Radial evolution of solar wind intermittency in the inner heliosphere

We analyzed intermittency in the solar wind, as observed on the ecliptic plane, looking at magnetic field and velocity fluctuations between 0.3 and 1 AU, for both fast and slow wind and for compressive and directional fluctuations. Our analysis focused on the property that probability distribution functions of a fluctuating field affected by intermittency become more and more peaked at smaller and smaller scales. Since the peakedness of a distribution is measured by its flatness factor we studied the behavior of this parameter for different scales to estimate the degree of intermittency of our time series. We confirmed that both magnetic field and velocity fluctuations are rather intermittent and that compressive magnetic fluctuations are generally more intermittent than the corresponding velocity fluctuations. In addition, we observed that compressive fluctuations are always more intermittent than directional fluctuations and that while slow wind intermittency does not depend on the radial distance from the sun, fast wind intermittency of both magnetic field and velocity fluctuations clearly increases with the heliocentric distance. We propose that the observed radial dependence can be understood if we imagine interplanetary fluctuations made of two main components: one represented by coherent, non propagating structures convected by the wind and, the other one made of propagating, stochastic fluctuations, namely Alfvén waves. While the first component tends to increase the intermittency level because of its coherent nature, the second one tends to decrease it because of its stochastic nature. As the wind expands, the Alfvénic contribution is depleted because of turbulent evolution and, consequently, the underlying coherent structures convected by the wind, strengthen further on by stream–stream dynamical interaction, assume a more important role increasing intermittency, as observed. Obviously, slow wind doesn’t show a similar behavior because Alfvénic fluctuations have a less dominant role than within fast wind and the Alfvénicity of the wind has already been frozen by the time we observe it at 0.3 AU. Finally, our analysis suggests that the most intermittent magnetic fluctuations are distributed along the local interplanetary magnetic field spiral direction while, those relative to wind velocity seem to be located along the radial direction.