On the Transmission of Turbulence Across Interplanetary Shocks: Observations and Theory

Collisionless magnetohydrodynamic shocks are ubiquitous in solar wind and in other space plasmas. They serve as natural sites where charged particles can be accelerated to supra-thermal energies via various Fermi acceleration mechanisms. Upstream and downstream fluctuations play a key role in these processes because they can act as scatter centers. Furthermore, the downstream turbulent plasma of strong shocks driven by coronal mass ejections can enhance the coupling of this plasma with the Earth’s magnetosphere. Understanding of how the fluctuations are transmitted across the shocks can provide an invaluable insight into many shock related studies.

In this paper, we investigate the interaction of fast forward (FF) shocks with magnetic island/flux rope mode fluctuations. We employ a recently developed framework of the Zank et al. (2021) transmission model. We analyze 378 FF shocks observed by the Wind spacecraft with varying upstream conditions and Mach numbers. We estimate upstream and downstream power spectra within one-hour intervals adjacent to the shock front and we calculate theoretically predicted downstream power spectrum. We analyze closely the difference between the observed and theoretically predicted spectra. On average, the model predicts the spectra with very good accuracy. We argue that large statistical spread of this difference is given mainly by the statistical uncertainties in the shock compression ratio, upstream power spectrum and by the turbulent evolution of fluctuations in the downstream region. Finally, our findings also suggest that Zank et al. (2021) model may estimate the downstream levels of fluctuations accurately even for a wider range of shocks than it was originally meant for.