X. N. Xing

Beihang University

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Co-Authors

C. M. Liu

Beihang University

Jinbin Cao

Beihang University

Y. Y. Liu

Ministry of Industry and Information Technology

B. N. Zhao

Ministry of Industry and Information Technology

C. T. Russell

University of California, Los Angeles

Z. Z. Chen

Sun Yat-sen University

C. J. Pollock

Denali Therapeutics (United States)

P. W. Daly

Max Planck Institute for Solar System Research

J. L. Burch

Southwest Research Institute

E. A. Kronberg

Ludwig-Maximilians-Universität München

Cooperative Institutions

Beihang University

Ministry of Industry and Information Technology

Swedish Institute of Space Physics

KTH Royal Institute of Technology

Planetary Science Institute

National Space Science Center

Chinese Academy of Sciences

University of California, Los Angeles

Institute of Geophysics

University College London

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Autogenous Electron Acceleration by Ion Flow Vortex in Space Plasmas

The Astrophysical Journal (2025)

C. M. Liu Jinbin Cao Y. Y. Liu E. A. Kronberg X. N. Xing

Abstract Plasma flow vortexes are ubiquitous in space and astrophysical settings and play a vital role in energy and mass transport, contributing to formation of stars and planets, launching and dissipation of astrophysical jets, and coupling between planetary magnetospheres and ionospheres. Here we present in situ observations showing that energetic electrons with energies up to 250 keV can be directly generated inside an ion flow vortex with finite spatial scale. The electron acceleration is achieved by establishment of Fermi acceleration trap and parallel electrostatic potential within the flow vortex, leading to dramatic enhancement of energetic electron flux by at least 1 order of magnitude. The accelerated electrons can drive mass and energy transport along local magnetic field lines and serve as a free energy source for broadband electrostatic emissions. Our observations, suggesting that the ion flow vortex can serve as novel electron accelerators, can be fundamentally important for understanding energetic phenomena in the cosmos, such as nonthermal emissions near astrophysical jets and killer electrons in geospace.

Astrophysical plasma

10.3847/1538-4357/ada269

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Field–particle energy transfer during chorus emissions in space

Nature (2025)

C. M. Liu B. N. Zhao Jinbin Cao C. J. Pollock C. T. Russell

Chorus waves are some of the strongest electromagnetic emissions naturally occurring in space and can cause radiation that is hazardous to humans and satellites1–3. Although chorus waves have attracted extreme interest and been intensively studied for decades4–7, their generation and evolution remain highly debated7. Here, in contrast to the conventional expectation that chorus waves are governed by planetary magnetic dipolar fields5,7, we report observations of repetitive, rising-tone chorus waves in the terrestrial neutral sheet, where the effects of the magnetic dipole are absent. Using high-cadence data from NASA's MMS mission, we present ultrafast measurements of the wave fields and three-dimensional electron distributions within the waves, which provides evidence for chorus–electron interactions and the development of electron holes in the wave phase space. We found that the waves are associated with resonant currents antiparallel to the wave magnetic field, as predicted by nonlinear wave theory. We estimated the nonlinear field–particle energy transfer inside the waves, finding that the waves extract energy from local thermal electrons, in line with the positive growth rate of the waves derived from an instability analysis. Our observations may help to resolve long-standing controversies regarding chorus emissions and in gaining an understanding of the energy transport observed in space and astrophysical environments. Whistler-mode chorus waves have been observed in the tail region of the terrestrial magnetosphere, where the magnetic field is not dipolar so that chorus waves were not expected, and their generation mechanisms have been tested with state-of-the-art observations.

Chorus

Particle (ecology)

10.1038/s41586-024-08402-z

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Citations (1)

Local Transition of Electron Pitch Angle Distribution within Flux Pileup Region behind Dipolarization Front

The Astrophysical Journal (2024)

X. N. Xing C. M. Liu Jinbin Cao Y. Y. Liu B. N. Zhao

Abstract Dipolarization fronts (DFs), earthward-propagating magnetic transients with a strong magnetic field, are important regions favorable for energetic electron acceleration in the magnetotail. The DF-driven electron acceleration usually generates coherent pitch angle distributions (PADs) inside flux pileup regions (FPRs), i.e., strong magnetic field regions behind the DFs, such as pancake, butterfly, and cigar distributions, which dominate at different tail regions and often occur separately. Here we present unique observations of electron PAD evolution inside the FPR, showing that electron PAD underwent local transition from cigar distribution, to butterfly distribution, then toward pancake distribution, forming a U-shaped distribution. During the local transition, electron perpendicular flux (relative to the local magnetic field) is anticorrelated with magnetic field strength, contrary to traditional expectation. The unexpected feature of the electron U-shaped distribution is associated with multiple physical processes at different scales, including local expansion of flux tubes and pitch angle variation near the neutral sheet. These atypical observations can advance our current understanding of electron acceleration and transport in the magnetosphere.

