An asymmetry of the electron foreshock due to the strahl



[1] We investigate statistically the dependence of electron foreshock Langmuir wave activity on the properties of the upstream electron distribution function, using wave and electron measurements from the Wind spacecraft. We find that the presence of a strong strahl beam in the upstream solar wind leads to a significant increase in the occurrence of Langmuir wave activity in the sunward wing of the electron foreshock, generating an asymmetry in the foreshock between the sunward and anti-sunward wings. This asymmetry of electrostatic wave occurrence is likely to be reflected in the radio emission properties of the two foreshock wings. The mechanism behind the asymmetry was first postulated in the context of the Venus foreshock, and may be relevant to recent observations of Langmuir wave activity in solar wind magnetic holes. Furthermore, the likely absence of a strahl-related enhancement at fast forward interplanetary shocks points out a key difference between the physics of electron acceleration at interplanetary shocks and bow shocks.