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In this letter, we have investigated the correlation between lower band chorus waves and electron beam-like distribution with long-term THEMIS data. Cold, hot, and beam electrons are assumed in the plasma. For quasi-parallel chorus waves, they are usually observed along with a relatively weak electron beam. Moreover, the density ratio between beam and hot electrons n(b)/n(h) is positively correlated with the wave parallel electric field and inversely correlated with the hot electron density, in agreement with a formation of the beam by Landau acceleration during wave propagation, while, along with oblique waves, there typically exists a strong electron beam. Notably, the n(b)/n(h) is found to be independent of both the parallel electric field and hot electron density, indicating that the electron beam-like distribution should be a precondition for exciting oblique waves rather than caused by waves. Our study provides some new insights into understanding the generation and evolution of chorus waves.

Plain Language Summary The interaction between whistler-mode chorus waves and electrons has long been a hot topic, which is important for understanding the evolution and generation of chorus waves. Both theoretical and simulation studies have indicated that quasi-parallel chorus waves are initially excited near the magnetic equator and then propagate toward high-latitude regions with increasing wave normal angles and enhanced parallel electric fields. During the propagation, the Landau resonance between chorus waves and electrons begins to take effect, causing the damping of waves and the formation of local electron beam. Within the equatorial region, there also exist a number of oblique chorus waves with large wave normal angles, which should be also locally excited. To generate oblique waves, a beam-like electron distribution is needed to suppress the Landau damping of oblique chorus waves, which should already exist before those waves. Although the two physical pictures are well known in this community, actually, there is still a lack of observational support. In this paper, we have made some new progress in understanding these two different scenarios by studying the correlation between chorus waves and electron beam-like distribution with long-term THEMIS data.

Key Points

We have investigated the correlation between lower-band chorus waves and electron beam-like distribution using THEMIS data. Along with quasi-parallel chorus waves, a weak elelctron beam can be observed, which is caused by Landau resonance with waves. But for oblique chorus waves, the electron beam-like distribution should be a precondition for exciting waves rather than caused by waves.