ISSN 1063-7737, Astronomy Letters, 2009, Vol. 35, No. 4, pp. 247–252. c ? Pleiades Publishing, Inc., 2009.
Original Russian Text c ? V.N. Obridko, B.D. Shelting, 2009, published in Pis’ma v Astronomicheski˘ ı Zhurnal, 2009, Vol. 35, No. 4, pp. 279–285.
Anomalies in the Evolution of Global and Large-Scale Solar Magnetic
Fields as the Precursors of Several Upcoming Low Solar Cycles
V. N. Obridko*and B. D. Shelting
Pushkov Institute of Terrestrial Magnetism, Ionosphere, and Radiowave Propagation, Russian Academy of
Sciences, Troitsk, Moscow oblast’ 142190, Russia
Received August 18, 2008
Abstract—Anomalies in the solar magnetic fields of various scales are studied. The polar magnetic field
strength is shown to have decreased steadily during the last three solar cycles.This is because the increase
in the dipole magnetic moment observed from 1915 to 1976 has changed into a decrease in the last three
cycles. At the same time, the intermediate-scale magnetic fields (like those of isolated coronal holes) have
beenunusuallystronginthe lastcycle.Asaresult,the tiltoftheheliosphericcurrentsheetisstillabout30◦.
Thelargeeffectivecontributionfromthe intermediate-scalefieldstothe total energyofthe large-scalefields
is also confirmed by our calculations of the effective multipolarity index. The aa-index at the cycle minima
is correlated with the height of the succeeding maxima. The set of data considered may be indicative of the
possible approach of a sequence of low solar cycles.
PACS numbers : 96.60.Hv
Key words: solar cyclicity, large-scale magnetic field.
The present solar cycle (23 according to the
in several respects, breaking many of the previously
established typical characteristics of solar cycles.
This primarily applies to the data on local magnetic
fields. Below, we list the best-known characteristics.
Violation of the Gnevyshev–Ohl Rule
As is well known, according to this rule, an odd
the adopted cycle numbering. This rule was formu-
lated in 1948 by Gnevyshev and Ohl for the sums of
monthly Wolf numbers and it had only one exception
over 26 cycles in the pair (4, 5) until the present
pair of cycles (22, 23). This rule was updated by
Kopeck ´ y (1950), who generalized it for the maximum
(in the cycle) Wolf numbers. Unfortunately, this rule
(which, for definiteness, should have been called the
Gnevyshev–Ohl–Kopeck ´ y rule) already has three vi-
olations in the pairs (−2, −1), (4, 5), and (8, 9). The
current pair of cycles (22, 23) violatesthis rulein both
A Very Long Cycle
By August 2008, there had been no reliable data
on the termination of cycle 23, but now it is clear that
its duration approaches 12 years and it is one of the
longest cycles or just the longest one in the recording
history of solar activity since 1848.
A more detailed list of anomalies in solar activ-
ity related to the behavior of sunspots and nonsta-
tionary processes can be found in Ishkov (2005). In
this paper, we would like to draw attention to some
peculiar features in the behavior of global and large-
scale magnetic fields in the present cycle, which, in
our opinion, suggest a transition to the period of low
First of all, let us define what we mean by the
large-scale and global magnetic fields. Looking at
modern high-resolution magnetograms, for example,
SOHO/MDI magnetograms, it is immediately ap-
parent that the magnetic fields have a small-scale
patchy pattern. However, we see at once that these
small-scale elements are distributed over the surface
not randomly but form extended regions in which
one of the polarities dominates, i.e., they form quasi-
unipolar regions of various scales. Undoubtedly, this
is due to the existence of weak large-scale (or global)
80 100 120 140 160 180 200
Fig. 5. Correlation between the geomagnetic aa-index in
the years of minimum solar activity (vertical axis) and the
relative sunspotnumber at the upcomingsolar maximum
(horizontal axis). The solid curve represents a quadratic
the maximum in 1928.4 was 78.1 in cycle 16). On the
other hand, in this paper, using the characteristics
of the large-scale field and geomagnetic activity, we
obtained moderate values for the maximum of cycle
24, 128 and 113 units, respectively. Using the data
on the geomagnetic field seems most reliable. Some
increase in the intermediate-scale fields associated
with the fields of extended unipolar regions must lead
to an increase in the geomagnetic disturbances.
The height of the upcoming maximum can be
correlated with the geomagnetic disturbance level at
the minimum. In Fig. 5, the yearly mean monthly
geomagnetic aa-indices for the year of minimum, one
year before, and one year after it are shown along
the vertical axis. This index can be determined from
thedataofseveral geomagnetic observatoriesandhas
been tabulated from 1968 until the present time. The
height of the upcoming maximum is shown along
the horizontal axis. A satisfactory quadratic depen-
dence with a correlation coefficient of 0.89 ± 0.03 is
date of the current minimum, not to mention the aa-
index in the next year. If we use the data for 2007 and
2008, then we will obtain 14–15 (these values are in-
dicated in the figure by the shaded rectangle) and this
corresponds to aheightofthenextmaximumof about
110. Analyzing the shift between the maximum of
the Wolf numbers and the maximum of the aa-index,
Georgieva (2008) obtained the same value. The shift
obtained in this work may be somehow related to the
the maximum of the sunspot number shown in Fig. 3.
Thus, in general, one may expect the beginning of
the 21st century to be characterized by one or two
cycles with a fairly low or just low intensity. A more
serious, Maunder-type decline of activity or at least
the decline that was observed at the beginning of the
20th century cannot be ruled out either.
This work was supported by the Russian Founda-
tion for Basic Research (project no. 08-02-00070).
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Translated by G. Rudnitskii
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