Pitch angle

10.3847/1538-4357/ad68fe

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Periodically Modulated Magnetic Reconnection

The Astrophysical Journal (2024)

C. M. Liu Jinbin Cao X. N. Xing Y. Y. Liu Boning Zhao

Abstract We present the first evidence for periodically modulated reconnection at the electron scale in space, using unparalleled, high-cadence data from Magnetospheric Multiscale spacecraft. The periodic modulation is attributed to finite magnetic trapping imposed by the X-line, which generates discrete, dispersive electron stripes. The dispersive stripes, well reproduced by a trapping-loss transition model, periodically break the frozen-in condition and drive energy dissipation. Such an electron transition effect eliminates free electrons, enhances electron mixing, and causes highly structured, three-dimensional distributions that generate intense radio emissions. These illuminating results, suggesting that reconnection hosts inherent periodicity determined by three-dimensional electron physics, provide crucial insights into understanding reconnection-driven energy transport in space and astrophysical plasmas.

10.3847/1538-4357/ad55c4

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Extended Energy Conversion and Electron Acceleration Behind Dipolarization Front

Journal of Geophysical Research Space Physics (2024)

X. N. Xing C. M. Liu Y. Y. Liu B. N. Zhao

Abstract Energy transfer and transport in the terrestrial magnetotails are primarily driven by dipolarization fronts (DFs) embedded inside plasma jets. The DF‐driven energy transfer has hitherto been believed to occur locally at the fronts. Different from the traditional knowledge, here we present the first observation of persistent energy conversion extended far behind a DF. The persistent energy conversion, which was dominated by energy loads and mainly contributed by electron currents, developed inside a turbulent, decaying flux pileup region (FPR), nearly 10 d DF (DF’s thickness) behind the DF. The energy transfer chain may be initiated by interaction between the ion flow and ambient plasmas and closed by electron dynamics, leading to electron acceleration perpendicular to magnetic field. These results highlight that electron physics in turbulent FPRs plays a crucial role in the energy transport in the planetary magnetospheres.

10.1029/2024ja032810

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Ultrafast measurement of field-particle energy transfer during chorus emissions in space

arXiv (Cornell University) (2024)

C. M. Liu B. N. Zhao Jinbin Cao C. J. Pollock C. T. Russell

Chorus is one of the strongest electromagnetic emissions naturally occurring in space, and can cause hazardous radiations to humans and satellites1-3. Although chorus has attracted extreme interest and been intensively studied for decades4-7, its generation and evolution remain highly debated, due to the complexity of the underlying physics and the limited capacity of previous spacecraft missions7. Chorus has also been believed to be governed by planetary magnetic dipolar fields5,7. Contrary to such conventional expectation, here we report unexpected observations of chorus in the terrestrial neutral sheet where magnetic dipolar effect is absent. Using unprecedentedly high-cadence data from the Magnetospheric Multiscale Mission, we present the first, ultrafast measurements of the wave dispersion relation and electron three-dimensional distributions within the waves, showing smoking-gun evidences for chorus-electron interactions and development of electron holes in the wave phase space. We estimate field-particle energy transfer inside the waves and find that the waves were extracting energy from local thermal electrons, in line with the wave positive growth rate derived from instability analysis. Our observations, opening new pathways for resolving long-standing controversies regarding the chorus emissions, are crucial for understanding nonlinear energy transport ubiquitously observed in space and astrophysical environments.

