The possibility is discussed of occasional larger changes in the state of the sun, lasting for some millions of years, which might be responsible for producing more drastic changes in earth's climate, called ice ages

Cameron, A. G. W.

English

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N76 14542
Solar Models in Relation to Terrestrial
Climatic Variations
A. G. W. CAMERON
Harvard College Observatory and Smithsonian Astrophysical Observatory
One of the suggested possibilities to explain the lack of observation of solar neutrinos is
that the Sun may have undergone a thermal expansion at the center, lasting a few million
years, with an accompanying decrease in luminosity, producing an ice age. A critical exam-
ination is given of this hypothesis.
Most of the papers at this meeting have dealt
with relatively small changes in the state of the
Sun that may or may not be accompanied by
relatively small changes in the state of Earth's
atmosphere. The present paper deals with the
possibility of occasional larger changes in the
state of the Sun, lasting for some millions of years,
that might be responsible for producing more
drastic changes in Earth's climate, called ice ages.
I have recently given a more complete summary
of this situation, and the reader interested in more
details and references is referred to this (Cam-
eron, 1973).
For some years, Raymond Davis, Jr., of the
Brookhaven National Laboratory has been at-
tempting to detect neutrinos emitted from the Sun.
He has been utilizing a large tank underground
in a mine in South Dakota that contains some
100000 gallons of commercial cleaning fluid,
C2CL,. The expected action of the more energetic
solar neutrinos is to convert some atoms of 37C1
into atoms of 37Ar, which is a radioactive nuclide.
Periodically, every month or two, the tank is
purged of rare-gas atoms, which are collected.
The argon is separated and any radioactive argon
atoms are detected by a carefully shielded counter.
The great sensitivity of this experiment may be
judged from the fact that Davis is looking for the
production of only a few radioactive argon atoms
per month in this large tank.
Davis's experimental results are usually quoted
in terms of a unit depending in part on the ex-
pected neutrino interaction cross section with
37C1 atoms. This unit is called the solar neutrino
unit (SNU). When the experiment was first
designed, model calculations had predicted that
Davis should obtain a signal equivalent to about
30 or 40 SNU. However, he did not detect any
signal, and with added effort, which has involved
increasing his detector sensitivity greatly, he has
pushed down the limit to the point where the solar
neutrino flux is not greater than about 1 SNU.
Meanwhile, there have been some revisions in
nuclear reaction cross sections, whose redeter-
mination has been motivated by these experi-
mental results, and current solar models predict
that he should detect a signal of about 7 SNU.
It is this discrepancy that has led to an intense
search for aspects of nuclear astrophysics, stellar
physics, or neutrino physics that might be an
error. Here I shall deal with only one of these
suggested methods for evading the solar neutrino
difficulty, that involving a temporary thermal
expansion of the center of the Sun. This idea was
originally suggested by W. A. Fowler.
Suppose that a considerable amount of thermal
energy is suddenly dumped into the center of the
Sun. This heats up the gas, increasing the pres-
sure, and causing the center of the Sun to expand.
This expansion, in turn, adiabatically cools the
143
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1;44 RELATIONSHIPS BETWEEN SOLAR ACTIVITY AND METEOROLOGICAL PHENOMENA
gas to a temperature lower than that which the
center of the Sun would normally have. This cuts
down the rate of the thermonuclear reactions
occurring there, and hence it will also greatly cut
down the emission of neutrinos from the central
regions of the Sun. This excess energy will diffuse
out of the center of the Sun, over the course of a
few million years, allowing the central region to
relax toward the normal condition.
There have been a number of discussions in the
last 2 yr of a possible way in which such a sudden
energy release might take place. To show sche-
matically how this happens, it is necessary to
consider the basic energy-producing reactions in'
the Sun and their temperature sensitivities. I shall
give here only the first of the so-called "proton-
proton reaction chains" that is probably respon-
sible for most of the energy generation in the
Sun, but which is not responsible for producing
neutrinos to which the Davis detector is sensitive.
The first step is the proton-proton reaction:
'HC/?, p+ v)2D. This reaction, involving a (3 decay,
is a rare one and has a relatively low temperature
sensitivity in the center of the Sun, about the
fourth power of the temperature at the center of
the Sun. This reaction is immediately followed by
another: 2D(p, y)3He. The deuterium formed in
the first reaction is almost instantaneously re-
