Abstract
In the past decades, there has been an increasing literature on the presence of an inertial energy cascade in interplanetary space plasma, being interpreted as the signature of Magnetohydrodynamic turbulence (MHD) for both fields and passive scalars. Here, we investigate the passive scalar nature of the solar wind proton density and temperature by looking for scaling features in the mixed-scalar third-order structure functions using measurements on-board the Ulysses spacecraft during two different periods, i.e., an equatorial slow solar wind and a high-latitude fast solar wind, respectively. We find a linear scaling of the mixed third-order structure function as predicted by Yaglom’s law for passive scalars in the case of slow solar wind, while the results for fast solar wind suggest that the mixed fourth-order structure function displays a linear scaling. A simple empirical explanation of the observed difference is proposed and discussed.
Highlights
The interplanetary medium is permeated by a supersonic, super-Alfvénic magnetized plasma flow, the solar wind, whose origin is in the outer solar atmosphere
Some slight discrepancies are observed in the case of the proton density especially in the case of the slow solar wind period
We study the passive scalar character of proton density and temperature in fast and slow solar wind by investigating the occurrence of Yaglom’s law for the third scalar-mixed moment
Summary
The interplanetary medium is permeated by a supersonic, super-Alfvénic magnetized plasma flow, the solar wind, whose origin is in the outer solar atmosphere It is characterized by large-amplitude, scale-invariant, turbulent, and intermittent fluctuations over a wide range of scales. The direct evidence of an inertial energy cascade, as predicted by MHD Alfvénic turbulence theory, has been only recently found and reported in a series of works on the linear dependence of the third-order structure function of Elsässer variables for high-latitude solar wind [5,6]. All these past and recent results make the solar wind a natural laboratory to study hydromagnetic turbulence and related phenomena
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