The combination of electron and heavy-ion energy distributions that make up category-2, suggestive of a downward current and category-4 with both up-going electrons and heavy-ions, is only considered for those electron precipitation signatures that have electron energy distributions with a significant asymmetry. For these signatures, up-going electrons and heavy-ions are the most common combination, while the combination of up-going electrons and down-going heavy-ions occur almost half as often. Even after discounting upward net flux of electrons from those signatures with isotropic electron energy distribution, category-4 still make up the second largest group and when added together with category-2 occur on 10% of MEX orbits.
MEX orbits
category-4 still make up the second largest group and when added together with category-2
10%
%