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| Titel |
On solar protons and polar cap absorption: constraints on an empirical relationship |
| VerfasserIn |
A. J. Kavanagh, S. R. Marple, F. Honary, I. W. McCrea, A. Senior |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
0992-7689
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| Digitales Dokument |
URL |
| Erschienen |
In: Annales Geophysicae ; 22, no. 4 ; Nr. 22, no. 4 (2004-04-02), S.1133-1147 |
| Datensatznummer |
250014828
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| Publikation (Nr.) |
 copernicus.org/angeo-22-1133-2004.pdf |
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| Zusammenfassung |
A large database of Solar Proton Events (SPE) from the period 1995 to 2001 is used to
investigate the relationship between proton flux at geostationary orbit and Cosmic Noise
Absorption (CNA) in the auroral zone. The effect of solar illumination on this
relationship is studied in a statistical manner by deriving correlation coefficients of
integral flux and absorption as a function of solar zenith angle limit, thus both an upper
limit on the angle and the best correlated integral flux of protons are determined (energies
in excess of 10MeV). By considering the correlation of various energy ranges (from the
GOES 8 differential proton flux channels) with CNA the range of proton energies for
which the relationship between flux and absorption is best defined is established
(15 to 44MeV), thus confirming previous predictions about which proton energy ranges are most
effective in giving rise to absorption during Polar Cap Absorption (PCA) events. An
empirical relationship between the square root of the integral proton flux and the
absorption, measured by the imaging riometer at Kilpisjärvi (IRIS), is determined and
departures from linearity and possible causes are examined. Variations in spectral
"hardness" and in collision frequency are demonstrated not to be significant causes of the
observed departures from a linear relationship. Geomagnetic activity may be a
significant factor in changing the relationship between the absorption and the square root
of the integral proton flux, although it is concluded that the cause is likely to be more
complex than a straightforward dependence on Kp. It is suggested that the most
significant factor might be a bias in the absorption estimates imposed by the presence of
Solar Radio Emission (SRE), which is not routinely measured at the operating frequency
of IRIS, making its precise effect difficult to quantify. SRE is known to be most prevalent
under conditions of high solar activity, such as those that might give rise to solar proton
events.
Key words. Ionosphere (particle precipitation; solar radiation
and cosmic ray effects; polar ionosphere) |
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