On Whistler Mode Wave Relation to Electron Field-Aligned Plateau Populations

Whistler mode waves are among the most intense electromagnetic emissions and play an important role in the energy redistribution between electron populations in the Earth inner magnetosphere through wave-particle resonant interactions. Usually generated by transversely anisotropic plasma sheet electron populations (similar to 10-30 keV) through cyclotron resonance, whistler mode waves can effectively accelerate a small fraction of the seed population of energetic electrons (similar to 100 keV) up to relativistic energies. However, these waves can be efficiently damped through simultaneous interactions with much more numerous suprathermal electrons (similar to 0.1-1 keV) via Landau resonance. Recent observations indeed show that electron distributions accompanied by intense whistler mode emissions often contain a plateau-like electron population at energies close to the energy of Landau resonance with the waves. However, simultaneous observations of these waves and of the related plateau population does not prove a causal relationship. Here, we test the hypothesis that such a plateau population may have been formed by whistler mode waves generated earlier, or by other types of waves. Combining analytical estimates and spacecraft observations, we show that this plateau population is often unlikely to be formed by whistler mode waves alone. We suggest three alternative scenarios that can lead to the formation of plateau populations and test these scenarios based on spacecraft observations. We show that a plateau population can be formed by ultralow frequency electric fields (carried by kinetic Alfven waves or time domain structures) often accompanying injections of plasma sheet electrons-the energy source for whistler mode waves. We also discuss the possible role of ionospheric secondary electrons.