Mathematics – Probability
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmsh43c..01m&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #SH43C-01
Mathematics
Probability
2102 Corotating Streams, 2134 Interplanetary Magnetic Fields, 2162 Solar Cycle Variations (7536), 2164 Solar Wind Plasma, 2784 Solar Wind/Magnetosphere Interactions
Scientific paper
During the declining phase of the solar cycle the evolution of the solar magnetic field produces large regions of unipolar magnetic field that create coronal holes and are the sources of high speed solar wind streams. These streams overtake and interact with slow solar wind emitted from the equatorial belt of closed magnetic field lines. These interaction regions corotate (CIR) with the Sun and sweep across the Earth once per solar rotation. Inside a CIR the properties of the solar wind are well organized by time relative to the interface between the two streams (epoch time). We have compiled lists of stream interfaces in solar cycle #22 (1994- 1996) and in cycle #23 (2003-2005) and used them to study the systematic properties of the solar wind and interplanetary magnetic field at 1 AU. We present the results as dynamic cumulative probability distributions (cdf) for different variables and as traces of the quartiles of these distributions as function of epoch time. We find that the solar wind is highly organized relative to the stream interface and therefore that geomagnetic activity driven by this wind is organized as well. This systematic behavior provides the basis for probabilistic forecasting by air mass climatology. If one can predict the arrival of a stream interface then within certain limits one can predict the probability that various measures of geomagnetic activity will lie within a given range. We also find that the climatology of the solar wind as measured at the Earth has a semiannual variation as a consequence of two different effects: the axial effect that causes the Earth to be at high or low heliographic latitude near equinoxes and the Rosenberg-Coleman (R-C) effect which states that the fraction of time the Earth samples a particular polarity of the IMF is dominated by the polar magnetic field of the Sun emanating from the hemisphere in which the Earth is located. A third phenomenon the Russell-McPherron (R-M) effect is important for the Earth and geomagnetic activity. In GSM coordinates the azimuthal component of the IMF (By) will have a negative projection on the Z-axis according to the rule "Spring to Fall away". A negative projection allows magnetic reconnection between the IMF and the Earth's magnetic field. The R-M effect in combination with the R-C and axial effects leads to the semiannual variation of geomagnetic activity. Another important effect is the 22-year Hale cycle of the solar magnetic field. Each solar cycle the dipole moment of the Sun reverses shortly after solar maximum. This has the effect of reversing the sector structure of the IMF at the Earth reversing the sign of the projection of By and hence decreasing the probability of reconnection. This makes the declining phase of even solar cycles (1995) slightly more geoeffective that of odd cycles (2006). Finally there is evidence that there is a real difference in the nature of solar magnetic activity between odd and even cycles further influencing the properties of the solar wind at 1 AU.
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