The low‐frequency continuum as observed in the solar wind from ISEE 3: Thermal electrostatic noise

The low-frequency (LF) continuum, observed in the solar wind by ISEE 3, is shown to be generated by local electrostatic (thermal) plasma waves and not by electromagnetic waves. The continuum has been investigated between 30 and 200 kHz utilizing the radio astronomy receivers and antennas aboard ISEE 3. This experiment has a noise level 5–20 dB lower than that of previous systems, as well as continuous frequency coverage from 30 to 80 kHz. Not only is the spacecraft itself very quiet but also it is stationed far from terrestrial noise sources. The spin plane wire antenna has a smaller photoemitting area and is consequently less noisy than tubular type antennas previously used. The observed LF noise continuum spectrum is a function of antenna length. For the spin plane S antenna, a 90 m dipole, a spectral index of −3 is found as compared to an index of −1.5 to −2.2 for the 2 to 5-m Z antenna deployed along the spin axis. This Z antenna shows no spectral cutoff even when one is observed on the S antenna. When an interplanetary shock passes the spacecraft, large noise enhancements occur within a few seconds. The plasma frequency fp and consequently the electron density is obtained from observations of the 2fp line with the S antenna. A low-frequency cutoff occurs very close to fp, and it is followed on the high-frequency side by a peak of variable intensity. A cutoff is not observed only when it happens to fall outside the receiver frequency range. The noise voltage variations as a function of fp are deduced from observations of the 2fp line. The observed noise levels are in close agreement with those obtained by Imp 6 and Imp 8 at locations nearer the earth, thus ruling out a magnetospheric origin. The phenomenon is interpreted as local thermal plasma noise. The antenna radiation resistance in the plasma electrostatic mode can be computed and then used to determine the noise. Theory shows, for example, that for f »fp the S antenna spectrum is fp²/f³ provided that the electron density Ne is sufficiently large that the antenna is longer than 7 Debye lengths. In general, the theoretical results for f < fp as well as for f »fp are in excellent agreement with observations. Previous observations are reviewed to show that many of them, postulating a magnetospheric radio source, can be better interpreted instead with the mechanism outlined in the present paper.