1. Moon-plasma interactions

The Moon offers a unique natural laboratory for studying fundamental plasma physics. As an airless, non-conducting body with no global intrinsic magnetic field, it cannot deflect or mass-load the incoming solar wind. Instead, when the supersonic solar wind flows past the Moon, it carves out a plasma void on the downstream side, the lunar wake, whose refilling is governed by kinetic plasma processes. Understanding this refilling dynamics is important not only for lunar science, but also for predicting plasma behavior around other airless bodies and in technologically relevant environments.

Our recent work has revealed the structure of electric potentials that form within the lunar wake, how these potentials mediate energy transfer between different forms of energy and across particle species, and how they leave distinct imprints on ion and electron velocity distribution functions.

Selected publications:

• An, X., Xu, S., Angelopoulos, V., Liu, T. Z., Poppe, A. R., Halekas, J. S., & Plaschke, F. (2026). Inferring lunar wake potentials from electron phase space densities. arXiv preprint arXiv:2604.17825.

• Liu, T. Z., An, X., Angelopoulos, V., & Poppe, A. R. (2025). ARTEMIS observations of electrostatic shocks inside the lunar wake. The Astrophysical Journal Letters, 990(2), L36.

• An, X., Angelopoulos, V., Liu, T. Z., Artemyev, A., Poppe, A. R., & Ma, D. (2025). Plasma refilling of the lunar wake: Plasma‐vacuum interactions, electrostatic shocks, and electromagnetic instabilities. Journal of Geophysical Research: Space Physics, 130(7), e2025JA034205.

3. Wave-induced particle precipitation

In Earth's inner magnetosphere, wave-induced scattering is the dominant cause of particle precipitation into the ionosphere and upper atmosphere. Such precipitation changes ionospheric conductances, which in turn modify the global magnetospheric convection. In addition, energetic particles penetrate down to the upper atmosphere and can impact its chemical composition, ionization, and temperature. Incorporating the observed wave characteristics, beyond the usually used averaged wave intensity, into an accurate, quantified model for particle precipitation and acceleration is the next challenging frontier, which requires a scheme beyond the quasi-linear resonant diffusion theory.

In a series of recent papers, we demonstrate that the precipitation energy range can be greatly enhanced by nonresonant effects, generally ignored in wave-particle resonant models. This result generalizes the classical quasi-linear diffusion model and takes a crucial step toward solving the observed nonresonant charged particle precipitation.

Selected publications:

• An, X., Artemyev, A., Angelopoulos, V., Zhang, X. J., Mourenas, D., Bortnik, J., & Shi, X. (2024). Nonresonant scattering of energetic electrons by electromagnetic ion cyclotron waves: spacecraft observations and theoretical framework. Journal of Geophysical Research: Space Physics, 129, e2023JA031863.

• An, X., Artemyev, A., Angelopoulos, V., Zhang, X., Mourenas, D., & Bortnik, J. (2022). Nonresonant scattering of relativistic electrons by electromagnetic ion cyclotron waves in Earth’s radiation belts. Physical Review Letters, 129(13), 135101.

4. Energy release and cascade around plasma boundaries

During geomagnetic active periods, fast plasma flows in the Earth's central plasma sheet penetrate the inner magnetosphere, generating dipolarization fronts. These dynamic events trigger a complex cascade of electromagnetic phenomena across multiple spatial scales. Energetic ions and electrons excite a diverse spectrum of waves, including kinetic Alfven waves, electromagnetic ion cyclotron waves, whistler-mode chorus waves, electron cyclotron harmonic waves, and electron-acoustic solitary structures. These waves and fast plasma flows ultimately dissipate their energy through thermal plasma heating and particle acceleration, effectively braking the plasma flows.

Our research extends beyond Earth's magnetosphere to explore similar cross-scale energy transfer mechanisms in other plasma boundaries. In magnetic discontinuities within the solar wind and magnetopause boundary layer, fluid-scale Alfven waves transfer energy to Debye-scale ion-acoustic solitary structures through ion beam acceleration. This Alfven-acoustic energy channeling represents an alternative pathway to the classical turbulent cascade.

The universal nature of cross-scale energy transfer is evident throughout the sun-Earth system, though its precise manifestations vary with specific plasma parameters. The next critical challenge in space plasma theory and simulation is to precisely quantify energy release and deposition across multiple scales around plasma boundaries.

Selected Publications:

• Shi, X., An, X., & Angelopoulos, V. (2026). Energy Transfer from Magnetohydrodynamic-scale Slow-mode Waves to Kinetic-scale Ion Acoustic Waves. The Astrophysical Journal Letters, 996(1), L8.

• An, X., Artemyev, A., Angelopoulos, V., Liu, T. Z., Vasko, I., & Malaspina, D. (2024). Cross-Scale Energy Transfer from Fluid-Scale Alfvén Waves to Kinetic-Scale Ion Acoustic Waves in the Earth’s Magnetopause Boundary Layer. Physical Review Letters, 133(22), 225201. [Editors' suggestion, Featured in Physics]

• Ma, D., An, X., Artemyev, A., Bortnik, J., Angelopoulos, V., & Zhang, X. J. (2024). Nonlinear Landau resonant interaction between whistler waves and electrons: Excitation of electron-acoustic waves. Physics of Plasmas, 31(2).

