- Cluster measurements of the terrestrial bow shock have resulted in the first ever obser- vation of a shock whose structure was based on quasi-electrostatic sub-shocks. Such shocks (sometimes referred to as viscous shocks) have been predicted theoretically but so far have only been observed in laboratory experiments and studied in the numer- ical simulations. Cluster observations have shown some surprising features of these sub-shock structures. Previously, for very high Mach numbers (above so called the third critical Mach Number), the shock was expected to be non-stationary. Cluster observations indicate that non-stationarity of the shock front might occur at lower Mach numbers i.e. between the first and the second critical Mach numbers as well. Observations of short scale quasi-electrostatic sub-shocks require a reconsideration of theoretical models developed to describe the electron dynamics at the shock front.