Cluster II Constraints on Electron Acceleration and Langmuir Waves at Earth's Bow Shock

Electron acceleration, Langmuir waves and radio emissions at multiples of the plasma frequency are associated with numerous shocks in the heliosphere, including Earth's bow shock and the shocks producing coronal and interplanetary type II radio bursts. Here Cluster data from the Whisper, FGM, and PEACE instruments are used to study electron acceleration at Earth's bow shock and by SLAMS, as well as the production of electron beams and Langmuir waves in Earth's foreshock. The results found include: (1) The most intense Langmuir waves are found where Cluster is magnetically connected to almost perpendicular regions of the shock with |θbn| > 70°. (2) The wave characteristics and analytic theory strongly imply that the `standard'' foreshock model is relevant: these electrons are produced by the magnetic mirror reflection/shock-drift acceleration (SDA) at the shock, the beams are produced by time-of-flight effects, and the waves are driven by beams with speeds greater than the electron thermal speed. (3) Weak bursts of broadband waves are found where |θbn| <70° and cutoff effects are unimportant. This is consistent with another electron acceleration or beam formation process being active. (4) Isolated SLAMS, even in periods between two SLAMS, produce only very weak, irregular bursts of Langmuir waves, implying that they are ineffective in accelerating electrons and/or forming beam distributions. This argues against SLAMS playing a role in type II bursts.