A set of sudden increases followed by slow decay has been observed in E region electron density on December 17, 1990 using the EISCAT UHF-radar. Simultaneous photometer observations indicate that the density enhancements are caused by impulsive particle precipitation with short duration (4¿1 s). Effective recombination coefficient profiles in the E region are calculated from decaying electron densities using the continuity equation, and the results are compared with previous observations. It is pointed out that short-lived particle precipitation affects the ion composition in the E region and therefore the effective recombination coefficient in this situation is different from the steady-state value. ¿ American Geophysical Union 1992