Coherent structures in magnetized turbulent plasma play an important role in mass transfer, energy dissipation and particle heating.The electron-scale coherent structure is a very small intermittent structure in which turbulent energy cascaded to the electron scale is dissipated to provide energy for the electron.A long unsolved problem in astrophysics and space physics is how energy is dissipated at the electronic scale.
To answer these questions, a team led by Professor Qiugang Zong of Peking University's School of Earth and Space Sciences recently published an article in the journal Nature Communications entitled "Electron scale coherent structure as micro accelerator in the Earth's magnetosheath."This newly discovered mechanism could help explain the energy dissipation at the electronic scale in space and celestial systems, as well as the acceleration of plasma heating.
Plasma turbulence is one of the basic physical phenomena that humans have not fully understood, and may exist throughout the universe, such as the early universe, crab-like pulsars, interstellar media, and planetary magnetosphere.In plasma turbulence, energy transfer from large to small scales and energy conversion processes between fields and particles are very complex.Plasma turbulence is thought to play a key role in the energyization of particles such as corona heating and cosmic radiation acceleration.A major challenge in turbulence research is multiscale coupling.Although magnetohydrodynamics (MHD) theory is a good description of large-scale physical processes, it is still unclear how energy is coupled at the plasma kinetic scale and at the electronic scale.
Various coherent structures, such as eddy currents and current plates, can be formed in turbulent flow with uneven energy transfer.Great efforts have been made to find coherent structures in space plasma, laboratory plasma and numerical plasma.These structures are thought to be directly related to turbulent energy cascades and dissipation mechanisms, and scales may vary from large to dynamic scales.
The recently launched Magnetospheric Multiscale (MMS) mission has propelled space exploration to the electrodynamic scale.Turbulence in magnetic sheaths measured by MMS can be quantitatively broken down into wave modes ranging from ionic to subelectronic scales: kinetic Alfon waves, whistle waves and ionic sound waves.Recent developments suggest that dissipation may also occur through wave-particle interactions.A series of electron-scale coherent structures, such as electron-scale magnetic holes and electron-scale current slices, have been identified and reported in space plasma environment.It is currently thought that turbulent energy may eventually dissipate at the electron scale, so the study of these structures is of great significance.However, in magnetized plasma, the minimum scale to which turbulent energy can be cascaded remains a fundamental problem.
In the study, Zong's team reported a novel electron-scale coherent structure by analyzing the data obtained from NASA's MMS in detail. As shown in Figures 1 and 2, the observations show distorted magnetic field lines and captured electrons.The scale size of the structure can be estimated by using the "electron cyclotron telemetry" method.The method uses "electron cyclotron anisotropy" as a measure of the boundary of space plasma structure, and takes the electron cyclotron radius as a "ruler" for measuring structure.The results show that the average distance from the boundary to the spacecraft is about 2.2 electron cyclotrons.The team developed an electron capture model that takes into account field-to-field potential drops and magnetic mirror forces, and found that electron capture and acceleration can be well attributed to potential changes along the field direction.In this model, electrons are captured in a bipolar parallel electric field at the center of the structure and accelerate in a magnetic field maximal region.At the end of the structure, a bidirectional electron jet is formed due to the combination of outward parallel electric field forces and outward magnetic mirror forces (Figure 3).