| Decameter-wavelength pulsar timing data are potentially sensitive to propagation effects which are difficult or impossible to detect at higher frequencies. These include refraction in the interstellar medium, ray-bending in the pulsar magnetosphere, and "magnetic sweepback" near the pulsar light cylinder. To search for such effects we made timing observations of eight nearby pulsars at multiple frequencies between 25 MHz and 4.8 GHz. Data obtained at 25 and 47 MHz were of unprecedented quality in terms of time resolution and signal-to-noise. We found that pulse arrival times below {dollar}sim{dollar}1 GHz were proportional to {dollar}nusp{lcub}-2{rcub}{dollar}, as expected for group delays in the tenuous interstellar plasma. Terms in the dispersion law proportional to {dollar}nusp{lcub}-3{rcub}{dollar} and {dollar}nusp{lcub}-4{rcub}{dollar} (due to density clumping, magnetic fields, and refraction) were absent, even at 25 MHz.; The absence of departures from the {dollar}nusp{lcub}-2{rcub}{dollar} dispersion law places stringent constraints on radius to frequency mapping models of the pulsar emission mechanism. An analysis including aberration, retardation and magnetic sweepback effects showed that the 47 and 4800 MHz emission zones were separated by no more than 200 km. Assuming a dipolar form for the pulsar magnetic field, the data indicate that 4800 MHz emission was produced less than {dollar}sim{dollar}100 km above the surface of the neutron star.; Observations at 47 and 430 MHz were conducted once a month over a two year period in order to monitor changes in the interstellar electron column density in the direction of seven pulsars. Fluctuations in the dispersion measure were detected toward all the objects we observed. Statistics of the fluctuations indicate a power-law spectrum of density irregularities covering a range of spatial scales from 10{dollar}sp9{dollar} to 10{dollar}sp{lcub}15{rcub}{dollar} cm. |
