Atmosphere-ionosphere conductivity enhancements during a hard solar energetic particle event

On 20 January 2005, a solar energetic particle (SEP) event caused the largest recorded solar proton ground level event since 1956. Serendipitously, a balloon-borne experiment intended to measure effects of relativistic electron precipitation was aloft over Antarctica (∼32 km; near 70°S, 345°E geographic) throughout the duration of the SEP event, including the fast (∼6 min) onset. The balloon instrumentation included dc electric field and scalar electrical conductivity sensors. The observed conductivity increased by nearly a factor of 20 above ambient with the SEP event onset and returned to within a factor of two above normal levels within 17 h. Results from a newly developed, globally applicable atmosphere–ionosphere conductivity model based on the Sodankylä Ion and Neutral Chemistry (SIC) model suggest that proton-induced ionization was directly responsible for the observed conductivity increase at the balloon. Model input for this event included estimates of ionization from energetic particle precipitation and rigidity cutoffs. Altitudes between 20 and 150 km were considered during model runs. The results show a maximum conductivity increase near 60 km of more than 600-fold directly after SEP event onset. Relatively small conductivity enhancements (two- to fivefold) are suggested to have occurred above 70 km as a result of SEP ionization, while almost no enhancement is thought to have occurred above 95 km. These results quantify the real effect that an SEP-event can have on atmosphere–ionosphere electrical conductivity on a large, nearly global scale and provide a detailed comparison to one of the few direct stratospheric conductivity observations made during an SEP event.

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