The Earth's magnetopause and plasma tops are the key outer and inner boundaries of the coupled solar wind-magnetosphere-ionosphere system (Figure 1). Plasma top dynamics is an important indicator of the coupling between the radiation band, the ring current, and the plasma layer. It is sensitive to geomagnetic activity, correlated with aurora characteristics, and affects the precipitation of energetic particles in ring currents and radiation belts through wave-particle interactions. Based on the theory of magnetohydrodynamics, Chen and Hasegawa (1974) proposed that pressure pulses acting on the magnetopause and plasma roof can excite these discrete frequency eigenmodes on the surface of the finite boundary layer rooted in the conjugate ionosphere. However, short-time-scale pulse drives often lead to complex superposition of ultra-low frequency wave patterns, thus hindering subsequent developments. It is only in recent years that direct and conclusive observational evidence of the existence of magnetopause surface eigenmodes (MSE) and plasma rooftop surface waves (PSW) has been captured through combined spaceborne and ground-based instruments.
图1 太阳风、内磁层和电离层耦合中的超低频波(摘自Usanova and Shprits，2016)
However, many key questions remain unclear about the excitation, evolution, frequency selection, space weather effects, and wave-particle interactions of PSW. Previous studies have shown that MSEs with peak frequencies of 1-2 mHz originating from the magnetopause on the solar side can naturally transport along the surface of the magnetopause toward the tail, possibly providing seed fluctuations, although not at the major growth frequencies that occur at the Kelvin-Helmholtz instability, which also subsequently grow, merge, and dissipate through the Kelvin-Helmholtz instability on the morning and dusk side. However, at the top of the plasma layer, the spatial evolution model of PSW has not been resolved, either by in-situ observations or numerical simulations.
In response to this problem, Zhou Yijia, a doctoral student at the Key Laboratory of the Institute of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, under the guidance of researcher He Fei, joined Yao Zhonghua Distinguished Professor, Rong Zhaojin, Wei Yong, Zhang Xiaoxin, National Satellite Meteorological Center, and Martin Archer, Imperial College London, and Y. Dr. X. Hao, a comprehensive analysis of the spatial evolution characteristics of PSW during a moderate geomagnetic storm (Figure 2). The study made clever use of observations from six satellites at the top of the plasma layer (Figure 3). Through the study of wave and particle characteristics, the source driving PSW was accurately identified, and the spatial evolution pattern of PSW in four stages, namely seed region, growth region, stable region and decay region, was determined for the first time (Fig. 4).
Fig.2 Solar wind parameters and satellite trajectories on July 16, 2017
The main conclusions of the article are summarized as follows:
(1) The periodic injection of high-energy protons and oxygen ions with energies between 0.1-10 keV can act on the steep plasma top boundary, resulting in the excitation of PSW. This view is confirmed by the quasi-periodic synchronous perturbations of proton and oxygen ion energy fluxes, the multiple peaks of the AE index, and the simultaneous observation of polar waves near the magnetotail perturbation. While previous studies have shown that PSW may be excited by high-energy particle injection during substorms, this study further identified the species of particles responsible for this phenomenon by simultaneously monitoring the magnetotail conditions and the twilight side PSW.
(2) The "seed zone" of PSW is characterized by weak seed disturbance in the angular magnetic field component, accompanied by strong extrusion on the top of the plasma layer. This may be consistent with the previous theory that extrusion or transverse magnetic field fluctuations at the low-latitude boundary layer on the solar side are likely to act as seed perturbations that excite Kelvin-Helmholtz waves on the magnetopause on the morning and dusk side, although they propagate in opposite directions.
(3) The "growth zone" of PSW is characterized by the dominant polar standing wave component and periodic modulation of the energy flux of high-energy electrons, as well as the irregular weak perturbation of cold electron density and the non-perturbation of heavy high-energy ions. In the "stable zone", the fully developed PSW is characterized by the most significant polar wave, the extruded polar wave, and the moderately strong co-rotating wave, which exhibits standing mode characteristics. This PSW can periodically adjust high-energy electrons, the cold electron density of the plasma layer, and heavy high-energy protons, oxygen, and even helium ions. By comparison, it can be concluded that the strong extruded polar (polar) standing wave of PSW is essential for the periodic regulation of cold electron density and various ionic energy fluxes (high-energy electron energy fluxes).
(4) The PSW in the "attenuation region" is presented in the form of a composite wave, which is dominated by a co-rotating wave and an unstable polar wave, supplemented by local extrusion characteristics, leaving sporadic perturbations on the plasma layer top, but this perturbation is still visible in high-energy electrons, cold plasma layer electrons and heavy high-energy ions. The sparsity of the PSW can be largely attributed to the reduction in the pressure of the high-energy ring current ions near the plasma top of the afternoon side, as indicated by the sporadic extrusion magnetic field component and the increased corotating magnetic field component.
Fig. 3 Overview of the time series data of the four-stage evolution of PSW measured by six satellites
Fig.4 Wavemode characteristics of magnetic and velocity field disturbances in the four-stage evolution of PSW
研究成果发表于国际学术期刊GRL(周一甲，何飞*，Martin O. Archer，张效信，Y. X. Hao，尧中华，戎昭金，魏勇. Spatial Evolution Characteristics of Plasmapause Surface Wave during a Geomagnetic Storm on July, 16,2017 [J]. Geophysical Research Letters, 2024. DOI:10.1029/2024GL109371)。 研究得到国家自然科学基金项目(42222408,41931073)、中国科技部重点研发计划(2021YFA0718600)、中科院青促进(Y2021027) 和UKRI (STFC/EPSRC) Stephen Hawking Fellowship (EP/T01735X/1)的联合资助。
Editor: Fu Shixu (East China Normal University)
Proofreading: Wan Peng