A survey study of sub-ion scale turbulent fluctuations in the solar wind - observational constraints for the 'Turbulent Dissipation Challenge'
Résumé
We present the beginnings of a large and detailed observational survey of the properties of near and sub-ion scale turbulent fluctuations in the near-Earth solar wind. We use approximately 20 intervals of spacecraft data from the Cluster mission - a quarter of which are from periods when the instruments were operating in burst mode and can thus access near-electron scales. We supplement this data from Cluster by a large and extensive subset of intervals from the entire Wind mission (1994 - to date), which sample the magnetic field for frequencies below 11 Hz and thus sample around the ion spatial scales. Both these data sets were chosen with stringent conditions e.g. in solar wind, not connected to bow shock, no CMEs or holes, sufficient sample size and time duration, stationarity of plasma parameters etc.; and crucially, that they sample a large variety of plasma conditions e.g. varying ion and electron plasma beta, fast and slow wind, periods throughout solar cycle etc. We will be exclusively focusing on plasma and magnetic field measurements. Thus, this study and the variety of plasma parameters represented in the observational data it uses, aligns well with the aims of the recently proposed and community-driven 'Turbulent Dissipation Challenge'. The results of the following investigations will be presented: 1./ Plasma beta variation of the magnetic compressibility ratio scanning from the inertial range to near and sub-ion scales; this is also linked to the component (variance) anisotropy of the fluctuations. Due to its ease of calculation from an observational view, it is also a useful measure (although slightly redundant) to distinguish between different wave modes - Whistlers or kinetic Alfven waves - in theories that suggest that the turbulence is mediated by the various linear waves supported by the plasma; although a more general description of the behaviour of the magnetic compressibility can be furnished by considering the Hall physics. 2./ A thorough and exhaustive study of whether the intermittency and scaling changes seen by some of the authors in earlier papers (Kiyani et al. 2009, 2013) and also seen by other kinetic simulation and observational studies (Wu et al. 2013) when crossing the inertial range to the dissipation range are ubiquitous in all intervals studied; thereby lending support to the hypothesis that this behavior is at least robust to parameter variations − if not universal to weakly collisional plasmas in general. Speculative comments will be made to why this scaling change occurs. 3./ Other solar wind variations of the above calculated quantities e.g. in varying wind speed, change in bulk parameters etc. 4./ Evidence to support the existence, if any, of a transition range between the inertial and dissipation ranges, and its behaviour in the light of points 1 and 2 above. It is our assertion that some of the above findings (especially point 2 above) can be used as an observational benchmark - albeit phenomenological - on theories of the dissipation of turbulence in weakly collisional plasmas, as well as for the various models and theories upon which simulations are founded.