SIII 39WIND observations of weak double layers in the solar wind and their relation to the interplanetary electric field.

C. Salem , C. Lacombe , J.-L. Bougeret , P. Kellogg 
In the solar wind at 1 AU, coherent electrostatic waveforms in the ion acoustic frequency range (fpi < f < fpe) have been recently observed by the WAVES/TDS instrument on WIND. Many of these structures have been interpreted in terms of Weak Double Layers (WDLs), since they sustain a net potential drop of roughly 1 mV directed towards the Earth. The TDS data are compared to the continuous measurements of thermal and non thermal electric spectra above 4 kHz obtained by the WAVES/TNR instrument : this allows to determine the frequency of occurrence of the WDLs at the L1 Langrange point. Extrapolating this result provides a total potential drop of about 450 V on the Sun-Earth distance, compatible with the potential needed to maintain the global charge neutrality in the solar wind. This suggests that the interplanetary electrostatic potential is not continuous but results from a succession of WDLs, distributed intermittently between the Sun and the Earth. We also find that the energy of the non thermal fluctuations on TNR between 4 and 6 kHz is correlated to the interplanetary electrostatic field, parallel to the spiral magnetic field, calculated with a two-fluid model, thus providing further evidence of a relation between the interplanetary electrostatic field and the electrostatic fluctuations in the ion acoustic range.