Physics
Scientific paper
Feb 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004jastp..66..323l&link_type=abstract
Journal of Atmospheric and Solar-Terrestrial Physics, Volume 66, Issue 3-4, p. 323-331.
Physics
13
Space Weather, Coronal Mass Ejections, Magnetic Clouds, Geomagnetic Storms
Scientific paper
Magnetic clouds are surveyed in interplanetary magnetic field (IMF) and solar wind data from ACE, WIND, ISEE3, and IMP8 spacecraft. The annual frequency distribution of the bipolar magnetic clouds combined with PVO results by Mulligan et al. (Geophys. Res. Lett. 25 (1998) 2959) shows that the occurrence of SN magnetic clouds prevails over NS magnetic clouds in an odd solar cycle, and the reverse is true for an even solar cycle. The prevailing polarity cases decrease in number towards the solar minimum, while the secondary polarity clouds start to increase in number, only becoming predominant after the later part of the declining phase. Therefore, the predominance of the magnetic cloud polarity reverses within the later part of the declining phase near the solar minimum, but does not coincide with either the solar minimum when the new polarity sunspots begin to emerge or the solar maximum when the large scale solar polar field reverses. The annual frequency distribution of the total number of bipolar magnetic clouds and total number of unipolar magnetic clouds is not well ordered by the solar cycle. Perhaps both solar polar field cycle and the Hale active region polarity cycle may affect the magnetic cloud polarity. Magnetic clouds from all four spacecraft during 1978-2002 are evaluated for their geoeffectiveness. Overall statistics show that NS and SN magnetic clouds are equally geoeffective. We also found that both the cloud portion and sheath portion during the magnetic cloud period are important in causing geomagnetic storms. The magnetic cloud disturbance periods are the primary cause of one-third of the total number of geomagnetic storms analyzed, but are responsible for the most intense storms.
Li Yan
Luhmann Janet
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