Cluster multispacecraft measurement of spatial scales of foreshock Langmuir waves

Abstract

In this article, we present the first multispacecraft study of spatial scales of Langmuir wave packets in the terrestrial foreshock. We used data from the Wideband Data (WBD) instrument on the Cluster satellites to identify simultaneous observations of the same wave packet on multiple spacecraft at very small separations. To overcome instrumental constraints, we developed a statistical technique for matching observations on different spacecraft by comparison of their Fourier spectra. In this article, we demonstrate an application of the technique on one multispacecraft foreshock event, and we afterward use it on a larger data set comprising several Cluster foreshock crossings. Our analysis shows that the typical scale of Langmuir wave packets in the direction transverse to the magnetic field falls between 40 and 100 km, and their size in the parallel direction is larger than 150 km. We also conclude that convection of wave packets may contribute to the large-scale temporal modulation of Langmuir waves observed in previous spacecraft studies, but the small-scale structure of the modulation must be attributed to other effects. Possible systematic errors and sources of uncertainty in our estimates are discussed in detail.