Weak Double Layers in the Solar Wind and their Relation to the Interplanetary Electric Field

In the solar wind at 1 AU, coherent electrostatic waveforms in the ion acoustic frequency range (between the ion and electron plasma frequencies) have been recently observed by the WAVES/TDS instrument on WIND. Many of these structures have been interpreted in terms of Weak Double Layers (WDL), 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 us to determine the frequency of occurrence of the WDL at the L1 Lagrange point. Extrapolating this result provides a total potential drop of about 300 to 1000 Volts 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 WDL, 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.