Observation and Simulation of Chorus Waves in Middle Magnetotail (r > 20 RE)

Abstract

Whistler-mode chorus waves, characterized by repetitive frequency-chirping elements, have traditionally been considered to be confined to near-planet dipolar magnetic fields. In contrast to this long-held belief, here we present the first observation of whistler-mode chorus waves in the middle magnetotail beyond 20 R_E from Earth, based on MMS measurements. These rising-tone chorus waves exhibit significantly higher chirping rates compared to those in radiation belts. Using the PIC simulation, we demonstrate these chorus waves can be locally generated, driven by perpendicular flux anisotropy and magnetic field inhomogeneity inside the pileup region. This region, resembling a magnetic bottle, shares shape similarities with the inner magnetosphere but provides a much greater inhomogeneity, resulting in higher chirping rates. These discoveries provide new inspirations for the generation of chorus waves and may lead to new insights into their behavior in space.