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We have observed a complex and continuous change in the integrated pulseprofile of PSR B2217+47, manifested as additional components trailing the mainpeak. These transient components are detected over 6 years at $150$ MHz usingthe LOw Frequency ARray (LOFAR), but they are not seen in contemporaneousLovell observations at $1.5$ GHz. We argue that propagation effects in theionized interstellar medium (IISM) are the most likely cause. The putativestructures in the IISM causing the profile variation are roughly half-waybetween the pulsar and the Earth and have transverse radii $R \sim 30$ AU. Weconsider different models for the structures. Under the assumption of sphericalsymmetry, their implied average electron density is $\overline{n}_e \sim 100$cm$^{-3}$. Since PSR B2217+47 is more than an order of magnitude brighter thanthe average pulsar population visible to LOFAR, similar profile variationswould not have been identified in most pulsars, suggesting that subtle profilevariations in low-frequency profiles might be more common than we have observedto date. Systematic studies of these variations at low frequencies can providea new tool to investigate the proprieties of the IISM and the limits to theprecision of pulsar timing.

Low-frequency pulse profile variation in PSR B2217+47: evidence for echoes from the interstellar medium