Abstract
Abstract. It is shown that the monthly smoothed sunspot number (SSN) or its rate of decrease during the final years of a solar cycle is correlated with the amplitude of the succeeding cycle. Based on this relationship, the amplitude of solar cycle 24 is predicted to be 84.5±23.9, assuming that the monthly smoothed SSN reached its minimum in December 2008. It is further shown that the monthly SSN in the three-year period from 2006 through 2008 is well correlated with the monthly average of the intensity of the interplanetary magnetic field (IMF). This correlation indicates that the SSN in the final years of a solar cycle is a good proxy for the IMF, which is understood to reflect the magnetic field in the corona of the sun, and the IMF is expected to be smallest at the solar minimum. We believe that this finding illuminates a physical meaning underlying the well-known precursor method for forecasting the amplitude of the next solar cycle using the aa index at the solar minimum or its average value in the decaying phase of the solar cycle (e.g. Ohl, 1966), since it is known that the geomagnetic disturbance depends strongly on the intensity of the IMF. That is, the old empirical method is considered to be based on the fact that the intensity of the coronal magnetic field decreases in the late phase of a solar cycle in parallel with the SSN. It seems that the precursor method proposed by Schatten et al. (1978) and Svalgaard et al. (2005), which uses the intensity of the polar magnetic field of the sun several years before a solar minimum, is also based on the same physical phenomenon, but seen from a different angle.
Highlights
Many estimates concerning the size of the approaching solar cycle have been performed using different methods
We further show that the monthly average sunspot number (SSN) in the late stage of a solar cycle is a good proxy for the intensity of the interplanetary magnetic field (IMF), and discuss the implications of this observation
This indicates that the monthly smoothed SSN in the late years of the decaying phase is a good proxy for the monthly average of the intensity of the IMF, which is thought to reflect the magnetic field in the corona of the sun, and the IMF should be smallest at the solar-cycle minimum
Summary
Many estimates concerning the size of the approaching solar cycle have been performed using different methods. The method is based on our finding that the rate of decrease of the SSN or the monthly smoothed SSN in the late years of the solar cycle is correlated with the amplitude of the succeeding cycle. Schatten (2005) performed a similar prediction by reexamining polar field data and using proxies of the sun’s polar field, such as soft X-rays from coronal holes and polar faculae, and forecasted fair space weather for cycle 24, meaning the amplitude will not be large, with a smoothed SSN of about Another well-known precursor method developed by Ohl and his group (Ohl, 1966) uses the relationship between the minimum of geomagnetic activity (where the aa index is usually used for this purpose) and the sunspot maximum of the cycle. We explain the reason why the empirical methods based on the geomagnetic activity tend to give large values
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