Upstream ULF waves and energetic electrons associated with the lunar wake: Detection of precursor activity

We present observations of precursor ULF wave activity and energetic electron flows detected by the WIND spacecraft just prior to entry of the lunar wake on 17 December 1994. This activity occurs upstream of the wake on field lines directly connected to the wake penumbra region. The activity ceases near the penumbra entrance. The observations of upstream ULF wave activity and solar wind counterstreaming electron flows is similar to observations made upstream of collisionless bow shocks. Analogously, the wake precursor region is characterized by thermalization and information propagation ahead of the wake structure.