Coupling Between Magnetotail Turbulence and the Ionosphere: Case Study and Numerical Experiment

The spectra of fluctuations in velocity and magnetic field observed by spacecraft in the magnetotail have the characteristics of fluid turbulence. The power spectral densities (PSDs) and probability distribution functions (PDFs) observed in the magnetotail are very similar to those found in the inertial range in both idealized MHD simulations and event case studies. The simulations also do a reasonably good job of representing the effect of dissipation at higher frequencies and yield dissipative scale lengths comparable to estimates from observations. In the case studies, the simulation is driven by time series of solar wind conditions measured by the WIND spacecraft. The results are then evaluated by comparisons with data streams from THEMIS spacecraft in the magnetotail. We found that the power levels in the simulations in the driving and inertial ranges are very similar. The PSDs in the inertial range for the idealized and case study simulations as well as the observations generally have typical spectral indices between -2 and -1.5. The fluctuation level in the MHD simulation, however, decreased more rapidly in the dissipative range than in the observations. In the MHD simulation, it is possible to conduct numerical experiments by varying ionospheric conductivity parameters. The results showed that the ionosphere can exert strong control over magnetotail turbulence. The typical ionospheric parameters, which are parameterized based on observations, yield the most realistic PSD results, and lower ionospheric conductivity resulted in greatly increased resistive dissipation in the magnetotail.