COMPOSITIONS OF ENERGETIC PARTICLE-POPULATIONS IN INTER-PLANETARY SPACE

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. Copyright 1979 by the American Geophysical Union.