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THEMIS observation of intermittent turbulence behind the quasi-parallel and quasi-perpendicular shocks

Turbulence is complex behavior that is ubiquitous in nature, but its mechanism is still not sufficiently clear. Therefore, the main aim of this paper is analysis of intermittent turbulence in magnetospheric and solar wind plasmas using a statistical approach based on experimental data acquired from space missions. The quintet spacecraft of Time History of Events and Macroscale Interactions during Substorms (THEMIS) allows us to investigate the details of turbulent plasma parameters behind the collisionless shocks. We investigate both the solar wind and magnetospheric data by using statistical probability distribution functions of Elsässer variables that can reveal the intermittent character of turbulence in space plasma. Our results suggest that turbulence behind the quasi-perpendicular shock is more intermittent with larger kurtosis than that behind the quasi-parallel shocks, which are immersed in a relatively quiet solar wind plasma, as confirmed by Wind measurements. It seems that behind the quasi-perpendicular shock the waves propagating outward from the Sun are larger than possibly damped waves propagating inward. In particular, we hope that this difference in characteristic behavior of the fluctuating space plasma parameters behind both types of shocks can help identify complex plasma structures in the future space missions. We also expect that the results obtained in this paper will be important for general models of turbulence.

Macek, W.; Wawrzaszek, A.; Sibeck, D.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 09/2015

YEAR: 2015     DOI: 10.1002/2015JA021656

intermittency; magnetosheath; shocks; Solar wind; Space plasma; turbulence

Broadband low frequency electromagnetic waves in the inner magnetosphere

A prominent yet largely unrecognized feature of the inner magnetosphere associated with particle injections, and more generally geomagnetic storms, is the occurrence of broadband electromagnetic field fluctuations over spacecraft frame frequencies (fsc) extending from effectively zero to fsc ≳ 100 Hz. Using observations from the Van Allen Probes we show that these waves most commonly occur pre-midnight but are observed over a range of local times extending into the dayside magnetosphere. We find that the variation of magnetic spectral energy density with fsc obeys inline image over several decades with a spectral break-point at fb ≈1 Hz. The values for α are log normally distributed with α = 1.9 \textpm 0.6 for fsc < fb andα = 2.9 \textpm 0.6 for fsc > fb. A is a function of geomagnetic activity with the largest values observed over intervals of decreasing Dst index during the main phase of geomagnetic storms. At these times these waves are nearly always present in the night-side inner magnetosphere and are commonly observed from L = 3 outward. The observed variation of the electric to magnetic field amplitude with fsc is well described by a dispersive Alfv\ en wave model under the assumption that fsc is primarily a consequence of the Doppler shift of plasma frame structures moving over the spacecraft. The robust anti-correlation between the time rate change of the Dst index and wave spectral energy density coupled with the ability of dispersive Alfv\ en waves to drive transverse ion acceleration suggests that these waves may boost ion energy density in the inner magnetosphere and intensify the ring current during storm times.

Chaston, C.; Bonnell, J.; Kletzing, C.; Hospodarsky, G.; Wygant, J.; Smith, C.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 09/2015

YEAR: 2015     DOI: 10.1002/2015JA021690

Alfven waves; Geomagnetic storms; ring current; turbulence; Van Allen Probes