INTERSTELLAR MODULATION OF THE FLUX-DENSITY AND ARRIVAL TIME OF PULSES FROM PULSAR B-1937+214

Observations of the millisecond pulsar B1937+214 made at Nancay over 6 years show 30% rms flux variations over 13 +/- 4 days due to Refracti ve Interstellar Scintillations. The arrival times (TOA) also show vari ations over a similar time scale 16 +/- 10 days with an rms amplitude of about 0.3 mu secs. These ''rapid'' TOA variations are anti-correlat ed (similar to -40%) with the flux and so are also caused by propagati on through the ionized interstellar medium. The correlation is such th at weak pulses tend to arrive late. While TOA modulations due to chang ing geometric delay should be positively correlated with flux, those d ue to small scale variations in the dispersive delay should be negativ ely correlated with the flux and so are presumed to be responsible in our observations. The level and time scales are shown to be consistent with expectations based on the Kolmogorov model of the interstellar d ensity spectrum. However, in the data there is a sequence of about 5 d iscrete events, in which the flux remains low over 10-30 days and the TOA is on average late but also shows rapid variations. Assuming that these are indeed discrete events, we interpret them as due to isolated regions of enhanced plasma density crossing the line of sight. Such ' 'Extreme Scattering Events'' make a major contribution to the TOA vari ations and their anti-correlations with the observed flux, They are se en against a background of the normal refractive scintillation. A mode l is proposed in which discrete sheets of plasma cross the line of sig ht and cause a ''de-focussing'' event when aligned parallel to the lin e of sight. The statistics of the events imply a surprisingly large sp ace density of the sheets; an alternative is that by chance we view PS R B1937+214 tangentially through a supernova shell which is fragmented and so causes multiple events.