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Ann. Geophys., 22, 1633-1648, 2004
www.ann-geophys.net/22/1633/2004/
© European Geosciences Union 2004
Polar cap absorption events of November 2001 at Terra Nova Bay, Antarctica
L. Perrone, L. Alfonsi, V. Romano, and G. de Franceschi
Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy
Abstract. Polar cap absorption (PCA) events recorded during November 2001 are investigated by
observations of ionospheric absorption of a 30MHz 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 >10MeV. 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 (~20dB) 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 84–500MeV protons in the case of the 4 November event and by 15–44MeV
protons for the event of 23 November, whereas in the entire event the contribution to the
absorption is due chiefly to 4.2–82MeV (4 November) and by 4.2–14.5MeV (23
November). Good agreement is generally obtained between observed and calculated
absorption by the empirical flux-absorption relationship for threshold energy E0=10MeV.
From the residuals one can argue that other factors (e.g. X-ray increases and geomagnetic
disturbances) can contribute to the ionospheric absorption.
Key words. Ionosphere (Polar Ionosphere, Particle precipitation)
– Solar physics (Flares and mass ejections)
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