Cyclicity of Solar Activity
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Figure 1
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[7] It is accepted to call solar activity the entire set of phenomena
in the solar atmosphere that change its radiation in various
ranges of electromagnetic waves and various-energy particle
fluxes. In the optical range these changes are exhibited
basically in the change of the number of various solar
structures; they are almost invisible in the optical radiation
(~0.1%). However, in soft X rays these changes are
observable both in the number of structures and in radiation: the
difference in the "background'' radiation level in the 1-8 Å
range (1-12.5 keV) at the solar activity maximum and minimum
exceeds two orders of magnitude (more than 100 times). The solar
activity state is characterized by some observational indices; of
these, the longest time series exists for the relative sunspot
number ( W ) introduced for the first time by R. Wolf. This index
was determined on a regular basis at first in Zürich and since
1849 until now in Brussels. This year may be considered as the
beginning of scientific observations of the Sun. In addition,
Wolf restored monthly average values of the given index since
1749 and annual averages since 1700 using rather fragmentary data
of individual European observers. Using literary data of
occasional telescopic observations, modern explorers have managed
to qualitatively prolong this series to 1611. The reliability of
all retrieved data is low; they enable us only to see a hint that
the cyclic solar activity existed also at that time. One of the
most remarkable features of the Sun are nearly-periodic changes
of intensity or number of different manifestations of solar
activity (SA)--solar activity cycles. For the first one, the
cycle that began in March 1755 is conventionally adopted.
Figure 1 shows the entire known series of relative sunspot
numbers since 1610 until present; the confidence of the
observational data is shown graphically; crosses: isolated
observations (qualitative estimate), light-gray curve: low
veracity; darker curve: reliable data. It should be noted that
the analysis and comparison of Wolf number series for the periods
1749-2006 (numbered series) and 1849-2006 (reliable series) has
shown obvious differences in the positions and magnitudes of the
fundamental harmonics and spectral parameters of these series. In
addition, the comparison of the series infers:
- a different behavior of the main characteristics on the interval 1749-1849;
- an increase in the length of the series results in a
poorer resolution of some significant spectral
characteristics (usually vice versa);
- an appreciable distortion of the "high frequency'' part of the spectrum.
[8] All this puts in question the correctness of using the retrieved
part of the series in the majority of applications. In other
words, to study the effect of solar activity on various long-term
processes, one can use only the reliable series of Wolf numbers
(1849-2006), from the middle of cycle 9 to cycle 23 of solar
activity. The small statistics of reliable solar cycles (14) and
absence of a physical model of a solar cycle development
puts so far insuperable barriers to the reliability of solar cycle prediction
prior to its beginning. However, the situation changes
with the beginning of a new cycle: after 18-24 months of its
development, it is already possible to determine its height, the
epoch of its maximum, and probable duration of the current solar
cycle. For the last two cycles the most successful prediction of
the solar cycle development its beginning was done by the
Waldmeier method and by the method of solar cycle sets similarity proposed by
the author [Ishkov, 2003, 2005]. Table 1 lists
the characteristics of all "numbered'' solar cycles.
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Figure 2
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[9] Almost eleven years of the development of the current cycle 23
(Figure 2) have completely uncovered its characteristics and the
majority of features of its evolution. The current cycle is the
second component of the physical 22-year solar cycle, and the
greatest surprise was that, contrary to the Gnevyshev-Ol' rule,
the trailing odd cycle 23, for the first time in the entire
realistic (since 1849) history of the research of solar cycles,
has become smaller than the leading even cycle 22. The main
stages of the current solar cycle development [Ishkov, 2005] are
as follows:
- minimum of the solar cycle 22: May 1996 ( W* = 8.0);
- beginning of the 23rd cycle phase of growth: September 1997;
- maximum of the relative sunspot number: April 2000 ( W* = 120.7);
- global polarity reversal of the solar global magnetic field: July - December 2000;
- secondary maximum of the relative sunspot number: November 2001 ( W* = 115.8);
- maximum of the radio flux at a wavelength of 10.7 cm: February 2002 ( F* = 197 sfu);
- phase of the current cycle maximum: October 1999 - June 2002;
- beginning of the decay phase: July 2002;
- beginning of the minimum phase: July 2006.
- The most powerful flare events in the current cycle: October - November 2003.
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