Pulsar Scintillation in the Local ISM: Loop I and Beyond

Recent pulsar scintillation measurements from Ooty, in conjunction with those from Parkes and other large radio telescopes, are used for a systematic investigation of the Local Interstellar Medium (LISM) towards the general direction of the Loop I Bubble. For several pulsars, clear evidence is found for an enhanced level of scattering which is over and above what can be accounted for by the enhanced scattering model for the Local Bubble. These results are interpreted in terms of enhanced scattering due to turbulent plasma associated with the Loop I shell. Useful constraints are obtained for the scattering properties of the shell. The inferred value for the scattering measure for the Loop I shell is found to be ~0.3 pc m^{-20/3}. Assuming a shell thickness ~5-10 pc, this implies an average strength of scattering in the shell that is ~100-200 times larger than that in the ambient ISM. An alternative explanation, where the enhanced level of scattering is due to a possible "interaction zone" between the Local Bubble and the Loop I Bubble, is also considered; it is found to be somewhat less satisfactory in explaining the observations. The best fit value for the scattering measure for such an interaction zone region is estimated to be ~1.1 pc m^{-20/3}. Further, several pulsars beyond ~1 kpc are found to show enhanced levels of scattering over and above that expected from this "two-bubble model." For some of the low-latitude pulsars, this is found to be due to enhanced scattering from plasma inside the intervening Sagittarius spiral arm. We discuss the implications of our results for the interpretation of scintillation data and for the general understanding of the LISM.