Physics
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufmsa31a..07a&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #SA31A-07
Physics
2162 Solar Cycle Variations (7536), 2169 Sources Of The Solar Wind, 2788 Storms And Substorms, 7524 Magnetic Fields
Scientific paper
Since the drag on the low altitude satellites orbiting the earth (and mission lifetime) depends on the level of solar activity, as measured by the sunspot numbers (SSNs), it is important that the forecasts be reliable and made sufficiently ahead of time to allow for a proper "planning." Following Ohl's (1966) lead, it is now recognized that "precursor" methods based on the temporal variations of the planetary index Ap (or its equivalent aa index) provide a superior forecast of SSNs at the cycle maximum (Rm), albeit they have a bias towards predicting higher values of Rm (Brown, 1992). In our opinion, this bias arises from an interpretation (by some researchers) of Ohl's recipe to look for an enhanced activity in the declining phase of the "old" cycle; for example, a late Ap peak that occurs just before the onset of the new cycle. This propensity led to numerous forecasts implying that cycle 23 will be more active than cycle 22; one of them predicted that cycle 23 activity might exceed that observed for cycle 19, which was the most active cycle ever in the history of SSN observations (Wilson, 1992).A new forecasting procedure was proposed by us based on our discovery that the annual mean minimum values of Ap (which typically occur one year after SSN minima) have embedded within them a quasiperiodicity of three cycles and they are linearly correlated with the amplitude of the smoothed SSN cycle (defined as the numeric difference between the maximum and minimum SSN values). Our procedure predicted that cycle 23 will be moderate (a la cycle 17) and its developmental time line will lie well below those for the most active cycles of the twentieth century (18, 19, 21, 22). As of the end of August 2001, our forecast appears to be right on the mark. Important lessons learned from this experience will be described and discussed.
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