Von Wilfried Schroeder
RESUMEN: Una revisión de las minimas de
actividad solar y la relaci6n entre la actividad auroral y geomagnética nos
permiten establecer que la actividad minima solar no es evidente en los datos
PALABRAS CLAVE: Aurora, minimum de Maunder,
ABSTRACT: A review of solar activity
minima and the relationship of the auroral and geomagnetic activities suggest
that solar minima are not evident in the available data.
KEY WORDS: Aurora,
Maunder minimum, solar activity.
A number of papers have discussed the problem of solar
variability (see Leon, Skumanich and White, 1992; Nesme-Ribes et al., 1993,
Mendoza, 1996). Eddy (1976), based on the ideas of the English astronomer
Maunder (see Schroeder, 1984). He proposed that a long solar minimum occurred in
the 17th century. The name "Maunder minimum" and other proposed minima
such as the Spoerer minimum (between 1460 and 1550) have been widely used in the
Eddy proposed that the Maunder minimum occurred between
1645 and 1710. His assumption was supported by a small amount of data; however,
later results in radio chemistry, archaeology and other fields seemed to support
The assumption of the Maunder minimum has been accepted
(e.g. Kippenhahn 1992), and criticized (Gleisberg, 1968; 1979; Landsberg, 1980).
Legrand et al. (1993) and Schlamminger (1993) have shown that the minima are not
supported by auroral data. The recently introduced Spoerer minimum, which is
supposed to have occurred between 1460 and 1550 (Kippenhahn, 1992; Suess, 1993)
is not consistent with evidence that the solar cycle did function from the 16th
to the 18th century and that there were continuously auroras in the middle
latitude with the usual frequency of occurrence (Schroeder, 1988).
Data from different catalogues of auroras and from
other sources (see Schroeder, 1984, 1998, 1996) are summarized in Tables
1 and 2 (cf. Boue, 1856; Fritz, 1873, Krivsky,
1988; Landsberg, 1980-, Legrand et al. 1993).
There is a fairly normal auroral activity for Central
Europe during the alleged Spoerer minimum as shown by Table
1. When interpreting such data, the present standards of observations cannot
be used. In late Medieval times the existing scientific institutions were not
interested in auroras. Such phenomena were assumed to possess some theological
origin and were neither observed nor recorded as natural ones. Few records date
from these centuries, but it is impossible to conclude that there were no
Even after the invention of printing there was no
systematic solar and auroral research. Records remained sporadic. There are no
data for some years, but it is possible that new data will be discovered in
sources which have not yet been studied.
Eddy's result seemed to be confirmed by the fact that
GeV energy range of cosmic rays is coupled to solar activity. In this energy
range, cosmic rays generate free neutrons in the atmosphere and influence the
isotope composition. Therefore the fluctuations of the solar activity must be
"fossil- ized" in the isotope composition (Suess, 1993).
Auroral frequency from 16th to 18th centtury from European data (middle latitudes)
solar "activity minima" exist?
Tables 1 and 2
cover a large historic time interval. There are years for which no auroras have
been reported until now. Does this mean that there were no auroras in these
years? By no means. As mentioned, the scientific and social conditions did not
favour the compilation of complete statistics. On the contrary, the evidence of
years with a high number of auroras (even if some events are questionable)
suggest that the solar activity was normal, i.e. that the solar cycle with a
period of about 11 years continued during these intervals (see Figure).
According to cosmic electrodynamics, the solar magnetic
field is the source of the corpuscular radiations which causes auroras. This
radiation consists of electrically charged particles which propagate with a
velocity up to 2W0 km/s, and reach the Earth within one day. The particles of
this solar corpuscular radiation extend the solar magnetic field to the vicinity
of the Earth and increase the shielding effect of the geomagnetic field against
cosmic radiation from space.
Cosmic radiation consists of positively charged nuclei,
mainly protons, which propagate nearly at the speed of light. The particles
cause nuclear reactions in the terrestrial atmosphere, which transform e.g. N14 to C14. Plants incorporate C14 into their structure; thus the
isotope composition of carbon in fossil wood reflects the intensity of cosmic
radiation during the lifetime of a tree.
