Wave mode identification of electrostatic noise observed with ISEE 3 in the deep tail boundary layer

Characteristics of VLF electrostatic noise observed with ISEE 3 in the low‐latitude boundary layer of distant geomagnetic tail (∼210 R e ) have been examined in detail. Gray tone dynamic spectra with high time resolution of 16 frequency channels of electric field exhibit successive spiky bursts with duration less than the wave sampling period (0.5 s) and with a wide frequency extent. A set of noise parameters such as peak amplitude, its frequency, and bandwidth has been determined from a Gaussian distribution function derived for each spiky noise spectrum by using a least squares fitting routine. In order to find a possible energy source for the noise generation, available correlations between the noise parameters and plasma electron parameters have been examined for a 2‐hour data interval when ISEE 3 was in the low‐latitude boundary layer. The analysis has been conducted on the basis of a hypothesis that the wide frequency extent of the noise spectra is composed of Doppler effects of waves propagating nearly omnidirectionally within the plasma rest frame, which is moving with the electron bulk speed. On the basis of the hypothesis the wavelength of the observed waves has been determined from the width of the frequency extent and the measured electron bulk speed. Although no obvious free energy source has been found from the analysis, the obtained dispersion relation for the electrostatic noise has demonstrated the validity of the hypothesis. The obtained dispersion relation is consistent with theoretical ion acoustic dispersions for the measured electron parameters. The result shows that the wavelength ranges from 2 to 8 times the plasma Debye length. The dispersion also indicates that the ion temperature is in a range from 1 to 4 times the electron temperature.