Full Wave Modeling of Small Scale Plasma Irregularities and Effects on Whistler Mode Chorus Propagation

Abstract:

Whistler mode chorus waves are known to play an important role in radiation belt dynamics. However, many aspects of chorus wave occurrence and propagation are still not fully understood and new observations often challenge long held assumptions. We focus on the propagation of chorus from an equatorial source region to midlatitudes and specifically on the impact of randomly distributed cold plasma field-aligned density irregularities. Whistler-mode wave propagation is modeled using full wave FDTD simulations and compared to multi-point spacecraft observations. The irregularities investigated are randomized positive and negative density perturbations between 1-10% of the nominal background density value with scales of ~50-100 km transverse and ~500-1500 km along the background magnetic field direction. We show that the density irregularities lead to spatial modulation and focusing of wave power accompanied by spreading of the wave normal angle distribution. Wave amplitudes can be locally increased by a factor of three inside density enhancements creating fine structure in the transverse mapping of the chorus element’s wave power distribution. The simulation results are in good agreement with the observed correlations of chorus power and variation of the plasma density from multi-point observations by the four MMS spacecraft and early observations on the CLUSTER spacecraft. The results challenge the often made assumptions of either a smooth plasmasphere or very large duct structures that extend along the entire field line. Cold plasma anisotropic inhomogeneities of sub-wavelength scales presumably can explain the observed characteristics of mid-latitude chorus waves, which are not always accurately modeled with raytracing.