Simultaneous observation of fundamental and second harmonic radio emission from the terrestrial foreshock

Abstract

There have been many reports of the terrestrial foreshock as a source of freely propagating electromagnetic (e.m.) radiation^1–7, identified as plasma emission at twice the solar-wind plasma frequency, 2f_p. The 2f_p 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 waves^6,8. These Langmuir waves^9,10 and the unstable electron distributions^11,12 have been observed. The mechanism suggested for generating the 2f_p radiation^5,6,13,14 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 models^8,14,16. 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.