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Cosmic Rays, Solar Protons in the Near-Earth Environment and their Entry into the Magnetosphere

Galactic cosmic rays (GCR) originating far outside our solar system are the most typical cosmic rays. They arrive evenly from all directions in the sky. They are the origin of the inner radiation belt. Their flux is modulated by solar activity. Enhanced solar wind shields the solar system from these particles. Most agree that cosmic rays with extreme high energies (GeV) are probably energized by  shock waves which expand from supernovas (Pieces of the Cosmic Ray Puzzle and Recent Observations).

 

Cosmic Ray and Supernova Dust Credit: M. DeBord, R. Ramaty and B. Kozlovsky (GSFC), R. Lingenfelter (UCSD), NASA

Solar energetic particle Events (SEPs) or Solar Proton Events (SPE) are made up of particles with MeV energyies and above. A SPE can originate from either a solar flare or the shock wave driven by a coronal mass ejection (CME). The accelerated particles travel toward and away from the Sun along interplanetary magnetic field lines (IMF) in the solar wind. However, a significant fraction of these particles are trapped near the propagating shock due to wave-particle interactions. Thus, once the shock reaches Earth, the energetic proton flux can increase suddenly by as much as two orders of magnitude, making this shock spike the most dangerous part of the solar particle event itself.

NOAA's Space Environment Center (SEC) declares an SPE to be in progress when the dose rate of particles with energies above 10 MeV (i.e., space-suit-penetrating) exceeds 10 particles cm-2s-1sr-1 (directional flux) for more than 15 minutes. When this happens, SEC alerts the Space Radiation Analysis Group (SRAG) at the Johnson Space Center (JSC). If the dose rate of particles with energies above 100 MeV exceeds 1 particle cm-2s-1sr-1, an "energetic SPE" is declared. These events can last several days.

At the Earth, SPEs can lead to polar cap absorption (PCA) events, so-called because increased ionisation leads to radio wave absorption in the HF and VHF bands. In addition, cosmic ray detectors may detect up to 10 times more energetic particles during these periods.

Anomalous cosmic rays (ACR) originate from the interstellar space beyond the heliospause -

  • leak into the heliosphere
  • get ionized by solar UV radiation or charge exchange with the solar wind
  • are picked up by the solar wind and convected back to the outer heliosphere
  • are accelerated, e.g., by the solar wind termination shock
  • diffuse and drift into the inner heliosphere as cosmic rays

Some Cosmic-Ray Web-Pages


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