As the shielding effect of the geomagnetic field is
less strong in the case of low solar activity than in the case of an active Sun,
more charged particles of cosmic rays reach the Earth. Thus more C14 is produced and this could be
detected from fossil wood. If the number of auroras and thus the corpuscular
solar radiation were not correlated with the sunspot number, no correlation
would exist with the amount of C14 in
However, it is not generally true that solar activity
always reduces the number of high energy particles in cosmic rays reaching the
vicinity of the Earth. Sunspots are possible sources of high-energy corpuscular
radiation, too. There are events of a strong increase of cosmic radiation
following magnetic storms one day later. On the other hand, the magnetic fields
of the Sun produce energetic rays at almost the velocity of light. This effect
is opposed to the former and has been investigated by Heisenberg (1953; cf. also
Smith, 1991). Kahler (1992) shows that solar protons do not reach the Earth
exactly from sunspots but from flares and/or mass ejections, often rooted in the
photosphere active regions and the associated sunspots.
Note that there is no theoretical basis for a secular
or periodic variation of the amplitude of the solar cycle, including the 11-year
cycle maxima. The 11 -year cycle itself and related phenomena are fairly well
explained by the dynamo models of magnetohydrodynamics. But these models are
based on linear (or quasi-linear) approximations (Eulerian equations of ideal
fluids, Maxwell-Lorentz equations of electrodynamics) which cannot give any
estimate about the absolute value of solar activity.
Thus the amplitude of the solar activity must be
assumed to remain constant as long as the physical parameters of the solar
atmosphere do not change. The nature of such changes and their existence are
Thus the supposed variation of the solar activity in
historic times is a purely empirical problem. Records and fossil data are
relevant at very different levels. Zhe records have the same problem as all
primary sources from ancient times. Some studies lack a critical evaluation of
the primary sources, and the conclusions drawn from incomplete secondary sources
are not well supported. Fossil data (see Suess, 1993) depend on the existence of
a quantitative connection with solar activity. Moreover, the synchronisation of
the data series (and perhaps of the historical records) is to be established.
The fundamental Parameter of solar radiation, and thus
of the global climate of the Earth, is the constant J. This constant depends on
the average distance between the Earth and the Sun, and on the total radiative
capacity of the Sun. The average Sun-Earth distance is the major axis of the
Earth's orbit which is a constant of celestial mechanics (Laplace's law). The
radiative power of the Sun is determined by the energy production of nuclear
fusion in the centre of the Sun, which is governed by the Helmholtz-Kelvin time
scale. It needs about 25 million years to reach the surface of the Sun. In the
present phase of the Sun's evolution this radiative power is constant over
several hundred million years.
This has been recently confirmed by extraterrestrial
measurements of the solar radiation (Froehlich, 1987). There are, however,
short-time fluctuations DJ (in the
solar minimum range) by fractions a thousand part of the total value. Changes of
a similar order of magnitude, DJ/ J =10-3 are experienced within the
11-year solar cycle. The average value J0 of the
solar constant has remained constant for decades. The instantaneous value is
given by the equation J = J0 - DJsin
(t/T), with T about 11 years.
The past changes of the global average temperature are
"frozen" into the annual rings or varves of trees, sediments, glaciers
and firns. Thus, temperature changes with a period of 11 years would reflect
variations in solar activity as a fossil indicator. No climatic changes are,
however, expected in the long run controlled from primary solar irradiation, as
the period of the solar constant is 11 years (except for smaller variations with
periods of N x 11 years).
It is important to note that the long-time constancy of
the amplitudes is nor relevant to the long-time constancy of the solar constant.
The solar constant of the quiet sun J is equal to the average value J0 of J over the total activity
period T. Without solar activity, J = J0 would
always hold and its value would be that of the extrema of a periodic activity (sunspot
maximum and minimum). Thus the secular temperature radiation of the sun is
independent of the solar activity.
At the present level of discussion it seems
nevertheless that there is some solar activity and that it may have changed in
the past, though we do not know the extent of such change. The possible change
of solar activity seems to imply some DJ which
might affect the Earth's temperature over decades, which is certainly relevant
for human beings.
Is the Sun a variable star? Many recent publications on
this point have a questionable basis. If the 11 year cycle is considered and the
available auroral data are collected they may be used similarly to the
geomagnetic activity as an indicator of an active Sun. Another matter is the
magnitude of this activity. This is an open question which needs more research
on solar and auroral data.
In conclusion, the level of auroral occurrence during
the Spoerer and Maunder minimum was similar to modern levels from Central
European data in the middle ages, mostly in Germany, Switzerland, Hungary and
Austria. Furthermore, it seems possible that the aurora occur with an
approximate 11 -year cycle. The problem is for determination of this solar cycle
to find more data from European, mostly unpublished sources.
More geophysical and historical studies are required
(Cf. Schroeder, 1984; 1988; Wittmann, 1978).
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