Time variability of pulsar dispersion measures

Energetic events like supernova explosions, stellar winds, and expanding H II regions create turbulent irregularities in the interstellar electron density. The dispersion measure (DM) of a pulsar fluctuates as these irregularities drift past the line of sight. To monitor changes in the interstellar electron density we conducted 47 and 430 MHz timing observations of seven pulsars, once a month for 2 yr with the Arecibo radio telescope. DM variations were detected toward six of the seven objects. The root mean square variations were largest for the most distant pulsars, confirming that the fluctuations were indeed interstellar. Empirically we found σ̂_DM ∝ DM^1.3±0.3 over a 10:1 range in DM. A structure function analysis showed that the electron density fluctuation spectrum was of the form P(q) = C_n² q^-α, over a range of spatial scales 2πq^-α≈ 10⁹ to 10¹⁵ cm. The inferred power-law indices were α = 3.77 ± 0.04 for PSR 0834 + 06, 3.87 ± 0.03 for PSR 0823 + 26, and 3.85 ± 0.03 for PSR 0919 + 06.