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We analyze dispersion measure (DM) variations of 37 millisecond pulsars inthe 9-year NANOGrav data release and constrain the sources of these variations.Variations are significant for nearly all pulsars, with characteristictimescales comparable to or even shorter than the average spacing betweenobservations. Five pulsars have periodic annual variations, 14 pulsars havemonotonically increasing or decreasing trends, and 13 pulsars show botheffects. Several pulsars show correlations between DM excesses and lines ofsight that pass close to the Sun. Mapping of the DM variations as a function ofthe pulsar trajectory can identify localized ISM features and, in one case, anupper limit to the size of the dispersing region of 13.2 AU. Finally, fivepulsars show very nearly quadratic structure functions, which could beindicative of an underlying Kolmogorov medium. Four pulsars show roughlyKolmogorov structure functions and another four show structure functions lesssteep than Kolmogorov. One pulsar has too large an uncertainty to allowcomparisons. We discuss explanations for apparent departures from aKolmogorov-like spectrum, and show that the presence of other trends in thedata is the most likely cause.

The NANOGrav Nine-Year Data Set: Measurement and Interpretation of Variations in Dispersion Measures