Evidence for inhomogeneous heating in the interplanetary plasma near current sheets dynamically generated by magnetohydrodynamic (MHD) turbulence is obtained using measurements from the ACE spacecraft. These coherent structures only constitute 19% of the data, but contribute 50% of the total plasma internal energy. Intermittent heating manifests as elevations in proton temperature near current sheets, resulting in regional heating and temperature enhancements extending over several hours. The number density of non-Gaussian structures is found to be proportional to the mean proton temperature and solar wind speed. These results suggest magnetofluid turbulence drives intermittent dissipation through a hierarchy of coherent structures, which collectively could be a significant source of coronal and solar wind heating.
Evidence for inhomogeneous heating in the interplanetary plasma near current sheets dynamically generated by magnetohydrodynamic (MHD) turbulence is obtained using measurements from the ACE spacecraft. These coherent structures only constitute 19% of the data, but contribute 50% of the total plasma internal energy. Intermittent heating manifests as elevations in proton temperature near current sheets, resulting in regional heating and temperature enhancements extending over several hours. The number density of non-Gaussian structures is found to be proportional to the mean proton temperature and solar wind speed. These results suggest magnetofluid turbulence drives intermittent dissipation through a hierarchy of coherent structures, which collectively could be a significant source of coronal and solar wind heating.
The energy required to explain the coronal heating problem and, hence, the existence of a solar wind [1,2] might be provided by magnetohydrodynamic (MHD) turbulence [3,4] driving a cascade [5] to scales where kinetic dissipation is efficient. This could account for the nonadiabatic expansion of heliospheric protons [6]. Analysis of higher order statistics from observations and numerical simulations indicates inertial range intermittency [7,8] is associated with coherent structures such as current sheets [9][10][11]. These may play a role in heating the solar corona by magnetic reconnection (or nano flares). Connections between the turbulence cascade and intermittent dissipation are fundamental in hydrodynamics [12], but are almost unexplored in space plasma physics, which relies heavily on linear Vlasov theory under the assumption of a uniform equilibrium plasma [13][14][15]. Recently, a statistical link was found between coherent magnetic field structures and elevated temperatures [16] (also see [17]), the interpretation of which has been questioned [18]. Here evidence is presented, based on correlations at two ranges of spatial scales and on global conditional averages, that temperature enhancements are locally linked to coherent structures and intermittency in solar wind turbulence.
Motivation for this investigation lies at the core of turbulence theory. The Kolmogorov 1941 framework [19] provides a useful description, but a more precise theory must account for large fluctuations in the energy dissipation rate [20]. The resulting intermittency can be defined as the increasing non-Gaussian character of increments with decreasing scale [12]. It is this connection between non-uniform dissipation and the statistics of increments that is embodied in the Kolmogorov Refined Similarity Hypothesis (KRSH) [21]. Since solar wind turbulence is well described by ideas that parallel its hydrodynamic antecedents -second order [22], third order [23,24], and higher order statistics [7] -it is perhaps paradoxical that interplanetary dissipation is not presumed to be highly spatially non-uniform. Indeed, we are not aware of an alternative rationale that provides a physical explanation for solar wind intermittency.
A major difficulty in linking energy dissipation to intermittency within low collisionality plasmas is that the underlying dissipation mechanisms are not unambiguously known. Often solar wind dissipation is framed in terms of Landau damping, cyclotron resonance, and instabilities in a uniform plasma where linear Vlasov theory is relevant. However, the observed consistency with intermittency theory implies deeper connections to KRSH. In order to pursue an unbiased assessment, without making assumptions about the dissipation function, proton temperature is adopted as a surrogate for the dissipation. Therefore, if the dissipation is non-uniform or intermittent, it will be reflected in the temperature distribution. In particular, we would expect to find sources of temperature enhancements within and around current sheets, which are the characteristic small scale coherent structures in MHD turbulence [9][10][11]25]. This is despite the collisional MHD framework not being applicable at the scales where the most intense current sheets would form. This lack of clarity regarding the balance between fluid and kinetic phenomena is partly responsible for the wide disparity of viewpoints concerning coronal and solar wind dissipation. The use of proton temperature as a surrogate side-steps this issue and addresses the statistics of dissipation without detailed knowledge of the mechanism.
We analyze the entire 64 s resolution magnetic field and proton temperature datasets from the MAG [26] and SWEPAM [27] instruments onboard the ACE spacecraft at 1 AU. Rapid changes in the magnetic field are described by vector increments:
where b(t) is the magnetic field time series and ∆t is the time lag. Here the temperature data cadence defines the lag ∆t = 64 s employed. Using Taylor’s hypothesis this corresponds to a spatial separation within the inertial range, which extends from about an hour to a second in the spacecraf
This content is AI-processed based on open access ArXiv data.
