Dynamics of the low‐altitude energetic proton fluxes beneath the main terrestrial radiation belts

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Abstract
At the interface between the upper atmosphere and the radiation belt region there exists a secondary radiation belt consisting mainly of energetic ions that have become neutralized in the ring current and in the main radiation belt and then re‐ionized by collisions in the inner exosphere. The time history of the proton fluxes in the 0.64–35 MeV energy range was traced in the equatorial region beneath the main radiation belts during the 3‐year period from February 21, 1984, to March 26, 1987, using data obtained with the High‐Energy Particle experiment on board the Japanese OHZORA satellite. During most of this period a fairly small proton flux of ∼1.2 cm−2 s−1 sr−1 was detected on geomagnetic field lines in the range 1.05 < L < 1.15. We report a few surprisingly deep and rapid flux decreases (flux reduction by typically 2 orders of magnitude). These flux decreases were also long in duration (lasting up to 3 months). We also registered abrupt flux increases, such that magnitude of the proton flux enhancements could reach 3 orders of magnitude and with an enhancement duration of 1–3 days. Possible reasons for these unexpected phenomena are discussed.