Chorus

Particle (ecology)

10.48550/arxiv.2408.13156

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Citations (1)

Observations of Electron Secondary Reconnection in Magnetic Reconnection Front

The Astrophysical Journal (2024)

C. M. Liu Jinbin Cao X. N. Xing Z. Z. Chen Hongtao Huang

Abstract Magnetic reconnection, a fundamental plasma process transforming magnetic field energy into particle energy, is ubiquitous in space and responsible for many explosive phenomena, such as solar flares and gamma-ray bursts. Recent numerical theories have predicted that reconnection fronts far from the primary reconnection region can host secondary reconnection in three-dimensional scenarios, different from the conventional two-dimensional diagram where only one X-line stands to sustain reconnection. In this study, we provide direct observational evidence for ongoing secondary reconnection in the reconnection front via the unprecedentedly high-cadence data from NASA’s MMS mission. The secondary reconnection is identified by the presence of an X-line, a super-Alfvénic electron jet, and nonideal energy dissipation. Different from the primary ion–electron reconnection, the secondary reconnection is electron-only, with its X-line quasi-perpendicular to the primary X-line. Hence reconnection, when evolving from local to global scales, becomes essentially three-dimensional with different patterns developed. These results provide crucial insights into understanding cross-scale energy transport driven by reconnection in space plasmas.

Solar flare

Current sheet

Nanoflares

Astrophysical plasma

10.3847/1538-4357/ad13f1

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Citations (5)

Autogenous Electron Acceleration by Ion Flow Vortex in Space Plasmas

Research Square (Research Square) (2024)

C. M. Liu Jinbin Cao Y. Y. Liu E. A. Kronberg X. N. Xing

Abstract Plasma flow vortexes are ubiquitous in space and astrophysical settings, and play a vital role in energy and mass transport, contributing to formation of stars and planets, launching and dissipation of astrophysical jets, and coupling between planetary magnetospheres and ionospheres. Here we present the first in situ observation showing that energetic electrons with energies up to 250 keV can be directly generated inside an ion flow vortex with finite spatial scale in space. The electron acceleration is achieved by establishment of Fermi acceleration trap and parallel electrostatic potential within the flow vortex, leading to dramatic enhancement of energetic electron flux by at least an order of magnitude. The accelerated electrons can drive mass and energy transport along local magnetic field lines, and serve as free energy source for broadband electrostatic emissions. The proposed acceleration mechanism driven by flow vortex can be fundamentally important for understanding energetic phenomena in the cosmos, such as nonthermal emissions near astrophysical jets and killer electrons in geospace.

10.21203/rs.3.rs-4657148/v1

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Citations (0)

Energy Conversion in the Dip Region Preceding Dipolarization Front

Geophysical Research Letters (2024)

B. N. Zhao C. M. Liu Jinbin Cao Y. Y. Liu X. N. Xing

Abstract Dipolarization fronts (DFs), characterized by sharp increases in the northward magnetic field and usually preceded by magnetic dips, are suggested to play an important role in energy conversion and transport in the magnetotail. It has been documented that strong energy conversion typically develops right at the fronts. Here we present spacecraft observations of electron‐scale energy conversion (EEC) developed inside the dip region ahead of a DF, by using high‐cadence data from the Magnetospheric Multiscale Mission. The EEC, with magnitude comparable to that at the front, is primarily driven by ion current and electron‐scale electric field. The electric field inside the dip is provided by electrostatic waves fed by lower hybrid drift instability, which experiences temporal decaying. Such decaying leads to nonhomogeneity of EEC along the dawn‐dusk direction. These results, uncovering a new channel for DF‐driven energy conversion, can provide important insights into understanding energy transport in the magnetotail.

Energy transformation

Polarity (international relations)

10.1029/2024gl109860

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Direct Observations of Reconnection Fronts in Earth's Turbulent Magnetosheath

The Astrophysical Journal (2023)

C. M. Liu X. N. Xing Jinbin Cao

Abstract Reconnection fronts (RFs), ion-scale magnetic transients characterized by dramatic enhancement of reconnected magnetic fields, have been documented as crucial energy transfer regions during magnetic reconnection. RFs have hitherto been observed only in the planetary (e.g., Earth, Saturn, Mars, and Venus) magnetotails. Whether RFs can exist in other magnetospheric regions remains unclear. Here, using high-cadence data from NASA’s Magnetospheric Multiscale mission, we present the first observation of successive RFs in Earth's turbulent magnetosheath. The RFs were detected inside an ion diffusion region and several di (ion inertial length) away from reconnection X-line. In addition, we find that the strongest energy conversion occurs at the RF rather than at the X-line. The present observation indicates that RFs may be universal in the planetary magnetosphere and play a crucial role in the reconnection dynamics.

Magnetosheath

Field line

10.3847/1538-4357/acf568

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Citations (2)