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

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 RE 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. Plain Language Summary Chorus wave is a type of whistler‐mode wave that is known for the characteristic of repetitive rising‐ and falling‐tones frequency chirping. These waves are usually formed in regions close to planets where the magnetic field is shaped like a dipole. In our study, however, we observed chorus waves in the middle magnetotail, by using data from MMS spacecraft. These waves exhibit a much higher chirping rate than those found close to Earth. Using computer simulations, we show that the magnetic configuration in the flux pileup region is responsible for generating these chorus waves with high chirping rates. This region resembles a magnetic bottle, similar in shape to the inner magnetosphere, but with greater inhomogeneity, leading to higher chorus rates. Our study provides new inspiration for the study of chorus waves. Key Points We present the first observation of chorus waves in the Earth's magnetotail These rising‐tone chorus waves have a chirping rate much higher than those in radiation belts We conducted a 1D PIC simulation model to reconstruct the generation of chorus waves