New twist on turbulence

Discovery of a distinct class of plasma vortex at the cusps of the Earth's magnetosphere provides new insight into the nature of turbulence in magnetized plasmas.

On 9th March 2002, a group of four closely spaced European research satellites known collectively as the Cluster array, passed upwards through a region of the Earth's magnetosphere where particles from the solar wind are able to enter the ionosphere. This provided an opportunity for researchers to study turbulent behaviour in the magnetosphere. In this week's Nature¹, David Sundkvist and colleagues report the results of this encounter, which reveal a new class of plasma vortex that occurs on length scales over a thousand times smaller than similar recently observed magnetospheric phenomena.

Near the surface, the lines of the Earth's magnetic field look like those of a simple dipole magnet. Further out, however, the topology of these lines is rather more complex. A peculiar aspect of this complexity is that at a distance of between five and ten Earth radii, the field lines that initially radiate out from near the poles bend towards the Sun (Fig. 1). The divergence of these field lines and their inclination towards the sun creates two funnel-like openings in the Earth's magnetic field — known as the polar cusps — that allow high-energy particles from the solar wind to cross the magnetosphere and enter the ionosphere. The interplay of the Earth's magnetic field and the solar wind at the cusps gives rise to large gradients in plasma density and velocity, which makes them ideal places to study complex turbulent flows.

Figure 1: Location of the Cluster spacecraft and the Earth's magnetic field lines.

Distances are in Earth radii. Reprinted with permission from ref. 1.

Owing to the variability of the solar wind and the fields in the vicinity of the cusps, it is vital to be able to distinguish variations in time from variations in space. As individual satellites are only able to take measurements at a single point in space at any given time, separating these temporal and spatial variations necessarily requires more than one satellite. Using data collected simultaneously by several closely located satellites in the Cluster array, Sundkvist et al. did just this, which led them to the discovery of a distinct class of plasma vortex within the region of a polar cusp, known as short-scale drift-kinetic Alfvénic (DKA). Significantly, they find these plasma vortices to have radii of around 25 km — much smaller than the Kelvin–Helmholtz (K-H) vortices², which are typically 40,000 km across, that have been observed on the flanks of the magnetosphere away from the cusps.

Both DKA and K-H vortices play an important role in allowing the transfer of particles and energy from the solar wind into the magnetosphere. As well as providing a more complete picture of the magnetosphere, the study of these effects should improve our understanding of related phenomena in other astrophysical and laboratory-based plasmas.