Quasi-periodic variations of low energy cosmic rays

Cosmic rays (CR) play an important role in space weather-related studies. Their temporal variability, both of a quasi-periodic character as well as an irregular one, has been studied from ground-based direct measurements, as well as from cosmogenic nuclides, over a long time. We attempt to describe the current knowledge of selected quasi-periodicities in CR flux in the energy range above the atmospheric threshold, from direct measurements. The power spectrum density (PSD) of the CR time series as measured by neutron monitors (NMs) and by muon detectors has a rather complicated character. Along with the shape (slope) of the PSD, knowledge of the contribution of quasi-periodic variations (q-per) to the CR signal is of importance for the modulation, as well as for checking the links of CR to space weather, and/or to space climate effects. The rotation of the Earth and solar rotation cause two types of mechanisms behind the certain q-per observed in secondary CR on the Earth's surface. Solar activity and solar magnetic field cyclicities contribute to the q-per signals in CR if studied over a longer time. The complexity of the spatial structure of the interplanetary magnetic field (IMF) and its evolution within the heliosphere, in addition to the changes in the geomagnetic field, cause variability in contributions of the q-per in CR. Wavelet spectra are useful tools for checking the fine structure of q-per and their temporal behaviour. Over a long time NMs and muon telescopes provide information about q-per in CR.