Can irregularities of solar proxies help understand quasi-biennial solar variations?
We 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 <i>N</i> = 162 to 648 days. The irregularity indices λ are computed within 4-year sliding wind...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2014-08-01
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| Series: | Nonlinear Processes in Geophysics |
| Online Access: | http://www.nonlin-processes-geophys.net/21/797/2014/npg-21-797-2014.pdf |
| Summary: | We 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 <i>N</i> = 162 to 648 days. The irregularity indices λ are
computed within 4-year sliding windows, with embedding dimensions <i>m</i> = 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 <i>R</i> 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
<i>N</i>. We identify two distinct regimes, called Q1 and Q2, defined mainly by the
evolution of <i>R</i> as a function of <i>N</i>: Q1, with increasing HSV behavior and <i>R</i> value
as <i>N</i> is increased, occurs before 1915–1930; and Q2, with decreasing HSV
behavior and <i>R</i> value as <i>N</i> 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 <i>T</i><sub>1</sub> and <i>T</i><sub>2</sub> (<i>T</i><sub>1</sub> = 11 years, and intermediate <i>T</i><sub>2</sub> is
tuned to mimic quasi-biennial oscillations QBO). The model can generate both
Q1 and Q2 regimes. When <i>m</i> = 1, HSV appears in the absence of <i>T</i><sub>2</sub>
variations. When <i>m</i> = 2, Q1 occurs when <i>T</i><sub>2</sub> variations are present,
whereas Q2 occurs when <i>T</i><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. |
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| ISSN: | 1023-5809 1607-7946 |