Time Variation of Galactic Cosmic Rays

Time variations in the flux of galactic cosmic rays are the result of changing conditions in the solar wind. Maximum cosmic ray fluxes, which occur when solar activity is at a minimum, are well defined. Reductions from this maximum level are typically systematic and predictable but on occasion are rapid and unexpected. Models relating the flux level at lower energy to tha t at neutron monitor energy are typically accurate to 20 percent of the total excursion at tha t energy. Other models! relating flux t o observables such as sunspot number, flare frequency, and current sheet tilt a re phenomenological bu t nevertheless can be quite accurate. Introduction. By definition, galactic cosmic rays are charged particles, electrons and nuclei. which occupy the local region of interstellar space. Unlike photons, the charged cosmic rays cannot travel in s t ra ight lines through the magnetic fields (typically one microgauss) which permeate the galaxy. Particles of energy less than about 10 GeV, which are the only ones numerous enough to contribute significantly to the radiation. background of a spacecraft , have radii of curvature in th i s field which are small compared to the dimensions of the solar system and tiny compared even to the distance t o the nearest star. Therefore even though these particles have speeds near t ha t of light their convoluted paths in the interstellar medium are such as to completely randomize their directions and smooth ou t any fluctuations in density. On the timescale of a human lifet ime it is inconceivable t h a t there could be any detectable t ime variation in. the fluxes of these particles. However, as Figure 1 shows. fluxes of galactic particles which penetrate into the solar system exhibit substant ia l time variations. Particles of different species and/or different energies show obviously related but nevertheless distinctly different variations. I t is my aim in this paper t o provide a brief and qualitative discussion of the causes of t ime variability of galactic cosmic radiation. I hope tha t I cause no offense by choosing not t o give references in the text. Instead I present a list of suggestions for further reading chosen in par t because they contain extensive references to the l i terature. Solar Modulation. The variability. or modulation, of the galactic cosmic ray fluxes was recognized some time ago as related to the general eleven year cycle of solar activity. hence the term "Solar hfodulation." Figure 1 shows approximately one complete cycle of this activity and illustrates the inverse nature of the relationship. Sunspots, solar flares, and the like were a t a maximum during the years 1980-1982 while the fluxes of cosmic rays were at a minimum. A naive explanation. t ha t the ex t ra "stuff" coming off the sun at solar maximum somehow drives the cosmic rays ou t of the solar system, is very close to the t ruth. Beyond this point however intuition fails and the details of the process are the subject of much scientific research and debate. Cosmic ray particle density and the particle density in the solar wind are so low tha t the particles almost never physically collide with one another. A11 of the 149 https://ntrs.nasa.gov/search.jsp?R=19890019098 2017-09-28T17:10:01+00:00Z