Polar cap absorption events of November 2001 at Terra Nova Bay, Antarctica

Abstract

Polar cap absorption (PCA) events recorded during November 2001 are investigated by observations of ionospheric absorption of a 30 MHz riometer installed at Terra Nova Bay (Antarctica), and of solar proton flux, monitored by the NOAA-GOES8 satellite in geo-synchronous orbit. During this period three solar proton events (SPE) on 4, 19 and 23 November occurred. Two of these are among the dozen most intense events since 1954 and during the current solar cycle (23rd), the event of 4 November shows the greatest proton flux at energies > 10 MeV. Many factors contribute to the peak intensity of the two SPE biggest events, one is the Coronal Mass Ejection (CME) speed, other factors are the ambient population of SPE and the shock front due to the CME. During these events absorption peaks of several dB (∼20 dB) are observed at Terra Nova Bay, tens of minutes after the impact of fast halo CMEs on the geomagnetic field. Results of a cross-correlation analysis show that the first hour of absorption is mainly produced by 84-500 MeV protons in the case of the 4 November event and by 15-44 MeV protons for the event of 23 November, whereas in the entire event the contribution to the absorption is due chiefly to 4.282 MeV (4 November) and by 4.2-14.5 MeV (23 November). Good agreement is generally obtained between observed and calculated absorption by the empirical flux-absorption relationship for threshold energy E0=10 MeV. From the residuals one can argue that other factors (e.g. X-ray increases and geomagnetic disturbances) can contribute to the ionospheric absorption. © European Geosciences Union 2004.