INTERSTELLAR FRINGES FROM PULSAR B0834+06

Observations at Jodrell Bank are reported of very fine modulations of the dynamic spectra of interstellar scintillation (ISS) from pulsar PS R B0834+06. These fringes have a period in radio frequency of 45 kHz, which is about 100 times smaller than the largest bandwidth due to nor mal ISS of the pulsar at the central observing frequency of 408 MHz. A nalysis shows good consistency with a model in which two ray-bundles i ntersect and interfere creating the fringes, from which a 22-mu s dela y difference is inferred. The separation between the bundles is greate r than 4 mas which is more than 10 times their angular diameter. Such a large ratio cannot be due to an inner scale that cuts off the pervas ive turbulent density spectrum responsible for the diffractive angular broadening. The fringing was observed on four occasions over 30 d and so is similar to the 'extreme scattering events' observed in other so urces. 3 au is the scale inferred, and various possible structures in the interstellar plasma density are considered. A stochastic refractio n model is proposed, in which many isolated regions in a layer of pars ec thickness randomly build up enhanced angles of refraction, Normally neighbouring ray-paths do not intersect. However, occasionally their angles are large enough to cause interference and the associated fring es. A specific model is proposed in which the ionized refracting layer surrounds a warm HI cloud, with anisotropic density irregularities of 0.25 electron cm(-3), which could be in equilibrium with the normal i nterstellar pressure.