  In separate series of observations of the Crab pulsar, pulse broadening due
to scattering was measured at 111 MHz, and variations of dispersion due to
pulse delay were measured at higher radio frequencies. In a remarkable event
lasting 200 days a large increase occurred in both parameters and with similar
time signatures. The increases in scattering and dispersion measure observed
over the 200 days MJD 53950 - 54150 are attributable to the effects of an
ionised cloud or filament crossing the line of sight. The cloud would be $10^11
- 10^12 m across, with electron density 10^3 - 10^4 cm^-3. The increased
scattering might originate within the cloud itself, or the moving filament
might induce turbulence in a separate higher density cloud in the line of
sight.

Astrophysics