Cross-Scale Effects in Solar-Wind Turbulence

The understanding of the small-scale termination of the turbulent energy cascade in collisionless plasmas is nowadays one of the outstanding problems in space physics. In the absence of collisional viscosity, the dynamics at small scales is presumably kinetic in nature; the identification of the physical mechanism which replaces energy dissipation and establishes the link between macroscopic and microscopic scales would open a new scenario in the study of turbulent heating in space plasmas. We present a numerical analysis of kinetic effects along the turbulent energy cascade in solar-wind plasmas which provides an effective unified interpretation of a wide set of spacecraft observations and shows that, simultaneously with an increase in the ion perpendicular temperature, strong bursts of electrostatic activity in the form of ion-acoustic turbulence are produced together with accelerated beams in the ion distribution function.

Figure 1

Perpendicular (black line) and parallel (red line) temperature versus $x$ at three different times.

Figure 2

Energy spectra at four different times: magnetic energy (black line), electric energy (blue line), kinetic energy (purple line), and density (red line).

Figure 3

(Top plot): parallel electric field (normalized to $E_0=m_i{V}_A{\Omega }_{\mathrm{ci}}/e$, $e$ being the electric charge) versus $x$ at $t=100$. (Middle plot): density fluctuations versus $x$ at $t=100$. (Bottom plot): $x\mathrm{\text{−}}{v}_x$ level lines of the reduced distribution $\widehat{f}$ at $t=100$.

Figure 4

Level lines (at the top) and surface plot (at the bottom) of $f$ in the velocity plane $v_x\mathrm{\text{−}}{v}_y$ at $t=200$.