https://insu.hal.science/insu-03581114Shapoval, A.A.ShapovalLe Mouël, J. L.J. L.Le MouëlIPGP - Institut de Physique du Globe de Paris - INSU - CNRS - Institut national des sciences de l'Univers - UPD7 - Université Paris Diderot - Paris 7 - UR - Université de La Réunion - IPG Paris - Institut de Physique du Globe de Paris - CNRS - Centre National de la Recherche ScientifiqueShnirman, M.M.ShnirmanIPGP - Institut de Physique du Globe de Paris - INSU - CNRS - Institut national des sciences de l'Univers - UPD7 - Université Paris Diderot - Paris 7 - UR - Université de La Réunion - IPG Paris - Institut de Physique du Globe de Paris - CNRS - Centre National de la Recherche ScientifiqueCourtillot, V.V.CourtillotIPGP - Institut de Physique du Globe de Paris - INSU - CNRS - Institut national des sciences de l'Univers - UPD7 - Université Paris Diderot - Paris 7 - UR - Université de La Réunion - IPG Paris - Institut de Physique du Globe de Paris - CNRS - Centre National de la Recherche ScientifiqueCan irregularities of solar proxies help understand quasi-biennial solar variations?HAL CCSD2014[SDU] Sciences of the Universe [physics]POTHIER, Nathalie2022-02-19 09:27:062023-05-01 03:56:352022-02-19 09:27:07enJournal articleshttps://insu.hal.science/insu-03581114/document10.5194/npg-21-797-2014application/pdf1We define, calculate and analyze irregularity indices λ<SUB>ISSN</SUB> of daily series of the International Sunspot Number ISSN as a function of increasing smoothing from N = 162 to 648 days. The irregularity indices λ are computed within 4-year sliding windows, with embedding dimensions m = 1 and 2. λ<SUB>ISSN</SUB> displays Schwabe cycles with ~5.5-year variations ("half Schwabe variations" HSV). The mean of λ<SUB>ISSN</SUB> undergoes a downward step and the amplitude of its variations strongly decreases around 1930. We observe changes in the ratio R of the mean amplitude of λ peaks at solar cycle minima with respect to peaks at solar maxima as a function of date, embedding dimension and, importantly, smoothing parameter N. We identify two distinct regimes, called Q1 and Q2, defined mainly by the evolution of R as a function of N: Q1, with increasing HSV behavior and R value as N is increased, occurs before 1915-1930; and Q2, with decreasing HSV behavior and R value as N is increased, occurs after ~1975. We attempt to account for these observations with an autoregressive (order 1) model with Poissonian noise and a mean modulated by two sine waves of periods T<SUB>1</SUB> and T<SUB>2</SUB> (T<SUB>1</SUB> = 11 years, and intermediate T<SUB>2</SUB> is tuned to mimic quasi-biennial oscillations QBO). The model can generate both Q1 and Q2 regimes. When m = 1, HSV appears in the absence of T<SUB>2</SUB> variations. When m = 2, Q1 occurs when T<SUB>2</SUB> variations are present, whereas Q2 occurs when T<SUB>2</SUB> variations are suppressed. We propose that the HSV behavior of the irregularity index of ISSN may be linked to the presence of strong QBO before 1915-1930, a transition and their disappearance around 1975, corresponding to a change in regime of solar activity.