The 1997 event in the Crab pulsar revisited

A complex event observed in the radio pulses from the Crab pulsar in 1997 included echoes, a dispersive delay and large changes in intensity. It is shown that these phenomena were due to refraction at the edge of a plasma cloud in the outer region of the Crab nebula. Several similar events have been observed, although in less detail. It is suggested that the plasma cloud is in the form of filaments with diameter around 3 x 1011 m and electron density of the order of 104 cm-3.