Cosmic rays

spaceweb@oulu.fi - last update: 1 December 1998, 0000 UT (RR)

Introduction

The high energy particles (mainly protons and alpha particles) found in the interplanetary space constitute what is called cosmic rays. There are three different types of cosmic rays (Mewaldt et al., 1994):

• galactic cosmic rays (GCR) originating far outside our solar system
• solar energetic particles (SEP) originating from the flares of our Sun
• "anomalous cosmic rays" (ACR) originating from the interstellar space beyond the heliopause

Galactic cosmic rays (GCR)

The source of the very energetic (GeV range) galactic cosmic rays is not known. Occasionally (one event per square kilometer per century) one measures energies as high as 10**20 eV (Schwarzschild, 1997), and no one knows yet how this can be achieved. One possibility is that shock waves from expanding supernovas are doing the trick.

GCR are the most typical cosmic rays, and their flux in the solar system is modulated by the solar activity: enhanced solar wind shields the system from these particles (Forbush, 1954; for more about the history, see Simpson, 1994). The effect is clearly seen from the cosmic ray measurements covering several solar cycles (see also global change). Quite interesting fact is that also the 22-year modulation is seen in the recovery of GCR intensities: during even cycles it is completed much faster than during odd cycles (Ahluwalia, 1979, 1980).

GCR form also one of the radiation belts.

Solar energetic particles (SEP)

The particles of tens of MeV energy originating from solar flares are causing, e.g., the polar cap absorption (PCA) events measured by riometers (while ten times more energetic particles can also be seen by cosmic ray detectors).

Anomalous cosmic rays (ACR)

The anomalous cosmic rays are most likely produced by neutral atoms in the interstellar space, which

• 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, and
• ...diffuse and drift into the inner heliosphere as cosmic rays

The anomalous cosmic rays differ from others by their composition. While in GCRs and SEPs there are much more protons than helium, and equal amounts of oxygen and carbon, in ACRs there are more helium than protons, and much more oxygen than carbon. This can be explained with the production scheme outlined above, which selects only those elements that are predominantly neutral in the interstellar medium: it is known that the first ionization potential of such atoms as oxygen and helium is high. It is thus possible to obtain information about the relative abundances of neutral atoms in the local interstellar medium by studying ACRs. Other difference is that ACR are singly charged, while GCRs have been stripped of their electrons during their millions year passage through the galaxy. Also, the energy of ACRs does not reach the highest values found in GCRs.

The anomalous cosmic rays are also creating their own radiation belt inside the created by GCRs.

Harmfull effects

Cosmic rays are creating the same, harmfull effects in near space around the Earth as are the radiation belts and, in addition, affect radio communication in polar regions (see also space weather).

References

• Ahluwalia, H. S., Eleven year variation of cosmic ray intensity and solar polar field reversals, Conf. Pap. Int. Cosmic Ray Conf. 16th, 12, 182-189, 1979.
• Ahluwalia, H. S., Solar polar field reversals and secular variations of cosmic ray intensity, in Solar and Interplanetary Dynamics, edited by M. Dryer and E. Tanderg-Hanssen, D. Reidel, pp. 79-89, Norwell, Mass., 1980.
• Forbush, S. E., Worldwide cosmic ray variations, 1937-1952, J. Geophys. Res., 59, 525-542, 1954.
• Mewaldt, R. A., A. C. Cummings, and E. C. Stone, Anomalous cosmic rays: Interstellar interlopers in the heliosphere and magnetosphere, EOS, 75, Number 16, 1994.
• Schwarzschild, B., Auger project seeks to study highest energy cosmic rays, Physics Today, February 1997, pp. 19-21, 1997.
• Simpson, J. A., A physicist in the world of geophysics and space, J. Geophys. Res., 99, 19159-19173, 1994.

See also

• ISPEC page on cosmic rays
• Sun and interplanetary models at NSSDC

and

• Space Physics Text Book of Oulu