Chorus Wave Within Wavelength‐Scale Density Irregularities

Abstract We present observations from the Van Allen Probes of lower band chorus waves interacting with wavelength‐scale density irregularities and gradients on the order of a few kilometers—comparable to the wavelength of the chorus waves themselves. High‐resolution electron density is derived from the upper hybrid resonance line in the High‐Frequency Receiver (HFR) merged spectrum, with a time resolution of 0.5 s. These observations show that density fluctuations modulate both the amplitude and wave normal angles of lower band chorus. High‐amplitude, quasi‐parallel waves are associated with regions of enhanced density, whereas very oblique waves with lower amplitude are found in regions of density depletion. The very oblique chorus waves are not generated locally by anisotropic electrons or shaped solely by propagation effects. One‐dimensional wave field calculations in a multilayered plasma demonstrate that wavelength‐scale density irregularities can scatter incident quasi‐parallel waves and produce very oblique waves at density depletions.