Unraveling the Formation Region and Frequency of Chorus Spectral Gaps

The present study addresses two basic questions related to banded chorus waves in the Earth's magnetosphere: 1) are chorus spectral gaps formed near the equatorial source region or during propagation away from the equator? and 2) why are chorus spectral gaps usually located below 0.5 f(ce) (f(ce): electron gyro-frequency)? By analyzing Van Allen Probes data, we demonstrate that chorus spectral gaps are observed in the source region where chorus waves propagate both in the parallel and anti-parallel directions to the magnetic field. Chorus spectral gaps below 0.5 f(ce) are associated with electron parallel acceleration at energies above the equatorial Landau resonant energies. We explain that initially generated chorus waves quickly isotropize the electron distribution through Landau resonant acceleration, and the isotropization occurs for higher energies at higher latitudes. The isotropized population, after returning to the magnetic equator, leads to a chorus gap typically below 0.5 f(ce) by suppressing wave excitation.