WIND observations of coherent electrostatic waves in the solar wind

The time domain sampler (TDS) experiment on WIND measures electric and magnetic wave forms with a sampling rate which reaches 120 000 points per second. We analyse here observations made in the solar wind near the Lagrange point L1. In the range of frequencies above the proton plasma frequency fpi and smaller than or of the order of the electron plasma frequency fpe, TDS observed three kinds of electrostatic (e.s.) waves: coherent wave packets of Langmuir waves with frequencies f ' fpe, coherent wave packets with frequencies in the ion acoustic range fpi f < fpe, and more or less isolated non-sinusoidal spikes lasting less than 1 ms. We con®rm that the observed frequency of the low frequency (LF) ion acoustic wave packets is dominated by the Doppler e€ect: the wavelengths are short, 10 to 50 electron Debye lengths kD. The electric ®eld in the isolated electrostatic structures (IES) and in the LF wave packets is more or less aligned with the solar wind magnetic ®eld. Across the IES, which have a spatial width of the order of '25kD, there is a small but ®nite electric potential drop, implying an average electric ®eld generally directed away from the Sun. The IES wave forms, which have not been previously reported in the solar wind, are similar, although with a smaller amplitude, to the weak double layers observed in the auroral regions, and to the electrostatic solitary waves observed in other regions in the magnetosphere. We have also studied the solar wind conditions which favour the occurrence of the three kinds of waves: all these e.s. waves are observed more or less continuously in the whole solar wind (except in the densest regions where a parasite prevents the TDS observations). The type (wave packet or IES) of the observed LF waves is mainly determined by the proton temperature and by the direction of the magnetic ®eld, which themselves depend on the latitude of WIND with respect to the heliospheric current sheet.