Considering a box filling with gas and moving at a constant speed initially. What will happen to the gas within the box if we stop the box by hand? A consequence is that the gas will be heated due to their collision with the box walls at the moment the box is stopped. The energy that is finally converted into the gas’s internal energy comes from the initial kinetic energy. This is a simplified (maybe oversimplified) example of converting macroscale, ordered energy into microscale, random energy. Similar cross-scale energy transfer processes occur throughout the universe, but usually are in a spontaneous manner rather than “by someone’s hand”. These processes contribute to the evolution of various systems, like Earth’s atmosphere and magnetosphere, the solar corona and solar wind, the interstellar medium, and the galaxy clusters. A full understanding of them is an indispensable step towards a general account of these systems.
Our interest here is how cross-scale energy transfer processes occur in space and astrophysical plasmas, which are usually collisionless due to their very low density. Currently, the most accepted model is the turbulent local cascade model, which assumes energy transfer generally proceeds via a cascade across similar spatial scales. Our question is whether there are other mechanisms.
To answer this question, we did a series of studies based on the observations obtained by NASA’s Magnetospheric Multiscale (MMS), and finally found that cross-scale wave-particle interactions, namely, simultaneous interactions between particles and plasma waves of different scales, can be a candidate for the mechanisms we search for (Fig. 1). Plasma waves are analogous to sound waves in the air. However, the difference of at least three orders of magnitude in the masses of the constituents of plasmas (e.g., protons and electrons) brings with them an enormous richness of spatial and temporal scales. Roughly, plasma waves can be divided into three classes according to their scales: fluid scale, ion-gyration scale and electron-gyration scale waves, with the first scale also termed macroscale and the last two termed microscale. Waves of different scales would interact with plasma particles in different ways. For example, magnetospheric ultra-low-frequency (ULF) waves, which are Alfven mode in nature and are generally of macroscales, can interact with particles through drift-bounce resonance. On the other hand, electromagnetic ion cyclotron (EMIC) waves, which are microscale waves as their frequency is close to ion gyro-frequency, generally modulate particle motion via cyclotron resonance. Wave-particle interactions can result in efficient energy exchange between waves and particles, no matter what the detailed forms are.
Our idea is rather simple. That is, when both macroscale and microscale waves are involved in wave-particle interactions, energy could first transfer from macroscale waves to particles, and then from particles to microscale waves (Fig. 1). As a result, cross-scale energy transfer occurs. The challenge is how to demonstrate this mechanism. Fortunately, the MMS spacecraft provided us with necessary observations to unambiguously confirm our scenario experimentally.
In the paper related to this blog post, we report two events, in which simultaneously interactions of ions, macroscale ULF waves and microscale EMIC waves are clearly observed (see Fig. 2 and 3 for observations in one event). The directly observed electric field and ion velocity distributions allow us to calculate the energy flow between ions and ULF waves, and the energy flow between ions and EMIC waves. The results show that, when energy flows from ULF waves to ions, ions’ velocity distributions become more anisotropic, which is a condition in favor of the excitation of EMIC waves. Indeed, EMIC waves with significant amplitude, together with energy flow directed from the anisotropic ions to EMIC waves, are observed at these times. Finally, the energy transferred to EMIC waves dissipates via further EMIC wave-induced ion energization (Fig. 3). It is interesting to note that in one of the two events (Fig. 2 and 3) the involved ions are helium ions, a minor species in terms of number density (2% of the number density of protons). Thus, both major and minor species can mediate cross-scale wave-particle interactions.
The main effect of cross-scale wave-particle interactions is the aforementioned energy flow from macroscales to microscales. However, two interesting “side” effects are also noted. First, in the interactions, waves of different scales are coupled with each other. In our events, this coupling process manifests as the strong modulation of EMIC waves' amplitude by ULF waves. This modulation could explain some space phenomena, like Pc 1 pulsations of pearl type and pulsating aurora. The second “side” effect is that the energy range of particles involved is expanded. For example, in the event involving helium ions, only ~100 eV helium ions can be energized by ULF waves, if there were no cross-scale wave-particle interactions. However, with cross-scale wave-particle interactions, helium ions can be accelerated to ~10000 eV.
Our study confirms that cross-scale wave-particle interactions can result in efficient energy transfer from macroscales to microscales. However, more detailed effects of this mechanism on space and astrophysical plasmas are currently not clear. More studies, both based on observations and simulations, are required to obtain a complete understanding. Nevertheless, we think cross-scale wave-particle interactions should be a very general process, since they can occur in any space and astrophysical systems as long as there is more than one type of plasma waves of different scales such that they can simultaneously interact with the same particle population.