Nonthermal electrons and high-frequency waves in the upstream solar wind, 2. Analysis and interpretation

A streaming instability caused by suprathermal electrons of bulk energy ∼1 kev and random widths from ∼60 ev to 4 kev is investigated as a model to explain the Ogo 5 observations of electric-field fluctuations with frequencies at or near the local electron plasma frequency upstream from the earth’s bow shock. In a previous paper, it was shown that the waves appeared in direct correlation with enhanced anisotropic electron fluxes having Ee≳700 ev. Here it is shown that the streaming instability produced by this type of suprathermal electron beam leads to a wave frequency nearly equal to the local electron plasma frequency, whereas the growth rates are still significant even if the beam thermal speed becomes comparable to the bulk speed.