Turbulence results from multi-scale nonlinear interactions and from instabilities of large-scale fluid motions involving many degrees of freedom. Collisionless space plasmas such as the solar wind or plasmas in planetary environments are in a turbulent non-equilibrium state, characterized by strong fluctuations of field and plasma parameters over multiple scales. The fluctuations are present from the largest energy injection scales through magnetohydrodynamic or fluid scales, where friction forces are negligible, to the smallest dissipation scales where the available energy is converted to heat. In the absence of collisions physical processes become increasingly more complex near the ion/electron kinetic scales. Near and over the kinetic scales the energy transfer/exchange between the electromagnetic fields, plasma motions and particles is possible via various channels of dynamics. Physical constraints in plasma turbulence lead to the generation of coherent intermittent structures such as (reconnecting) current sheets, vortices, discontinuities or flux tubes. Although plasma turbulence is considered to be highly nonlinear, it is often hypothesized that linear physics remains important for the turbulence dynamics and the system may retain some properties of linear wave modes. However, over the kinetic scales the waves become dispersive and dissipative also exhibiting anisotropies with respect to the mean magnetic field. Particularly interesting questions which has to be addressed in near ion or sub-ion scale space plasma turbulence are: (a) What kind of wave modes (co-)exist under different plasma conditions? (b) What kinds of intermittent spatial structures do the turbulent motions and fields exhibit?

At IWF both theoretical and experimental studies dedicated to space plasma turbulence are carried out. The space missions targeted in these studies are the multi-spacecraft Cluster and MMS missions and inner heliospheric missions such as Solar Orbiter and BepiColombo. Multi-point and single-point wave analysis methods are developed to distinguish between linear wave modes in sub-ion scale compressive or incompressive turbulence such as kinetic slow waves, kinetic Alfven waves or ion-Bernstein magnetosonic waves. In this effort the high resolution electron density obtained and calibrated from the spacecraft potential was very useful. Multi-point Cluster and MMS data are useful to observe current sheets and understand better energy conversion at kinetic scales in the magnetosheath. Reconnecting small-scale current sheets in turbulent magnetosheath are associated with whistler emissions and lower-hybrid drift waves. The coherent structures can be responsible for turbulence intermittency, however, using the techniques proposed by the group members, it was also shown that the presence of wave activity can potentially reduce intermittency at sub-ion scales. Theoretical investigations help us to understand the observed scalings on spatial scales smaller than the ion inertial length. Hall turbulence appears to be the likely candidate to explain the steepening of the magnetic energy spectra. Analytical calculations based on the linear Vlasov theory allowed to derive the dielectric tensor of plasma containing various fluid picture processes in the lowest order. It is expected that the predicted transport ratios offer a diagnostic tool to study and identify the kinetic Alfven mode in the inner heliosphere for Solar Orbiter data.

Cluster and MMS missions and inner heliospheric missions such as Solar Orbiter and BepiColombo. Multi-point and single-point wave analysis methods are developed to distinguish between linear wave modes in sub-ion scale compressive or incompressive turbulence such as kinetic slow waves, kinetic Alfven waves or ion-Bernstein magnetosonic waves. In this effort the high resolution electron density obtained and calibrated from the spacecraft potential was very useful. Multi-point Cluster and MMS data are useful to observe current sheets and understand better energy conversion at kinetic scales in the magnetosheath. Reconnecting small-scale current sheets in turbulent magnetosheath are associated with whistler emissions and lower-hybrid drift waves. The coherent structures can be responsible for turbulence intermittency, however, using the techniques proposed by the group members, it was also shown that the presence of wave activity can potentially reduce intermittency at sub-ion scales. Theoretical investigations help us to understand the observed scalings on spatial scales smaller than the ion inertial length. Hall turbulence appears to be the likely candidate to explain the steepening of the magnetic energy spectra. Analytical calculations based on the linear Vlasov theory allowed to derive the dielectric tensor of plasma containing various fluid picture processes in the lowest order. It is expected that the predicted transport ratios offer a diagnostic tool to study and identify the kinetic Alfven mode in the inner heliosphere for Solar Orbiter data.