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| Titel |
Observations of recurrent cosmic ray decreases during solar cycles 22 and 23 |
| VerfasserIn |
P. Dunzlaff, B. Heber, A. Kopp, O. Rother, R. Müller-Mellin, A. Klassen, R. Gómez-Herrero, R. Wimmer-Schweingruber |
| 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 ; 26, no. 10 ; Nr. 26, no. 10 (2008-10-15), S.3127-3138 |
| Datensatznummer |
250016259
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| Publikation (Nr.) |
 copernicus.org/angeo-26-3127-2008.pdf |
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| Zusammenfassung |
| During solar cycle 22, the modulation of several hundred MeV galactic cosmic
rays (GCRs) by recurrent and transient cosmic ray decreases was observed by
the Ulysses spacecraft on its descent towards the solar south pole. In solar
cycle 23, Ulysses repeated this trajectory segment during a similar phase of
the solar cycle, but with opposite heliospheric magnetic field polarity. Since
cosmic ray propagation in the heliosphere should depend on drift effects, we
determine in this study the latitudinal distribution of the amplitude of
recurrent cosmic ray decreases in solar cycles 22 and 23. As long as we
measure the recurrent plasma structures in situ, we find that these decreases
behave nearly the same in both cycles. Measurements in the fast solar wind,
however, show differences: in cycle 22 (A>0) the recurrent cosmic ray
decreases show a clear maximum near 25° and are still present beyond
40°, whereas we see in cycle 23 (A<0) neither such a pronounced
maximum nor significant decreases above 40°. In other words: the
periodicity in the cosmic ray intensity, which can be clearly seen in the slow
solar wind, appears to vanish there. Theoretical models for drift effects,
however, predict quite the opposite behaviour for the two solar cycles. To
closer investigate this apparent contradiction, we first put the visual
inspection of the data onto a more solid basis by performing a detailed Lomb
(spectral) analysis. The next step consists of an analysis of the resulting
periodicities at 1 AU in order to distinguish between spatial and temporal
variations, so that we can obtain statements about the question in how far
there is a correlation between the in-situ data at 1 AU and those measured by
Ulysses at larger latitudes. We find a good correlation being present during
cycle 22, but not for cycle 23. As one potential explanation for this
behaviour, we suggest the difference in the coronal hole structures between
the cycles 22 and 23 due to a large, stable coronal hole structure, which is
present during cycle 22, but not in cycle 23. We support this possibility by
comparing Yohkoh SXT and SOHO EIT maps. |
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