Electron velocity space hole modes and narrowband electrostatic noise in the distant tail

Narrowband electrostatic noise (NEN) is frequently observed upstream of the slow shocks in the distant tail. We suggest that NEN represents a non‐standard plasma wave mode which results from a hole in the low energy electron velocity distribution. A very simple model which plausibly indicates how a hole might be formed by the interaction of the electrons with the slow shock is presented. The hole mode is destabilized by a positive slope in the parallel electron distribution and/or by a weak beam of energetic ions which escape upstream from the slow shock.