Observation of inertial energy cascade in interplanetary space plasma

Direct evidence for the presence of an inertial energy cascade, the most characteristic signature ofhydromagnetic turbulence (MHD), is observed in the solar wind by the Ulysses spacecraft. After a briefrederivation of the equivalent of Yaglom’s law for MHD turbulence, a linear relation is indeed observedfor the scaling of mixed third-order structure functions involving Elsasser variables. This experimentalresult firmly establishes the turbulent character of low-frequency velocity and magnetic field fluctuationsin the solar wind plasma.