Spectral analysis of neutron monitors and muon telescope daily averages counts shows a significant higher harmonic (∼9 day) of the 27 day variation of galactic cosmic rays (CRs). This quasiperiodicity is also present in the time series of daily averages of the product of interplanetary magnetic field magnitude and the square of solar wind speed (BV2). The wavelet spectrum density of the third harmonic of the 27 day variation of CRs is weakly correlated with the quantity BV2. This result reflects the coupling between galactic CR modulation and interplanetary parameters. Key Points Nine day periodicity is evident in the spectra of galactic cosmic rays Nine day periodicity is also evident in the spectra of the quantity BV2 Coupling between galactic cosmic ray modulation and interplanetary parameters