Observations of magnetospheric radio noise by the Goddard Space Flight Center radio experiment on the Imp 6 spacecraft have revealed a quasi-continuous component at frequencies between 30 and 110 kHz. When the spacecraft is in the interplanetary medium or the magnetosheath, a low-frequency cutoff often characterizes the otherwise power law (f-&agr;) spectrum of this noise. A positive correlation is observed between this cutoff frequency fco and the solar wind plasma frequency fsw, deduced from the Los Alamos plasma experiment on the same spececraft; on the average, fco?1.3fsw. If one pictures the magnetosheath as a homogeneous layer of plasma lying between the radio noise source (at L~4-7) and the spacecraft in the interplanetary medium and having an electron density 2-3 times that of the solar wind, then one will expect fco?21/2fsw-31/2fsw. Within the limits of the experimental error this simple model correctly accounts for the observations. A rough calculation shows that radio wave scattering by electron density fluctuations in the magnetosheath plasma is likely to be important for frequencies below 200 kHz. However, the effects of such scattering cannot be detected in the Imp 6 observations considered here because neither concurrent measurements nor sufficiently accurate models of the necessary magnetosheath plasma parameters are presently available.