The different populations of backstreaming ions found earlier with ISEE instruments outside the earth's bow shock were organized by the orientation of the IMF to the shock in the plane containing BSW and the solar ecliptic X axis (the B-X plane). The associations of ion classes with field geometry and shock structure are shown by means of computer-drawn three-dimensional sketches. Particles of several keV, or about 10 times the solar wind streaming energy, left the shock as ‘reflected’ beams in a guiding center direction determined by BSW and the interplanetary drift velocity. Their area of departure was the part of the shock where the transition from quasi-perpendicular to quasi-parallel structure began. Slightly farther toward the quasi-parallel portion of the shock, particles of lower energy escaped upstream, widening the ion spectra, exciting small magnetic waves, and forming a class of ‘intermediate’ ions. Finally, in direct connection with the quasi-parallel structure, ions leaving the shock formed the ‘diffuse’ group, containing a wide distribution of energies including high values well beyond any recorded in the other two classes. Sources of the various ion groups are postulated.
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