No abstract

The best method was sought for defining locally an electrostatic or an electromagnetic turbulence in a space plasma, based on the simultaneous measurement of several wave field components at several points in space. The turbulent field is supposed to be stationary in time and homogeneous in space. An estimate of a local field energy distribution, denoted F(ω,k), which specifies how the field energy observed by a limited number of satellites is distributed with regard to the angular frequency ω and to the wave number k, is derived from a generalization of the Capon [1969] high‐resolution method. The Quality of this estimate is deemed sufficient, as shown by examples of application to synthetic data.

[1] A set of experimental data is presented for a highMach-number (M f = 5) quasiperpendicular (q Bn = 81°) bow shock layer crossed by Cluster spacecraft on 24 January 2001 at 07:05 -07:09 UT. The measurements of magnetic field, spectra of electric field fluctuations, and ion distributions reveal that the shock is highly nonstationary. In particular, the magnetic field profiles measured aboard different spacecraft differ considerably from each other. The mean frequency of downshifted waves observed upstream of the shock ramp oscillates with a characteristic time comparable with the proton gyroperiod. In addition, the reflection of ions from the shock is bursty and a characteristic time for this process is also comparable with the ion gyroperiod. All of these features in conjunction are the first convincing experimental evidence in favor of the shock front reformation. Citation: Lobzin,