Early Observations of MHD Turbulence

Abstract

Here we briefly present the history, since the first Mariner missions during the 1960s, of the main steps towards the completion of an observational picture of turbulence in interplanetary space. This retrospective look at all the advances made in this field shows that space flights allowed us to discover a very large laboratory in space. As a matter of fact, in a wind tunnel we deal with characteristic dimensions of the order of L ≤ 10 m and probes of the size of about d ≃ 1 cm. In space, L ≃ 10⁸ m, while “probes” (say spacecrafts) are about d ≃ 5 m. Thus, space provides a much larger laboratory but most of the available data derive from single point measurements. The ESA-Cluster project at the beginning of the past decade and, recently, the NASA-MMS project are the only space missions that allow multiple measurements, i.e. 3D measurements. In this context, after a short definition of the main reference systems in which data is provided, it is useful to recall the basic statistical concepts and numerical tools used to describe MHD turbulence in space.