Abstract
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)
Highlights
In the 1940’s and 1950’s the emissions of solar protons associated with solar flares, as particles in many cases, leave the Sun within a few minutes after the solar flare’s maximum (Cane et al, 2002)
The role of Coronal Mass Ejections (CMEs) in producing major solar proton emission was emphasised to the point that a small group of solar particle physicists consider major solar proton events (SPE) to be completely independent of flares (Kahler, 2001; Reames, 1999)
They are associated with large solar flares (X1 and M9.9) originating from the Sun’s western hemisphere and with high CME speeds
Summary
In the 1940’s and 1950’s the emissions of solar protons associated with solar flares, as particles in many cases, leave the Sun within a few minutes after the solar flare’s maximum (Cane et al, 2002). Routine monitoring of ionospheric absorption is possible since the riometric technique was introduced (Little and Leinbach, 1959) This instrument measures the amount of cosmic noise absorbed by the ionosphere at operating frequencies in the range 20–50 MHz. During daytime when PCA event ionisation conditions exist, the dependence of electron density Ne on ionisation rate q is relatively simple; an excellent correlation has been observed between simultaneous measurements of HF absorption and proton fluxes. During daytime when PCA event ionisation conditions exist, the dependence of electron density Ne on ionisation rate q is relatively simple; an excellent correlation has been observed between simultaneous measurements of HF absorption and proton fluxes This prompted the development of empirical formulas connecting these two quantities for estimating one from the other when only a single measurement is available.
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