WIND observations of coherent electrostatic waves in the solar wind

Abstract

Abstract. 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 v1. In the range of frequencies above the proton plasma frequency f pi and smaller than or of the order of the electron plasma frequency f pe , TDS observed three kinds of electrostatic (e.s.) waves: coherent wave packets of Langmuir waves with frequencies f 9 f pe , coherent wave packets with frequencies in the ion acoustic range f pi f`f pe , 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 eect: the wavelengths are short, 10 to 50 electron Debye lengths k h . 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 925k h , 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.

Spaced measurements and progress in understanding space plasma waves

“…During high altitude ͑25 000 to 45 000 km͒ passes through the polar magnetosphere, wave electric field detectors respond to coherent electrostatic structures. 21,22 Similar structures have been observed in many different regions of the magnetosphere, including the magnetotail, 23,24 the bow shock, 25,26 the solar wind, 27 and the auroral acceleration region. [28][29][30][31] Thus, the structures are observed in a wide range of plasma environments covering ␤Ӷ1 to ␤Ϸ1.…”

Debye-scale solitary structures measured in a beam-plasma laboratory experiment

“…However the short delay between the probe tips do not allow to find out if this structure conserves it shapes as it propagates or whether it is only transitory. Nevertheless the tripolar electric field and potential drop are reminiscent of observations in the auroral region (Temerin et al, 1982;Bostrom et al, 1988;Pickett et al, 2004b) or the solar wind (Mangeney et al, 1999) which were interpreted as weak double layers. Tripolar structures have also been interpreted in terms of nested ion and electron holes (Pottelette and Treumann, 2005), although the estimated velocity of the structure is here probably too large for such an interpretation.…”

Nonlinear MHD Waves and Turbulence

“…While the first observations were limited to the ecliptic plane, recent measurements by the URAP instrument onboard Ulysses show that this activity is also present at high heliographic latitudes, with, perhaps a lower intensity (MacDowall et al, 1996). Some properties of the waves observed in this frequency range, the so-called "«ora acoustic range'^ (referred to in what follows as the lAC range) are now well established ( see for example the review by Gurnett, I99I, and the recent work by Mangeney et al, 1999).…”

“…At higher time resolution (Kurth et al, 1979) the emission begun to show some structure in the form of brief narrowband bursts with rapidly drifting center frequency. More recently, Mangeney et al (1999) analyzed waveform data obtained with the Time Domain Sampler (TDS) experiment on the WIND spacecraft (Bougeret et al, 1995) with a time resolution reaching 120,000 points per second. At this temporal resolution, the wave activity in the ion acoustic range appears to be highly coherent, made of a mixture of electrostatic wave packets (W.P.)…”