Abstract
The possible influence of MHD turbulence on the energy distributions of ions in the Earth's plasma sheet was studied using data taken by the THEMIS satellites. Turbulence levels were traced using eddy diffusion coefficients (D), of which we measured one for each Geocentric Solar Magnetospheric (GSM) coordinates every 12 min. Ion fluxes between 1.75 and 210.5 keV during the same time windows that correspond to mainly suprathermal populations were fitted to Kappa distribution functions, which approximate a Maxwellian distribution when theκ-index (κ) is large. We found that the distribution of the eddy diffusion coefficients is bimodal, independently of both the eddy diffusion component and the plasma beta (β) parameter, which is defined as the ratio between plasma and magnetic pressures. The main peak corresponds to turbulent plasma flows withD> 103km2s−1. In such cases, the impact of turbulence on theκindex depends on the value ofβand also on the direction of the turbulent transport. For eddy diffusion perpendicular to the neutral sheet, the values ofκdecrease asDzzincreases forβ< 2; while for higher values ofβ,κincreases withDzz. For the other two directions, the values ofκdecrease asDincreases. This last tendency is stronger forβ~ 1 but almost null forβ~ 10. The secondary peak in the distribution ofDvalues might represent quasi-laminar flows forming part of very large vortices, correct detection and description of which is beyond the scope of this study.
Highlights
Since the beginnings of the Space Age, it is known that various regions of the Earth’s magnetosphere are filled with turbulent plasmas
The secondary maximum might correspond to nearly laminar flows (D ∼ 102 km2/s), which could be part of vortices larger than the maximum vortex size that we are able to detect
The rms velocity would be low, and the autocorrelation time would exceed 12 min, could not be measured correctly. This method may be insensitive to very large vortices, especially considering that the satellite velocity is often of the same order of magnitude as the plasma velocity averaged over turbulent eddies
Summary
Since the beginnings of the Space Age, it is known that various regions of the Earth’s magnetosphere are filled with turbulent plasmas (see, for example, Sonett, 1960; Ness et al, 1961). Successful direct observations of eddy vortices in the plasma sheet are only possible if measurements are carried out fairly simultaneously by two or more satellites separated by long distances, such as THEMIS satellite mission (Time History of Events and Macroscale Interactions during Substorms; Angelopoulos, 2008). Plasma vortices of such scales were observed by Keika et al (2009), Keiling et al (2009), Panov et al (2010) using THEMIS data. The eddy diffusion coefficients are a useful way to quantify MHD turbulence. Borovsky et al (1998) proposed a method for calculating this coefficient using data from a single satellite as a function of the root mean square velocity and autocorrelation time
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