Turbulence on a small scale

摘要

The four-spacecraft Cluster mission has identified small-scale vortices in Earth's magnetosphere. The observation reveals processes that transfer energy and momentum from the solar wind to the magnetosphere. Turbulence is ubiquitous. It can be seen by gently stirring cream into coffee, or by observing the white caps and surf at the beach, and it causes the drag on cars and aeroplanes. The phenomenon is also widespread in magnetized plasmas — ionized gases that contain a magnetic field — such as the interstellar and intergalactic media, the solar wind and Earths magnetosphere. Although magnetic fields complicate the description of turbulence, certain characteristics, among them vortices, can arise in fluid and magnetofluid turbulence. On page 825 of this issue, Sundkvist et al. report the detection of small vortices in the 'dayside cusp' of Earth's magnetosphere by the four-spacecraft Cluster, a joint European Space Agency and NASA mission. The authors describe their observations as evidence for a phenomenon known as drift-kinetic Alfvenic turbulence.