A theory for the radiation at the second harmonic of the plasma frequency f_p observed near the earth's bow shock is advanced in which the dominant plasma emission mechanisms are the processes L + L ± S→t, proceeding in two three‐wave steps, L ± S→L′ and L + L′→t, where L, S, and t denote Langmuir, ion sound, and electromagnetic waves, respectively. This theory receives strong observational support from the correct prediction of the existence and frequencies of a class of low‐frequency ion‐sound‐like waves associated with Langmuir waves in the earth's foreshock. Three predictions of the theory which may be suitable for observational testing are stated. The observed brightness temperature of the 2f_p source is calculated to be of the order of 10¹¹ K. It is shown that Fung et al.'s (1982) theory cannot explain either the 2f_p radiation due to an intrinsic brightness temperature limit of 3×10⁹ K for their model or the observed levels of Langmuir waves in the foreshock region.