Morphology of Langmuir wave active sites in the Earth's foreshock observed from the four points CLUSTER constellation : analysis of a case event at small separation

High frequency fluctuating electric field upstream from the Earth bowshock, at frequencies near the local electron plasma frequency Fpe, but varying within a considerable range (~ 0.1 to 1.1 Fpe), have been observed in situ by several plasma wave instruments. Referred to as `Langmuir waves' when found near the plasma frequency, as `upshifted'or `downshifted' oscillations when found above or below Fpe, they are associated with the flux of electrons which move upstream from the bowshock. The Cluster constellation offers a unique opportunity to perform a spatio-temporal analysis of the active sites of such electrostatic emissions when at small (~ 100 km) average separation. We present a case event taken in this context, over a few minutes time interval. The presence of so called 30 s (period) ULF waves in DC magnetic field and plasma density indicates a position in the ion foreshock. The HF electrostatic fluctuations are measured in the 2 - 80 kHz band with a good time resolution: the instrument performs signal acquisitions in snapshots of 6.3 ms duration at a recurrence of 13.3 ms, transmitting to ground a survey of large band signal variations at resolution 13.3 ms, and, at 107 ms recurrence, the accumulation of frequency spectra (at 160 Hz resolution) derived on board for all the snapshots. The HF fluctuations, as observed from the accumulated spectra, can be sorted in several classes: (a) thermal noise above the plasma frequency, the position of which is precisely measured by the sounder at regular intervals,(b)Langmuir waves in the close vicinity of Fpe, sometimes associated with emissions at 2 Fpe, (c) downshifted waves, (c)broadband noise at the low frequency end of the Whisper range.In many instances, more than one class (excluding thermal noise, always present) are associated in the same accumulated spectrum. Although the emissions are quite sporadic in term of amplitude variations, the global characteristics, in term of frequency signatures, can persist over a few seconds, the changes being seen on all spacecraft at nearby time boundaries. We can thus define regions, or active sites, of a given class (simple or composite) of frequency signatures. We shall present and discuss an analysis of a) the size and velocity of given regions (from simple boundary time analysis), b) occurrence of a given site as compared to global plasma parameters, such as DC magnetic field configuration, solar wind velocity, or global features in the ion and electron populations, c) directivity properties (in the antenna spin plane) of the spectral signatures, compared to DC magnetic field and solar wind velocity orientation.