Abstract
Abstract. We report the results of MHD simulations of Earth's magnetosphere for idealized steady solar wind plasma and interplanetary magnetic field (IMF) conditions. The simulations feature purely northward and southward magnetic fields and were designed to study turbulence in the magnetotail plasma sheet. We found that the power spectral densities (PSDs) for both northward and southward IMF had the characteristics of turbulent flow. In both cases, the PSDs showed the three scale ranges expected from theory: the energy-containing scale, the inertial range, and the dissipative range. The results were generally consistent with in-situ observations and theoretical predictions. While the two cases studied, northward and southward IMF, had some similar characteristics, there were significant differences as well. For southward IMF, localized reconnection was the main energy source for the turbulence. For northward IMF, remnant reconnection contributed to driving the turbulence. Boundary waves may also have contributed. In both cases, the PSD slopes had spatial distributions in the dissipative range that reflected the pattern of resistive dissipation. For southward IMF there was a trend toward steeper slopes in the dissipative range with distance down the tail. For northward IMF there was a marked dusk-dawn asymmetry with steeper slopes on the dusk side of the tail. The inertial scale PSDs had a dusk-dawn symmetry during the northward IMF interval with steeper slopes on the dawn side. This asymmetry was not found in the distribution of inertial range slopes for southward IMF. The inertial range PSD slopes were clustered around values close to the theoretical expectation for both northward and southward IMF. In the dissipative range, however, the slopes were broadly distributed and the median values were significantly different, consistent with a different distribution of resistivity.
Highlights
For northward interplanetary magnetic field (IMF) we found that the power spectral densities (PSDs) have slopes that were almost all between −2.4 to −1.3 in the inertial range with a median value of −1.90 and a standard deviation of 0.30
We calculated PSDs in the central plasma sheet and found that the results were consistent with observations, and that they had interesting physical implications
The MHD simulation exhibited nested vortices on multiple scales, with the largest scales associated with reconnection outflows and the diversion of high speed flows in the near-Earth region for southward IMF (El-Alaoui et al, 2010)
Summary
Several studies have found evidence for turbulence in the plasma sheet (Consolini et al, 1996, 1998; Borovsky et al, 1997; Borovsky and Funsten, 2003; Lui, 2001, 2002; Weygand et al, 2005, 2006, 2007). Borovsky et al (1997) and Borovsky and Funsten (2003) examined the temporal fluctuations of the plasma sheet magnetic field and flow velocity. Borovsky (2004) suggested that large-scale shear flows at the magnetopause are a source for turbulence Both local (Nykyri and Otto, 2001; Nykyri et al, 2006b) and global MHD simulation studies have shown vortices forming at the magnetopause (e.g., Walker et al, 1998, 2006; AshourAbdalla et al, 1999, 2002; White et al, 2001; Slinker et al, 2003; Hasegawa et al, 2004; Fairfield et al, 2000; Collado-Vega et al, 2007; Claudepierre et al, 2008; Hwang et al, 2011). The study found that a vigorous spectrum of fluctuations and eddies occurred under steady southward IMF driving conditions They computed PSDs and PDFs and found results consistent with insitu observations and with theory.
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