Probing the ISM using almost a decade of pulsar observations below 100 MHz

Recent advancements in low-frequency radio observations of pulsars are providing new insights into the properties as well as the effect of the interstellar medium. Pulsars are strongly polarized radio sources that probe the density and magnetic field variations along the line of sight to the Earth. For example, one can monitor the effect of the interstellar medium on the pulsar Dispersion Measure (DM) which has an inverse square dependence on the observing frequency or the pulsar Rotation Measure (RM) which is proportional to the product of electron density and galactic magnetic field, making low-frequency observations preferable for such measurements. The Long Wavelength Array (LWA) has an active pulsar monitoring program, where we regularly observe a set of ~100 pulsars below 100 MHz, studying these effects in time and frequency. We have obtained dispersion measure variation in more than 50 pulsars and rotation measure of more than 20 pulsars along with their long term RM variation in at least 10 pulsars, over this period. Here we present a systematic study of these objects over almost a decade of observations, and what we have learned about the local interstellar medium