Compositions of energetic particle populations in interplanetary space

Abstract

Although the study of the composition of low‐energy particles in the interplanetary medium is a relatively new subject, our knowledge in this field has been advancing rapidly. Discoveries of pronounced anomalies in the compositions of solar flare particles and low‐energy “cosmic‐rays”, and of large‐scale interplanetary acceleration regions beyond several AU have been followed by more systematic investigations aimed at characterizing each of the components of the interplanetary energetic particle population. The discoveries may be attributed to a combination of three favorable circumstances: (a) availability of sophisticated instruments capable of performing composition measurements at low energies, (b) properly instrumented deep space probes exploring the outer and inner heliosphere, and (c) a favorable time period in the solar activity cycle during which the discovered phenomena were especially prominent. Composition measurements not only have played a key role in the initial discoveries, but have also contributed significantly to our understanding of the acceleration processes responsible for the production of energetic particles which populate interplanetary space.

Life under Conditions of Ionizing Radiation

“…They are composed predominantly of protons with a minor contribution from helium ions (~10%) and an even smaller part (1%) heavy ions and electrons [7]. SPEs develop rapidly and generally last for no more than some hours, however some proton events may continue over several days.…”

“…The space ionizing radiation environment of our galaxy [3][4][5][6][7][8][9] is dominated by energetic, highly penetrating ions and nuclei. These particles constitute the primary radiation hazard for life in space.…”

The Acceleration and Propagation of Solar Flare Energetic Particles

“…Spacecraft observations (e.g. Van Hollebeke 1979, Gloeckler 1979 can determine the energy spectra of the various particle species including electrons, the abundances of essentially all elements from H through Ni, and isotopic and ionic abundances for a few abundant elements.…”

Predicting and Modeling Solar Flare Generated Proton Fluxes in the Inner Heliosphere

“…There are few predictors of the slope of the solar proton spectrum. Some of the early work attempting this prediction from the maximum and minimum flux values in the radio peak power spectrum (Bakshi and Barron 1974, 1975, 1979 has not proven extremely reliable. The statistical tendencies of the exponent of the proton energy spectral slope noted by Van Hollebeke et al (1975) shown in Figure 7 have demonstrated usefulness.…”