Comment on "Fingerprint structure PC 1 geomagnetic pulsations"

Sato et al. [1987] recently drew attention to a new form of Pc 1 pulsations, which they refer to as ‘fingerprint structure’ Pc 1 pulsations because of their distinctive appearance in frequency‐time (f‐t) spectrograms. The new pulsations were observed on moderately high latitude geomagnetic field lines (L≈6) and the authors suggested that the considerable number of spectral lines and their constant frequency separation might provide evidence for non‐linear effects in the Pc 1 pulsation generation region. It is suggested here, on the contrary, that the fine frequency structure is not a non‐linear effect but is simply a manifestation of field line resonances of the kind suggested by Fraser‐Smith [1967].