First determination of whistler wave dispersion relation in superhot (Te > 5 keV) plasmas

A whistler wave is a fundamental electromagnetic radiation. In the universe, it transmits information through diverse mediums at audio frequencies. To know the properties of such a radio, one should determine how its frequency (energy) changes with its wavelength (momentum). The relation between them is called the dispersion relation, which encodes essential information on the kinetics. Although the dispersion relation governs the majority of the whistler properties, the experimental determination of their entire dispersion is still a challenge today. Especially in high-temperature mediums, the group velocity dispersion properties of whistlers are still unverified by spacecraft experiments despite their practical importance. Here, for the first time, we experimentally determine the dispersion relation of whistler waves in superhot (T-e > 5 keV) plasmas by the magnetospheric multiscale mission located in a magnetotail plasma sheet. Our result unmasks the characteristics of whistler waves at elevated temperatures and exhibits the evolution of their group velocity dispersion from positive to negative, which agrees well with kinetic theory, opening the door to controlled radio excitation and transport.