Ciclo 24, un massimo davvero al minimo

Un ciclo solare, il numero 24 attualmente in corso, che dovrebbe raggiungere un massimo poco prominente, tra circa due anni e mezzo. Queste sono le prime previsioni formulate dagli esperti di fisica solare del Marshall Space Flight Center della NASA, disponibili qui. Allo stato le previsioni dicono che il massimo raggiungerà un numero di macchie normalizzato pari a 64 (davvero poco) verso il luglio 2013. Proseguendo il suo cammino, il ciclo che è da poco iniziato fornirà ulteriori indizi. Ecco un riassunto delle tre tecniche utilizzate dagli scienziati per questo tipo di proiezioni.

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Among the most reliable techniques are those that use the measurements of changes in the Earth's magnetic field at, and before, sunspot minimum. These changes in the Earth's magnetic field are known to be caused by solar storms but the precise connections between them and future solar activity levels is still uncertain.

Of these "geomagnetic precursor" techniques three stand out. The earliest is from Ohl and Ohl [Solar-Terrestrial Predictions Proceedings, Vol. II. 258 (1979)] They found that the value of the geomagnetic aa index at its minimum was related to the sunspot number during the ensuing maximum. The primary disadvantage of this technique is that the minimum in the geomagnetic aa index often occurs slightly after sunspot minimum so the prediction isn't available until the sunspot cycle has started.

An alternative method is due to a process suggested by Joan Feynman. She separates the geomagnetic aa index into two components: one in phase with and proportional to the sunspot number, the other component is then the remaining signal. This remaining signal has, in the past, given good estimates of the sunspot numbers several years in advance. The maximum in this signal occurs near sunspot minimum and is proportional to the sunspot number during the following maximum. This method does allow for a prediction of the next sunspot maximum at the time of sunspot minimum.

A third method is due to Richard Thompson [Solar Physics 148, 383 (1993)]. He found a relationship between the number of days during a sunspot cycle in which the geomagnetic field was "disturbed" and the amplitude of the next sunspot maximum. His method has the advantage of giving a prediction for the size of the next sunspot maximum well before sunspot minimum.

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