In a new study published in Physical Verification Letters, researchers from Japan show that high-frequency plasma waves in geospace can generate low-frequency plasma waves through wave-particle interactions by heating low-energy ions, revealing a new energy transfer path in collision-free plasma.
A prominent signature of plasma – a state of matter characterized by freely roaming charged particles interacting via electromagnetic forces – is the generation of “plasma waves” resulting from an instability in the plasma distribution. Fast Magnetosonic Waves (MSWs) are a type of electromagnetic plasma waves found in geospace. MSWs result from hot protons and are considered “radio frequency waves”.
Another type of wave commonly generated in geospace is the “electromagnetic ion cyclotron” (EMIC) wave, which is considered a “low frequency” wave. Recently, satellite observations in geospace have shown that MSWs and EMIC waves often occur together. However, the mechanism underlying this co-occurrence remained unclear.
Now, a team of researchers led by Professor Yoshizumi Miyoshi from the University of Nagoya, Japan, has unraveled this mechanism somewhat. “MSWs are known to occur together with the heating of low-energy protons, and studies have shown that MSWs can heat these ‘cold’ ions. Given the recent observational data from the Arase satellite showing simultaneous EMIC waves, we wondered whether the occurrence of EMIC waves is indeed coupled to the MSW-mediated ion heating process,” Prof. Miyoshi explains the motivation behind the study.
Accordingly, the team applied a wave-particle interaction analysis method to the MSWs and EMIC waves – which they observed using the Arase satellite – to study the ‘cross-energy coupling’ between them through ion heating. The results were instructive: they found that while MSWs transferred energy to the ‘cold’ protons to heat them up, some of the transferred energy from the protons went into excitation of the EMIC waves. Considering that MSWs were in turn excited by hot protons, they actually acted as a mediator for energy transfer to the EMIC waves, i.e. a cross-energy coupling between hot protons, cold protons, MSWs and EMIC waves.
While these results are exciting in their own right, Prof. Miyoshi explains how important they are to our knowledge of geospace as well: “EMIC waves cause significant scattering and loss of ‘killer electrons’ in the Van Allen radiation belts, which often lead to satellite malfunctions. The new energy transmission route for excitatory EMIC waves uncovered in our study could contribute to improved space weather forecasting and make satellite operations in the Van Allen radiation belts safer.”
Materials provided by Nagoya University. Note: Content can be edited for style and length.