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| Titel |
Spectrum analysis of short-period K index behaviour at high and mid-latitudes |
| VerfasserIn |
P. B. Kotzé |
| 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 ; 33, no. 1 ; Nr. 33, no. 1 (2015-01-09), S.31-37 |
| Datensatznummer |
250121145
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| Publikation (Nr.) |
 copernicus.org/angeo-33-31-2015.pdf |
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| Zusammenfassung |
| Geomagnetic activity levels during the declining phase and solar minimum
period of the solar cycle are considerably different from those during the
solar maximum phase. Previous studies revealed variations in the pattern of
recurrent activity from cycle to cycle as well as variations in the average
geomagnetic activity levels during a solar cycle. During the declining phase
of a solar cycle (and solar minimum), the solar and interplanetary causes of
geomagnetic activity are substantially different from those during the solar
maximum phase. Co-rotating fast solar wind streams originating from large
polar coronal holes, extending towards the Sun's equator, interact with the
Earth's magnetosphere, resulting in recurrent geomagnetic activity
particularly during solar cycle minimum periods. This is a well-known
phenomenon with respect to 27.0- and 13.5-day recurrence geomagnetic
activity, and it is well-known to be related to sectorial (non-axial) poloidal
magnetic field structure in the Sun. Published results of the recent
solar-cycle-23 minimum showed that the presence of 9.0- and 6.7-day recurrent
geomagnetic activities can be attributed to the sectorial spherical harmonic
structure present in the solar magnetic field. In this study we performed a
wavelet and Lomb–Scargle analysis of the geomagnetic activity K index at
Lerwick (LER), Hermanus (HER) and Canberra (CNB) for the period between 1960
and 2010, overlapping with solar cycles 20 to 23. Daily mean K indices are
used to identify how several harmonics of the 27.0-day recurrent period
change during each solar cycle when comparing high and mid-latitude
geomagnetic activity, applying a 95% confidence level. In particular the
behaviour of the second (13.5-day), third (9.0-day) and fourth (6.7-day)
harmonics are investigated by doing a wavelet analysis of each individual
year's K indices at each location. Results obtained show that particularly
during solar minima the 27.0-day period is no longer detectable above the
95% confidence level, and that geomagnetic activity is in fact dominated
by higher harmonics like 13.5-, 9.0- and 6.7-day periods. These findings in
fact are in line with previous investigations and confirm the results
obtained by researchers using other geomagnetic activity indices like \textit{aa} and
C9. The wavelet-spectrum analysis also reveals that during the downward
phase of cycle 23 and the very long minimum of 23–24 between 2002 and 2008,
the 27.0-day activity period drops below the 95% confidence level. This
is confirmed by Lomb–Scargle analyses of every year's K index activity.
Results obtained in this study support evidence by other investigations that
this can be attributed to the lack of coronal-mass ejection (CME)-dominated solar activity during
solar minima, periods characterized by strong solar dipolar magnetic fields,
less sunspot numbers than at solar maxima, and multiple prominent
co-rotating solar wind streams present. This analysis further confirms
previous studies by other authors that the pattern of recurrent activity is
dictated by the configuration of coronal holes which give rise to related
high-speed streams during a solar cycle by analysing K indices at both high-
and mid-latitude magnetic observatories. |
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