Solar activity: Towards a standard classification of solar phases from cycle 1 to cycle 24

Abstract

This paper aims to develop a standard classification method of solar phases from Cycles 1 to 24. The purpose is to establish new solar cycle cutting criteria. The maximum sunspot number values (SNmax) of previous criteria were examined. In addition, the evolution of sunspot appearance latitudes (maximum latitude (λ_max) and minimum latitude (λ_min)) was taken into account. This has enabled new criteria to be developed. These criteria take into account the actual Sun' behavior. They also resolve past inconsistencies between classification criteria and new sunspot data. Four phases have been retained: minimum, increasing, maximum and decreasing phases. The new sunspot number (SN) data computed by the Belgian Observatory have been taking into account. SN data set have been smoothed to obtain normalized curves that allowing to represent solar sunspot numbers evolution as a trigonometric function. The mathematical hypotheses formulated concern sunspot appearance latitudes and the identification of the solar cycle. The results show that the different phases do not have the same duration. The increasing phase is quite fast compared to the decreasing one. In fact, the method proposed in this work presents a new cutting criterion as follow: (i) minimum phase: SN(t) < 0.122 × SNmax; (ii) increasing phase: 0.122 × SNmax ≤ SN(t) ≤ 0.73 × SNmax; (iii) maximum phase: SN(t) > 0.73 × SNmax and (iv) decreasing phase: 0.73 × SNmax ≥ SN(t) > SNmin (next cycle).