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There have been many reports of the terrestrial foreshock as a source of freely propagating electromagnetic (e.m.) radiation1–7, identified as plasma emission at twice the solar-wind plasma frequency, 2fp. The 2fp source is close behind the curved surface defined by the interplanetary magnetic field lines tangent to the Earth's bow shock. Electrons accelerated at the shock over a wide range of energies propagate along these field lines where, from a time-of-flight effect6, they have a distribution that is unstable to the production of Langmuir waves6,8. These Langmuir waves9,10 and the unstable electron distributions11,12 have been observed. The mechanism suggested for generating the 2fp radiation5,6,13,14 by the terrestrial foreshock is closely related to present models of type III solar radio emission and rely on some of the energy in the Langmuir waves converting to e.m. waves. The e.m. emission of type III bursts is known to be produced in the solar wind at both the fundamental and second harmonic15, and whether fundamental emission is produced in the terrestrial foreshock is of considerable interest. Comparison of nearby fundamental and harmonic emissions and the Langmuir wave energy density in the source will help discriminate between the different conversion models8,14,16. We present here unambiguous measurements made with the sounder experiment aboard the International Sun–Earth Explorer-1 (ISEE-1) and the radio experiment on ISEE-3 of simultaneous fundamental and second harmonic e.m. emission from the Earth's foreshock.

MANY experiments to measure the radio noise intensity in the topside ionosphere have found an intense “band” of noise in the range of frequency between the plasma frequency and the upper hybrid resonance frequency1–4. Various explanations have been suggested, including plasma instability, radiation by energetic particles5 and anomalous behaviour of the antenna impedance4. Data received by the UK-2 (Ariel 2) satellite suggest that the energy for this noise comes from the particles trapped in the inner Van Allen radiation belt.

ON April 17, 1949, during the course of a survey of the shore known as the Gates at the southern tip of Lundy, a piece of a Floridean alga, too small to identify with certainty, was collected in order that its population of epiphytic red algae might be examined. It occurred in a pool very near extreme low water springs, the spring tides at this period being some of the biggest of the year.