High‐latitude proton precipitation and light ion density profiles during the magnetic storm initial phase

Measurements of precipitating protons and light ion densities by experiments on Ogo 4 indicate that widespread proton precipitation occurs in predawn hours during the magnetic storm initial phase from the latitude of the high‐latitude ion trough, or plasmapause, up to Λ > 75°. A softening of the proton spectrum is apparent as the plasmapause is approached. The separation of the low‐latitude precipitation boundaries for 7.3‐kev and 23.8‐kev protons is ≲1°, compared with a 3.6° separation that has been computed by using the formulas of Gendrin and Eather and Carovillano. Consideration of probable proton drift morphology leads to the conclusion that protons are injected in predawn hours, widespread precipitation occurring in the region outside the plasmapause. Protons less energetic than ∼7 kev drift eastward, whereas the more energetic protons drift westward, producing the observed dawn‐dusk asymmetry for the lower‐energy protons.