• An, X., Bortnik, J., & Zhang, X.J. (2021). Nonlinear Landau resonant interaction between kinetic Alfvén waves and thermal electrons: Excitation of time domain structures. Journal of Geophysical Research: Space Physics, 126(1), p.e2020JA028643.

• An, X., Li, J., Bortnik, J., Decyk, V., Kletzing, C., & Hospodarsky, G. (2019). Unified View of Nonlinear Wave Structures Associated with Whistler-Mode Chorus. Physical Review Letters, 122(4), 045101.

5. Laboratory experiments on whistler-mode chorus excitation

Whistler-mode chorus waves are natural electromagnetic emissions in Earth's magnetosphere, playing a key role in electron acceleration and scattering within the Van Allen radiation belt encircling the Earth. Although whistler-mode chorus waves have been observed by satellites for almost six decades, the origin and excitation mechanisms of these waves are poorly understood because there are not enough appropriate observations. The nontraditional, but quite perspective approach for investigation of chorus waves is laboratory modeling, which forms a large part of my PhD thesis. This led to the first observation of chirping chorus waves in a laboratory plasma and the measurements of their resonant mode structures. Using the capacity for reproducibility and precise control in the laboratory, we produce the wave signatures that are characteristically observed in space and confirmed by numerical simulations.

More recently, using the technique of electron cyclotron resonance heating, we generated electron pitch-angle anisotropy, the subsequent whistler anisotropy instability, and the repetitive whistler wave emissions in this continuously driven system, analogous to realistic magnetospheric environment.

Selected publications:

• Ma, D., An, X., Han, J., Tripathi, S., Bortnik, J., Artemyev, A. V., ... & Pribyl, P. (2025). Excitation of whistler‐mode waves by an electron temperature anisotropy in a laboratory plasma. Geophysical Research Letters, 52(22), e2025GL118392.

• An, X., Bortnik, J., Van Compernolle, B., Decyk, V., & Thorne, R. (2017). Electrostatic and whistler instabilities excited by an electron beam. Physics of Plasmas, 24(7), 072116.

• Van Compernolle, B., An, X., Bortnik, J., Thorne, R. M., Pribyl, P., & Gekelman, W. (2016). Laboratory simulation of magnetospheric chorus wave generation. Plasma Physics and Controlled Fusion, 59(1), 014016.

• An, X., Van Compernolle, B., Bortnik, J., Thorne, R. M., Chen, L., & Li, W. (2016). Resonant excitation of whistler waves by a helical electron beam. Geophysical Research Letters, 43(6), 2413-2421.

• Van Compernolle, B., An, X., Bortnik, J., Thorne, R. M., Pribyl, P., & Gekelman, W. (2015). Excitation of chirping whistler waves in a laboratory plasma. Physical Review Letters, 114(24), 245002.

6. Miscellaneous topics

My focus on a deep integration of kinetic plasma simulations and modern spacecraft observations allows a natural expansion of the research activity to different topics, to mention a few: moon-plasma interactions, development of new computational methods, formation of foreshock transients in the day-side magnetosphere, and observational evidence for an intrinsic threshold of whistler anisotropy instability.

Selected publications:

• An, X., Chang, M., Cao, H., Angelopoulos, V., & Artemyev, A. (2026). Ion pickup and velocity space thermalization at outer planet moons. arXiv preprint arXiv:2604.10083.

• Liu, T. Z., Angelopoulos, V., An, X., & Madanian, H. (2024). ARTEMIS observations of lunar crustal field‐solar wind interaction and impact on reflected plasma under weak radial IMF. Journal of Geophysical Research: Space Physics, 129(12), e2024JA033217.

• An, X., Artemyev, A., Angelopoulos, V., Lu, S., Pritchett, P., & Decyk, V. (2022). Fast Inverse Transform Sampling of Non-Gaussian Distribution Functions in Space Plasmas. Journal of Geophysical Research: Space Physics, 127(5), e2021JA030031.

• Liu, T. Z., An, X., Zhang, H., & Turner, D. (2020). Magnetospheric Multiscale Observations of Foreshock Transients at Their Very Early Stage. The Astrophysical Journal, 902(1), 5.

• An, X., Liu, T. Z., Bortnik, J., Osmane, A., & Angelopoulos, V., (2020). Formation of foreshock transients and associated secondary shocks. The Astrophysical Journal, 901, 73.

• An, X., Yue, C., Bortnik, J., Decyk, V., Li, W., & Thorne, R. M. (2017). On the parameter dependence of the whistler anisotropy instability. Journal of Geophysical Research: Space Physics, 122(2), 2001-2009.

• Yue, C., An, X., Bortnik, J., Ma, Q., Li, W., Thorne, R. M., ... & Kletzing, C. A. (2016). The relationship between the macroscopic state of electrons and the properties of chorus waves observed by the Van Allen Probes. Geophysical Research Letters, 43(15), 7804-7812.