Current measurements in space plasmas and the problem of separating between spatial and temporal variations in the field of a plane electromagnetic wave

A description of an instrument for measuring a current density vector is presented. This instrument uses 3 orthogonal Rogovskii coils. The need for such and instrument is discussed. Together with magnetic measurements it is possible to determine the wave vector of a plane electromagnetic wave using the formula B × CurlB = kB 2 = B × [ j + ∂ E ∂t ] using direct on-board processing and assuming that ∂ E ∂t can be neglected in this frequency range. The real frequency of the wave can be calculated using Doppler's law ω = Ω + k υ ̇ , where υ is the vehicle velocity and Ω the measured frequency. Test results for prototype coil are presented, together with a discussion of further developments.