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| Titel |
On the relationship between magnetic cloud field polarity and geoeffectiveness |
| VerfasserIn |
E. K. J. Kilpua, Y. Li, J. G. Luhmann, L. K. Jian, C. T. Russell |
| 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 ; 30, no. 7 ; Nr. 30, no. 7 (2012-07-13), S.1037-1050 |
| Datensatznummer |
250017242
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| Publikation (Nr.) |
 copernicus.org/angeo-30-1037-2012.pdf |
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| Zusammenfassung |
| In this paper, we have investigated geoeffectivity of near-Earth magnetic
clouds during two periods concentrated around the last two solar minima. The
studied magnetic clouds were categorised according to the behaviour of the
Z-component of the interplanetary magnetic field (BZ) into bipolar
(BZ changes sign) and unipolar (BZ maintains its sign) clouds. The
magnetic structure of bipolar clouds followed the solar cycle rule deduced
from observations over three previous solar cycles, except during the early
rising phase of cycle 24 when both BZ polarities were identified almost
with the same frequency. We found a clear difference in the number of
unipolar clouds whose axial field points south (S-type) between our two study
periods. In particular, it seems that the lack of S-type unipolar clouds
contributed to relatively low geomagnetic activity in the early rising phase
of cycle 24. We estimated the level of magnetospheric activity using a Dst
prediction formula with the measured BZ and by reversing the sign of
BZ. We found that bipolar clouds with fields rotating south-to-north (SN)
and north-to-south (NS) were equally geoeffective, but their geoeffectiveness
was clearly modified by the ambient solar wind structure. Geoeffectivity of
NS-polarity clouds was enhanced when they were followed by a higher-speed
solar wind, while the majority of geoeffective SN-polarity clouds lacked the
trailing faster wind. A leading shock increased the geoeffectiveness of both
NS- and SN-polarity clouds, in particular, in the case of an intense storm.
We found that in 1995–1998, SN-polarity clouds were more geoeffective, while
in 2006–2011 NS-polarity clouds produced more storms. A considerably larger
fraction of events were trailed by a higher-speed solar wind during our
latter study period, which presumably increased geoeffectivity of
NS-polarity. Thus, our study demonstrates that during low and moderate solar
activity, geoeffectivity of opposite polarity bipolar clouds may depend
significantly on the surrounding solar wind structure. In addition, different
polarities also give different temporal storm evolutions: a storm from an
SN-polarity cloud is expected to occur, on average, half-a-day earlier than a
storm from an NS-polarity cloud. |
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