Interball-tail observations of vertical plasma motions in the magnetotail

International audienceThe Interball spacecraft configuration favors, in contrast to previous experiments, investigation of vertical ion flows (GSM Vz ). We use measurements of the CORALL instrument for the statistical study of Vz and Vy plasma flows in the mid-tail plasma sheet. In agreement with the previous observations, the mean Vy was positive on the dusk side and negative on the dawn side. When IMF was southward, the mean Vz consisted of the convection flow towards the equatorial plane ~ 7 km/s and the northward flow ~ 8 km/s. When IMF was northward, both components nearly vanished. The velocity variance was much larger than the mean values. The Vz variance maximized on the dawn flank and was always 15?20% smaller than the Vy one. The Vy variance maximized in the pre-midnight sector closer to the neutral sheet. We conclude that velocity fluctuations are composed with the inherent high-beta plasma turbulence contributing to all components, and the BBF-related activity contributing mainly to Vy in the pre-midnight plasma sheet

Variability of magnetic field spectra in the Earth's magnetotail

We investigate the variability of magnetic fluctuation spectra below 1 Hz in the Earth's plasma sheet using specially selected long observation intervals by Geotail spacecraft. The spectra can be generally described by a negative power law with two kinks. The range between kinks ~0.02–0.2 Hz has the most stable power law index ~2.4–2.6. Indices at the lower and the higher frequencies are more variable and generally increase with power of fluctuations. In the sub-second range fluctuations are strongly localized and indices are closer to 3. At the lower-frequency end indices are about 1.5. The lower kink is usually well defined on average spectra and its frequency tends to increase with activity. Combination of spectrum index α and fractal dimension δ is expected to follow the Berry relation α+2δ=5, but actually is ~5.5

Weibel-dominated quasi-perpendicular shock: hybrid simulations and in situ observations

We directly compare hybrid kinetic simulations and in situ observations of a high Mach number high-$\beta$ shock in the Solar wind. We launch virtual probes to demonstrate that the model quantitatively reproduces the observations. The observed wave properties are caused by the ion Weibel instability in the shock foot. Parameters of reflected ions in the shock foot are extracted from simulations, and their coordinate dependencies are linearly approximated. These approximations could be used in analytical models. Due to strong magnetic variations at ramp the reflected ions density can be locally very high (nearly that of the incoming flow), which makes favourable conditions for the instability.Comment: 11 pages, 10 figure