Modeling a large solar proton event in the southern polar atmosphere

We have modeled the effects of the Sodankylä Ion Chemistry model (SIC) electron density profiles on VLF propagation across the southern polar region during the first few days of the 4–10 November 2001 solar proton event (SPE). The results show that the SIC model is accurately reproducing the changes in ionization during the SPE. These results were obtained by approximating the SIC electron density profiles to the Wait β and h′ profiles where the densities were below 1000 el cm−3, a limitation that means during SPEs the technique is typically sensitive in the altitude range 50–60 km. The calculated values of β and h′ were applied to the part of the propagation path poleward of the L = 4 boundary for the Hawaii (NPM)–Halley great circle path. Comparing the change in amplitude of NPM at Halley during the SPE with the GOES satellite proton flux measurements, we observe a good correlation and thus conclude that the variability observed in the VLF data is primarily caused by >50 MeV proton fluxes. This suggests that the SPE produced ionization dominates all other precipitation sources at these altitudes during 4 and 5 November 2001. Consequently, these results suggest that the assumption made during the SIC modeling runs of 4–5 November 2001 of only having proton precipitation and no significant energetic electron precipitation was reasonable. Our work strongly suggests that VLF subionospheric propagation is a reliable tool for the study of SPEs and that it is particularly effective when used in conjunction with an atmospheric model such as SIC.

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