Two variations on this method circumvent the timing problem. Feynman (1982) noted that geomagnetic activity has two different sources – one due to solar activity (flares, CMEs, and filament eruptions) that follows the sunspot cycle and another due to recurrent high speed solar wind streams that peaks during the decline of each cycle (see Section 3.7 and Figures 18 and 19). She separated the two by finding the sunspot number dependence of the base level of geomagnetic activity and removing it to reveal the “interplanetary” component of geomagnetic activity. The peaks in the interplanetary component prior to sunspot cycle minimum are very good indicators for the amplitude of the following sunspot cycle as shown in Figure 40.

Hathaway and Wilson (2006) used a modification of this method to predict cycle 24. At the
time of that writing there was a distinct peak in aa_{I} in late 2003. This large peak led to a
prediction of R_{max}(24) = 160 ± 25. While this method does give predictions prior to sunspot number
minimum it is not without its problems. Different smoothings of the data give very different
maxima and different methods are used to extract the sunspot number component for the data
shown in Figure 39a. Feynman (1982) and others chose to pass a sloping line through the
two lowest points. Hathaway and Wilson (2006) fit a line through the 20 lowest points from
20 bins in sunspot number. These variations introduce significant uncertainty in the actual
predictions.

Thompson (1993) also noted that some geomagnetic activity during the previous cycle served as a
predictor for the amplitude of the following cycle but, instead of trying to separate the two, he simply
related the geomagnetic activity (as represented by the number of days with the geomagnetic Ap index
≥ 25) during one cycle to the sum of the amplitudes of that cycle and the following cycle (Figure 41).
Predictions for the amplitude of a sunspot cycle are available well before minimum with this method. The
number of geomagnetically disturbed days during cycle 23 gives a prediction of R_{max}(24) = 131 ± 28.
Disadvantages with this method include the fact that two cycle amplitudes are involved, the fact that longer
cycle will have more disturbed days simply due to their length, and the standard errors are
larger.

Hathaway et al. (1999) tested these precursor methods by backing-up in time to 1950, calibrating each precursor method using only data prior to the time, and then using each method to predict cycles 19 – 22, updating the data and recalibrating each method for each remaining cycle. The results of this test were examined for both accuracy and stability (i.e. did the relationships used in the method vary significantly from one cycle to the next). An updated (including cycle 23 and corrections to the data) version of their Table 3 is given here as Table 6. The RMS errors in the predictions show that the geomagnetic precursor methods (Ohl’s method, Feynman’s method, and Thompson’s method) consistently outperform the other tested methods. Furthermore, these geomagnetic precursor methods are also more stable. For example, as time progressed from cycle 19 to cycle 23 the Gleissberg cycle period changed from 7.5-cycles to 8.5-cycles and the mean cycle amplitude changed from 103.9 to 114.1 while the relationships between geomagnetic indicators and sunspot cycle amplitude were relatively unchanged.

Prediction Method | cycle 19 | cycle 20 | cycle 21 | cycle 22 | cycle 23 | RMS |

Mean Cycle | –97.4 | –1.6 | –55.4 | –46.7 | –6.9 | 54.4 |

Even–Odd | –60.1 | ? | –26.7 | ? | 61.4 | 52.0 |

Maximum–Minimum | –109.7 | 24.9 | –18.6 | –8.1 | 5.2 | 51.2 |

Amplitude–Period | –75.3 | 18.4 | –73.5 | –25.6 | 15.0 | 49.6 |

Secular Trend | –96.4 | 14.6 | –40.6 | –25.4 | 18.9 | 49.3 |

Three Cycle Sawtooth | –96.5 | 14.6 | –38.5 | –25.4 | 18.8 | 49.0 |

Gleissberg Cycle | –64.8 | 48.0 | –36.9 | –31.8 | –0.9 | 42.1 |

Ohl’s Method | –55.4 | –5.9 | 2.3 | –9.1 | 10.5 | 28.7 |

Feynman’s Method | –43.3 | –22.4 | –1.0 | –14.8 | 25.9 | 28.6 |

Thompson’s Method | –17.8 | 8.7 | –26.5 | –13.6 | 40.5 | 27.0 |

The physics behind the geomagnetic precursors is uncertain. The geomagnetic disturbances that produce the precursor signal are primarily due to high speed solar wind streams from low latitude coronal holes late in a cycle. Schatten and Sofia (1987) suggested that this geomagnetic activity near the time of sunspot cycle minimum is related to the strength of the Sun’s polar magnetic field which is, in turn, related to the strength of the following maximum (see next Section 7.4 on dynamo based predictions). Cameron and Schüssler (2007) suggest that it is simply the overlap of the sunspot cycles and the Waldmeier Effect that leads to these precursor relationships with the next cycle’s amplitude. Wang and Sheeley Jr (2009) argue that Ohl’s method has closer connections to the Sun’s magnetic dipole strength and should therefore provide better predictions.

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