Professor Zong Qiugang's team discovered a new mechanism for cross-scale energy transfer in space

There are many different scales of physical behavior in space, ranging from microscopic scales characterized by electron cyclic motion (<0.
1 km) and ionic cyclotron motion (~1 km) to macroscopic scales comparable to planets (>¹⁰⁷ km), spanning more than 8 orders of magnitude
.
How are these physical processes at different scales coupled? How is energy transported between them?

In order to answer these questions, the research team led by Professor Qiugang Zong of the School of Earth and Space Sciences of Peking University recently published an article entitled "Simultaneous Macroscale and Microscale Wave-ion Interaction in Near Earth Space Plasmas", in the international academic journal Nature Communication (IF: 17.
69).
It is proposed that in space and celestial plasmas, cross-scale fluctuation-particle interactions can lead to rapid transport
of energy from the macroscopic to microscopic scales.
This newly discovered mechanism helps explain the problem of energy dissipation in space and celestial systems and the heating acceleration of plasma
.

Plasma fills the entire universe and makes up a wide variety of celestial and space systems, such as the planetary magnetosphere, the solar corona, the solar wind and heliospheric layers, and the interstellar medium
.
An important question for understanding the history and evolution of these plasma systems is how the energy of macroscopic and directed motion is converted into microscopic and randomly moving energies
.
Because the density of space and celestial plasma is generally very low, thermal collisions that normally play a dominant role are difficult to occur
.
For example, in the solar system, a charged particle that moves from the sun to the Earth will only experience about one collision
.
Because, in various space and celestial systems, the energy conversion from macroscopic to microscopic is generally done
through electromagnetic interactions.
As a long-range interaction, electromagnetic interactions can function over long distances, and therefore can link
sparsely distributed charged particles.

For a specific spatial and celestial plasma system, electromagnetic interactions take many different forms, allowing for the cross-scale transport
of energy in many different ways.
At present, the mainstream cross-scale energy transfer mechanism is the turbulent cascade model, which believes that energy is gradually transported from the macroscopic scale to the microscopic scale through a series of similar scales
.
In order to better understand the energy processes in various space and celestial systems, the search for cross-scale energy transfer mechanisms outside the turbulent flow series is one of the current research hotspots in the field of
space physics and astrophysics.

Recently, Zong Qiugang's team confirmed through the analysis of observational data that cross-scale fluctuation-particle interaction, that is, the interaction of charged particles with plasma fluctuations at different scales at the same time is a possible cross-scale energy transport mechanism
.
Similar to the atmosphere filled with sound waves, space and celestial plasmas are also filled with various plasma fluctuations
.
But because the masses of the components that make up a plasma (usually protons and electrons) vary widely, plasma fluctuations have a variety of different temporal and spatial scales
.
Most coarsely, plasma fluctuations can be divided into three broad categories according to scale: fluid-scale fluctuations, ionic scale fluctuations, and electron-scale fluctuations
.
The former is also known as macroscopic scale fluctuations, and the latter two are collectively referred to as microscopic scale fluctuations
.
Plasma fluctuations at different scales interact
with charged particles in different ways.
For example, the ultra-low frequency fluctuation in the Earth's magnetosphere is a typical macro-scale fluctuation (~¹⁰⁵ kilometers) that can accelerate charged particles through drift-bounce resonance, resulting in killer electrons that can endanger the safety of spacecraft and astronauts
.
The electromagnetic ion gyroscope wave is a typical microscopic scale fluctuation (~¹⁰³ km), which often interacts with charged particles through cyclotron resonance, and one of the results of this action is to cause charged particles in space to settle into the Earth's atmosphere, and these settled particles can also produce auroras and other phenomena
through subsequent processes.
But regardless of the specific mode of action, wave-particle interactions can lead to an exchange of energy between electromagnetic fields and charged particles
.

In the study, Zong's team analyzed the data obtained by NASA's Magnetospheric Multiscale mission in detail and found that ions in space can simultaneously interact with ultra-low frequency waves at the macroscopic scale and electromagnetic ion cyclones at the microscopic scale (as shown in Figures 1 and 2).

Through this interaction, energy is first transferred from the ultra-low frequency wave to the ion, then from the ion to the electromagnetic ion echo, and finally dissipated by the electromagnetic ion echo-ion gyronomic resonance (see Figure 3
).
Unlike traditional turbulence cascade models, in this cross-scale fluctuation-particle interaction, energy can be transferred directly from the macroscopic to the microscopic scale without intermediate-scale mediation
.
Quantitative analysis of observational data shows that the time scale of cross-scale fluctuation-particle interaction is about 1 minute, which is much smaller than the time scale of various spatial and celestial energy processes, which proves that it is an effective mechanism for transporting energy across scales (see Figure 1).

Figure 1: Cross-scale fluctuation-particle interactions result in the transfer of energy directly from the macroscopic to the microscopic scale

Figure 2: Cross-scale interaction
between ions and ultra-low frequency waves (ULF waves) at the macroscopic scale and electromagnetic ion cyclones at the microscopic scale (EMIC wave).
This interaction results in the transport of energy directly from the macroscopic scale to the microscopic scale
.
This image shows the observation of the Earth's magnetosphere from the Magnetospheric Multiscale satellite

Figure 3: Ion phase capture due to electromagnetic ion cyclic (EMIC)-ion resonance (top) and energy change (bottom)

In addition to mediating the cross-scale transport of energy, the study also found that cross-scale fluctuation-particle interactions can lead to the coupling of dynamic processes at different scales, as well as the heating and acceleration
of spatial plasma.
These findings provide new ideas
for further understanding auroras, geomagnetic pulsations, and the creation of high-energy particles in space.

Liu Zhiyang, a doctoral candidate at the Institute of Space Physics and Applied Technology at the School of Earth and Space Sciences of Peking University, is the first author of this paper, and Zong Qiugang is the corresponding author
of the paper.
This work has been supported
by the National Key Research and Development Program "Key Scientific Issues of Transformative Technology", the Civil Aerospace Technology Advance Research Project, and the National Natural Science Foundation of China.

References:

Liu, ZY.
, Zong, QG.
, Rankin, R.
et al.
Simultaneous macroscale and microscale wave–ion interaction in near-earth space plasmas.
Nat Commun 13, 5593 (2022).
https://doi.
org/10.
1038/s41467-022-33298-6