Hourly data are represented as percentage relative to the average level  . Physically, the data placed here are proportional to the number of primary particles incident on the atmospheric boundary. Thus, during the solar flare of September 29, 1989, a maximum hourly value made up +188% of a quiet level. At the time of some magnetic storms, the cosmic-ray intensity decreases (Forbush-effect). For instance, during the March 24, 1991 magnetic storm, a minimum hourly value made up -18% of a quiet level.

Cosmic rays (CR) represent fluxes of high-energy particles (predominantly protons) arriving at the Earth approximately uniformly from all directions from space. The main source of CR is considered to be provided by supernovae - galactic CR (GCR) ; some of CR arrive from the Sun after powerful solar flares - solar CR (SCR). GCR and SCR are primary cosmic rays.

10-15 years after the discovery of cosmic radiation (1919) there emerged two main avenues of investigation in the physics of cosmic rays (CR): 1) nuclear - elucidation of the origin of elementary particles and nuclear interactions; and 2) astrophysical (space physics) - solution of the problem related to the origin and variations (changes in intensity) of CR. Accumulation of experimental data on CR variations and interpretation of these data on the basis of results on space electrodynamics and plasma physics demonstrated a tremendous potential built into this branch of the CR physics lying at the junction with geo-, helio- and astrophysics. These investigations have shown that CR provide a highly sensitive sound probing the electromagnetic conditions in the space environment. The energy spectrum and nuclear composition of CR are variable in space and time, i.e. there occur various time variations in energy spectrum and nuclear composition. Besides, the CR flux in interplanetary space is, strictly speaking, not isotropic. The presence of a spatial anisotropy leads - as a result of Earth rotation - to diurnal variations of CR. The CR intensity undergoes a strong influence of solar activity (SA), especially in the region of low energies where the particle flux changes nearly by a factor of 10. Substantial variations of the above CR characteristics occur in periods of solar flares and magnetic storms. Charged particles (primary CR), prior to being incident on the ground, propagate through the Earth’s magnetosphere and through a large layer of air, and by interacting with its nuclei, generate secondary CR (elementary particles of different types). Thus the CR intensity observed on the ground undergoes the influence of the processes on the Sun and in interplanetary space as well as in the Earth’s magnetosphere and atmosphere. In the cosmic-ray physics, the CR intensity variations observed on the ground are subdivided into variations of interplanetary, magnetospheric and atmospheric origin.

CR intensity variations of interplanetary origin include 11-year and 27-day variations, Forbush-decreases (Forbush-effect), and SCR outbursts which are associated with the interaction of SCR with plasma flowing out of the Sun (solar wind), coronal mass ejections, and other like phenomena on the Sun. Sporadic processes on the Sun are the cause of the arrival at the Earth of large SCR, give rise to geomagnetic storms and, hence, radio communication disturbances, technogenic emergencies, impairment of health of chronically sick persons, etc. In addition to solving problems in basic science, the study of CR variations solves the problems of diagnostics of interplanetary processes and of making predictions of solar activity and sporadic phenomena on the Sun.

CR intensity observations on the ground are carried out at the
worldwide network of cosmic-ray stations (currently about 40 stations) equipped with installations which are large in area and are designed for recording both the mu-meson CR component (meson telescopes) and the neutron component (neutron monitor). Observational data are collected at world data centers and are made available to the scientific community at large.