron-scale waves were embedded in low frequency ion-scale waves, indicating the plasma waves on electron and ion scales can be coupled in a coherent way.

The electron-scale waves observed in this event are recognized as whistler mode waves. Whistler mode waves are often seen at the magnetotail region, and are believed to be associated with the interaction of magnetized electrons with an electromagnetic wave. Even though the propagation of whistler mode waves can be confined by magnetic structures or density ducts, they have not been reported to be coupled with ion-scale waves in the magnetotail region. Our observation reveals the multi-scale nature of the energy release process in the magnetotail, covering from ion to electron scales, through wave modulations.

Figure 1. Schematic diagram of wave modulation processes in the magnetotail. The solid lines with arrows represent the magnetic field, while the dashed line mark the neutral sheet. The content in the solid-line box shows detailed features of the ion-scale waves in the dash line box.

Results

Figure 1 shows a schematic diagram of the wave modulation process in a tailward plasma flow at ~54 RE down the magnetotail. The large amplitude ion-scale wave was observed several seconds after the flow leading edge (Dipolarization Front). A tailward streaming hot ion beam was found to generate the wave, which is likely a kinetic Alfvén wave. During this time interval, whistler mode waves were also observed periodically at each cycle of the ion-scale wave, indicating they have been modulated by the ion wave. Detailed analyses show that these whistler mode waves were caused by the field-aligned streaming electron populations that might be accelerated by the ion-scale waves. The streaming electrons only occurred on the field-aligned direction with no bouncing electrons, because of the magnetic field configuration on the tailward side, leading to an asymmetrical distribution to provide free energy for the whistler mode waves to grow.

Figure 2. Wave analyses and corresponding particle behaviors. (a) Magnetic field components, (b) Total magnetic field strength, (c) Magnetic field fluctuations filtered above 10 Hz in field-aligned coordinate system, (d) magnetic field wave spectrum with the white line that marks 0.4 fce, (e) Electron flux anisotropy in parallel and antiparallel directions calculated from the parallel electron differential flux (F+∥, pitch angle: 0°-30°) minus the antiparallel electron differential flux (F-∥, pitch angle: 150°-180°) and then normalized byF-∥. (f) electron flux anisotropy between perpendicular (F⊥ pitch angle: 60°-120°) and parallel (F∥ = (F+∥ + F-∥)/2) directions calculated by F⊥/F∥. The pink and white colors separate the six wave cycles marked by 1–6 in panel (a). The boxes marked by 1–6 below panel (f) are the hodograms corresponding to 1–6 of the wave cycles. The normal direction of the hodograms points out of the paper and the blue diamond is the starting point.

Figure 2 shows the detailed observation of wave characteristics detected by the ARTEMIS–P1 spacecraft. When the waves appear, the magnetic field shows large amplitude fluctuations with a frequency close to the local ion cyclotron frequency (Figure 2a), and is thus called an ion-scale wave. At each cycle of the ion-scale waves, higher frequency activities appeared periodically (Figure 2c). These high frequency waves are identified as whistler mode waves since they are right-hand circularly polarized with a frequency below 0.5 times the electron cyclotron frequency (Figure 2d). The whistler mode waves are associated with parallel electron flux enhancement, while the anisotropies are not as pronounced outside the wave intervals (Figure 2e). The transverse flux anisotropies are also calculated and shown in Figure 2f, with no recognizable anisotropies when the waves appear. Therefore, the enhanced parallel electron fluxes are considered to be the energy source for the whistler mode waves, and this hypothesis is supported by linear wave dispersion analyses based on the electron phase-space distributions.

Conclusion

We have observed wave modulations between whistler mode waves and ion-scale waves at ~54 RE down the magnetotail. The ion-scale waves, most likely kinetic Alfvén waves, are excited by the interaction of the ion beam and the background ions. The ion-scale waves periodically generated field-aligned electron populations, which drove whistler mode waves at each cycle of the ion-scale waves. These observations have suggested that plasma dynamics on electron and ion scales can be coupled through waves.

References

Zhao, D., Fu, S., Parks, G. K., Chen, L., Liu, X., Tong, Y., et al. (2020). Modulation of whistler mode waves by ion-scale waves observed in the distant magnetotail. Journal of Geophysical Research: Space Physics, 125, e2019JA027278. https://doi.org/10.1029/2019JA027278

Zhao, D., Fu, S., Parks, G. K., Sun, W., Zong, Q., Pan, D., Wu, T. (2017). Electron Flat-top Distributions and cross-scale wave modulations observed in the current sheet of geomagnetic tail. Physics of Plasma, 24(8), 082903. https://doi.org/10.1063/1.4997765

Biographical Note

Duo Zhao is a postdoctor and got his PhD from Peking University in Space Physics. His research focuses on the wave activities and associated nonlinear physics in the magnetotail.

Suiyan Fu is a professor in School of Earth and Space Sciences, Peking University. Her research area covers magnetotail, magnetosphere-ionosphere coupling, and oxygen ion dynamics.

Please send comments/suggestions to
Emmanuel Masongsong / emasongsong @ igpp.ucla.edu