Unexpected ubiquity of chorus emissions in terrestrial magnetotail

Chorus waves are one of the strongest electromagnetic emissions widely occurring in planetary space, and have been documented to play a key role in mediating extreme space climate and driving spectacular auroras. Chorus waves have hitherto been believed to develop typically inside planetary inner magnetospheres, where magnetic dipolar fields dominate and govern their generation and propagation. Here, in contrast to such conventional knowledge, we show that chorus waves are surprisingly ubiquitous in terrestrial neutral sheet where magnetic fields are not dipolar, by surveying years of data from NASA’s Magnetospheric Multiscale spacecraft. We discover that chorus waves in the neutral sheet are mostly falling-tone, unlike inner magnetosphere chorus waves which are predominantly rising-tone, suggesting that global magnetic field topology can mediate the wave pattern. In addition, we find that the falling-tone chorus waves mostly propagate along magnetic field lines, different from obliquely-propagated falling-tone chorus in the inner magnetosphere. We further reveal that local magnetic field inhomogeneity associated with the chorus waves is basically negligible, posing observational constraints for theoretical modeling of chorus waves. These results uncover an unexpected source region for inner-magnetosphere chorus, and provide novel insights into understanding wave-driven space weather and climate.