Evidence of Small Scale Plasma Irregularity Effects on Whistler Mode Chorus Propagation

The impact of randomly distributed field‐aligned density irregularities on whistler‐mode wave propagation is investigated using full‐wave simulations and multipoint spacecraft observations. The irregularities are modeled as randomized density perturbations between 1% and 10% of the nominal background density value with scales of ∼10–60 km transverse and ∼50–500 km along the background magnetic field. The density irregularities affect whistler wave propagation and lead to spatial modulation of wave average power density accompanied by spreading of the wave normal angle distribution. Wave power variation is shown to statistically increase with the depth of density irregularities. The simulation results are in good agreement with the observed correlations of chorus power and variation of the plasma density from multipoint observations by the four Magnetosphere MultiScale spacecraft. The change in fundamental wave properties from scattering from these irregularities affects the efficiency of wave‐particle interactions in the radiation belts and needs to be incorporated into large‐scale energetic‐particle flux models. Plain Language Summary Electromagnetic waves in the near‐Earth space environment are a major contributor to space weather processes that can affect a large array of technological platforms in space and on the ground. A key class of waves in near‐Earth space are so called whistler mode waves and it is important to accurately model and predict how these waves propagate. Whistler mode wave propagation is affected by the background plasma and in this work we simulate propagation of these waves in the presence of small scale (smaller than a wavelength) irregularities of the plasma medium. Past simulations of these waves have focused on a smooth background or very large plasma density structures. We simulate small structures and compare our results to observations made with multiple spacecraft that fly in close formation. Agreement between the simulations and observations suggests that the plasma density in near‐Earth space may be filled with many small irregularities that need to be taken into account. Key Points Full wave (finite difference time domain (FDTD)) model used to quantify the effect of sub wavelength sized irregularities on chorus wave parameters Comparison of model results to Magnetosphere MultiScale (MMS) spacecraft observations suggests that irregularities of 6%–10% cause wave amplitude fluctuations of ∼50% Wavelength and subwavelength i