Upstream energetic ions and electrons: Bow shock‐associated or magnetospheric origin?

Abstract

We have analyzed 35 proton bursts observed with the Max‐Planck‐Institut/University of Maryland sensor system on ISEE 3 far upstream of the earth's bow shock. These upstream bursts fall into two distinctive groups: the first group is accompanied by energetic electrons (≳75 keV) and the proton spectrum extends up to energies ≳300 keV and higher and bends over towards lower energies (≲30 keV). The second group, which is unaccompanied by energetic electron bursts, exhibits spectra which can be represented very well by exponentials in energy with a mean e‐folding energy of ∼15 keV. We have supplemented these data with measurements obtained with an identical sensor system on ISEE 1 close to the bow shock or within the geomagnetic tail. Whenever ISEE 1 is upstream of the bow shock and ISEE 3 observes a burst of group 2, ISEE 1 observes diffuse upstream ions. When ISEE 3 observes bursts of group 1, ISEE 1 observes either no bursts, diffuse ions, or highly anisotropic distributions. It is suggested that group 1 is of magnetospheric origin, while group 2 is bow shock‐associated.