LASP Magnetosphere Seminars

Abstract

Turbulence is an important mechanism for energy conversion in weakly collisional plasmas. In fluid turbulence, energy injected into the system at large spatial scales is transferred to smaller scales until it is dissipated as heat through collisions. In the interim between energy injection and dissipation, kinetic effects can also mediate the dissipation of energy below characteristic fluid (ion) scales. There is a longstanding question concerning plasmas with low collisional rates: How does this energy conversion process end without collisions? This classical understanding of turbulence is, in nature, based on a velocity-averaged theory of Vlasov-Boltzmann systems. The energy in consideration is that of the plasma bulk motion; the transfer process is one through space. Since a Vlasov-Boltzmann description concerns both space and velocity, we can ask the question: Is there a conjugate spatial-averaged theory of turbulence, one that describes a turbulent energy transfer instead through velocity?

In this talk, we explore these two questions regarding turbulence in space and in velocity with MMS observations in two regions in the Earth’s magnetosphere. In the magnetotail, explosive large-scale magnetic reconnection generates strong turbulence with low density and background field. The electromagnetic field spectra are well resolved below electron scales, suitable for turbulence studies of kinetic effects. We show evidence of a sub-electron kinetic range where the energy transfer process appears to complete. In the magnetosphere, the distribution function is well resolved with MMS instruments, suitable for turbulence studies of fine structures in velocity space near fluid scales. We show the first statistically established observational evidence of velocity-space cascade using data from the MMS unbiased magnetosheath campaign.