Cosmic Ray Data

There are many possible mechanisms that can accelerate particles up to the CR range of energy. When the acceleration process is associated with solar flares and/or Coronal Mass Ejections (CMEs), the CR particles released to the interplanetary medium are called Solar Cosmic Rays (SCRs) or Solar Energetic Particles (SEPs). This phenomenon can cause Solar Radiation Storms (measured, studied and categorized by NOAA). The flux and composition of SEPs is strongly variable; energies are generally below 1 GeV, rarely reaching 10 GeV. 

CR Flux

The CRs with energies above 10 GeV measured near Earth originate from outside of the Solar System, either in our Galaxy or outside of it. These are called Galactic Cosmic Rays (GCRs). Both GCR and SEP compositions have a major contribution of fully ionized protons and alpha-particles with a smaller, but important, contribution of heavier nuclei.

Since the primary CR flux is composed of charged particles, it is expected that any magnetic irregularity and/or turbulence affects their large-scale transport and consequently their energy spectrum. Through its journey to Earth, the CR flux is modulated inside the Heliosphere by the Interplanetary Magnetic Field (IMF) and solar wind conditions, which vary according to the Solar Cycle Progression. Near Earth, the CR fluxes are almost isotropic but the CR particle’s trajectories will be affected by Lorentz forces due to the geomagnetic field.  This results in a latitudinal-dependency from their interaction with our atmosphere. The portion of the CR spectrum that reaches the Earth's atmosphere is controlled by the geomagnetic cutoff which varies from a minimum (theoretically zero) at the magnetic poles, to a vertical cosmic ray cutoff of about 15 GV (ranging from 13 to 17) in the equatorial regions. (Note,  a GeV is a unit of energy, and GV is a unit of magnetic rigidity.)

Using ground-based, or underground detectors, or deploying balloons for sounding, different devices can be used to measure each component of the CR’s cascade from underground to high altitude. The near ground measurements of the flux of secondary CR particles from the cascade provides information concerning the different modulation processes suffered by the primary CR during their transport in the heliosphere, magnetosphere and atmosphere, and also the journey of the secondary CR particle. This is why CR particles are considered “Celestial Messengers.”

Neutron Monitors

In order to detect the nucleonic component at the Earth’s surface, a proportional gas-filled counter called a Neutron Monitor (NM) is used. The International Geophysical Year (IGY), in 1957, and the International Quiet Sun Year (IQSY), in 1964, marked the implementation of a global network of standardized NM stations.

Today, most of the data from past and current worldwide NM stations can be found at the Neutron Monitor Database website (NMDB) and at  WDC-CR : World Data Center for Cosmic Rays, Geospace Research Center, Solar-Terrestrial Environment Laboratory, Nagoya University.

Forbush Decrease and GLEs

Forbush Decreases are abrupt decreases of the background galactic cosmic ray intensity as observed near ground typically by neutron monitors. They are associated with major plasma and magnetic field enhancements in the solar wind at or beyond the Earth. These magnetic field enhancements deflect the background CR particles.  GLEs are sharp increases in the ground-level CR count to at least 10 percent above background, associated with solar protons of energies greater than 500 MeV. GLEs are relatively rare, occurring only a few times each solar cycle. When they occur, GLEs begin a few minutes after flare maximum and last for tens of minutes to hours. Intense particle fluxes at lower energies can be expected to follow this initial burst of relativistic particles.