Context : During their journey to perihelion , comets may appear in the field-of-view of space-borne optical instruments , showing in some cases a nicely developed plasma tail extending from their coma and exhibiting an oscillatory behaviour .

Aims : The oscillations of cometary tails may be explained in terms of vortex shedding because of the interaction of the comet with the solar wind streams .
Therefore , it is possible to exploit these oscillations in order to infer the value of the Strouhal number St , which quantifies the vortex shedding phenomenon , and the physical properties of the local medium .

Methods : We used the Heliospheric Imager ( HI ) data of the Solar TErrestrial Relations Observatory ( STEREO ) mission to study the oscillations of the tails of the comets 2P/Encke and C/2012 S1 ( ISON ) during their perihelion in Nov 2013 , determining the Strouhal numbers from the estimates of the halo size , the relative speed of the solar wind flow and the period of the oscillations .

Results : We found that the estimated Strouhal numbers are very small , and the typical value of St \sim 0.2 would be extrapolated for size of the halo larger than \sim 10 ^ { 6 } km .

Conclusions : Despite the vortex shedding phenomenon has not been unambiguously revealed , the findings suggest that some MHD instability process is responsible for the observed behaviour of cometary tails , which can be exploited for probing the physical conditions of the near-Sun region .