Phenomena, Measurement, and Causes
Spots from multiple cycles can co-exist for some time, and since it was discovered that the sun reverses magnetic polarity from one solar half cycle to the next, spots from different cycles can be torn apart. However, it takes some months before a definite decision can be made as to the true date of solar minimum. One of the principal authorities that determine the date of the solar minimum is SIDC (the Solar Influences Data Analysis Center), which is located in Belgium and works with agencies such as NASA and ESA.
The most important information today comes from SOHO (a project of international cooperation between ESA and NASA), such as the MDI magneto gram, where the solar "surface" magnetic field can be seen.
The basic causes of the solar variability and solar cycles are still under debate, with some researchers suggesting a link with the tidal forces due to the gas giants Jupiter and Saturn, or due to the solar inertial motion. Another cause of sun spots can be solar jet stream "torsional oscillation".
Patterns have been noted in solar cycles. For example, the Waldmeier effect is the phenomenon that cycles with larger maximum amplitudes tend to take less time to reach their maxima than cycles with smaller amplitudes; there is also a negative correlation between maximum amplitudes and the lengths of earlier cycles, which allows a degree of prediction.
Read more about this topic: Solar Cycle