Sub-Auroral Ionospheric and Stratospheric Electric Field Responses to an Extreme Solar Energetic Particle Event

This paper will report on the global effects of one of the most extreme space weather events of the spacecraft era. On January 20th, 2005, a solar energetic particle (SEP) event caused the largest recorded ground level event since 1956. Serendipitously, a balloon-borne experiment intended to measure relativistic electron precipitation and its effects was aloft over Antarctica (~32 km; near 70º S, 345º W geographic) throughout the duration of the SEP event, including the fast (~6 minute) onset. The balloon instrumentation included an x-ray scintillation counter, dc electric field, and scalar electrical conductivity sensors. Intense worldwide energetic proton precipitation and large increases in the energetic proton population of the outer radiation belts were observed by a global array of observatories and spacecraft. 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 hours. Decreases to near zero of both the vertical and horizontal electric field components were observed in conjunction with the increase in particle flux at SEP onset. Combined with an atmospheric electric field mapping model, these data are consistent with a shorting out of the global electric circuit and point toward substantial ionospheric convection modifications. 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. This paper will summarize the current interpretation of the electric field observations. First, it is shown that the conductivity profile predicted by the model does not shield the balloon payload at 32 km from the ionospheric horizontal field. Thus, the data really do indicate a very low level of ionospheric convection over the balloon during the 6 hours following the SEP event. Second, we have tried to use the conductivity model and AMIE model outputs to interpret the changes in the vertical field as indicators of large scale convection changes. The results indicate that we have not yet understood all of the physics responsible for the observations.