A ∼1.7‐year quasi‐periodicity in cosmic ray intensity variation observed in the outer heliosphere

The wavelet transform technique has been applied to investigate the periodicities longer than a year in cosmic ray intensity variations observed at the Voyager spacecraft and at the Earth. We found significant ∼1.7‐ and ∼1.3‐year periodic components in the cosmic ray intensity variation in the outer heliosphere during the 1980s (solar cycle 21) and the 1990s (solar cycle 22), respectively. This property is consistent with the result of the wavelet analysis of the cosmic ray intensity variations observed at several Neutron Monitors (NMs) and also with previous reports about these periodicities using several plasma parameters, such as the solar wind speed and geomagnetic indices, for example. As comparing with the interplanetary magnetic field (IMF) and solar flare indices, causal relation between energy release from the Sun and the ∼1.7‐year quasi‐periodicity is suggested.