moved and converted to 3He by this reaction. The
3He builds up until there is enough of it present
for it to react with itself: 3He (3He, 2p)4He. This
reaction has a much higher temperature sensitivity,
something like the 20th power of the temperature
near the center of the Sun.
. As a result of the different temperature sensi-
tivities of these reactions, the amount of 3He
which will be present under steady-state condi-
tions will increase as one goes away from the
center of the Sun. This results from the fact that
much larger amounts of it are needed to compen-
sate the relatively smaller reaction rate at lower
temperatures in the Sun.
Therefore it is evident that if some mechanism
could produce a large-scale and sudden mixing of
the central regions of the Sun, the amount of 3He
at the center would be greatly increased. The
amount would then be much in excess of that
needed to produce 4He at the steady-state rate
established by the basic proton-proton reaction.
Hence the. excess 3He would more rapidly be
destroyed in the central region of the Sun, releas-
ing energy at higher than the normal rate and
providing the source .for the relatively sudden
release of energy that has been postulated.
It is necessary to emphasize that we do not
know of a suitable mixing mechanism that would
be needed, to produce this effect. The only detailed
mechanism suggested is an oscillatory overstability
of the central regions of the Sun, leading to mix-
ing, proposed by Dilke and Gough. However, this
mechanism has come under severe criticism by
Ulrich and others. Thus at the present time we
have nothing to suggest for a driving mechanism
that would cause the mixing,-and this is the funda-
mental weak point in this whole approach. All we
can do is suppose that the mixing happens, and
inquire as to the consequences. This simply recog-
nizes that there is'a considerable amount of strange
behavior associated with the dynamics of rotating
fluids that we do not yet understand, so that
perhaps it may be possible in the future to find a
driving mechanism for the mixing if the conse-
quences should look interesting.
Let me cite a specific numerical example, cal-
culated by Ezer and Cameron (1972). In this
experiment; 56 percent of the central mass of the
Sun was suddenly mixed, which meant that its
composition was rendered uniform. This led to
an increase of 3He near the center, and the addi-
tional energy released by destruction of this nu-
clide caused the center of the Sun to expand over
a period of about 2 million years. Following an
initial neutrino flash immediately after the mixing,
the neutrino production fell off markedly through-
out the Sun, and the expected detection by Davis
dropped to about 0.5 SNU. The photons then
gradually diffused out of the center of the Sun,
allowing the solar core to relax back toward nor-
mal conditions over the following 4 million years.
The total time involved in the core expansion was
thus 6 million years, and during this period of
time the solar luminosity dropped to a minimum
of about two-thirds of normal. There was a small
overshoot in luminosity at the end of the recovery
period, which would gradually die out over a
somewhat longer period.
It is reasonable to expect that the large de-
crease in luminosity of the Sun would produce an
SOLAR MODELS IN RELATION TO TERRESTRIAL CLIMATIC VARIATIONS 145
ice age. We are presently involved in an ice age,
which has lasted for a few million years. As long
as the poles of Earth are covered by ice, this is to
be regarded as an ice age, and we are not con-
cerned with the motion of the ice sheet back and
forth between high and low latitudes. It appears
that Earth was free of polar icecaps throughout
most of geologic history. Thus the numerical exam-
ple that I have just cited cannot be expected to be
truly representative of the situation. If something
like this were to happen, we would identify the
present as a period of reduced solar luminosity so
that the normal solar luminosity would be con-
siderably higher than at the present time, perhaps
50 percent greater. This would have burnt more
hydrogen in the central region of the Sun, leading
to a rising level of the normal solar neutrino flux,
and the current dip in this neutrino flux would
not be as great as indicated in the example. A
more realistic calculation would probably bring '
the minimum down only comparable with Davis's
upper limit on the neutrino flux.
To judge from the geologic record, this sort of
mixing would have to occur about four times per
billion years throughout the history of the Sun.
If this should prove to be an explanation for the
terrestrial ice ages, then I wish to emphasize the
restrictions imposed on the process by these cal-
culated time scales. These calculations seem to
pin down the total duration involved in the
luminosity excursion quite well; I would not
expect this duration to be much affected by any
details of the mixing mechanism that might be
determined in the future, with one exception that
will be discussed.
Therefore, it is important that the geologic
record does not seem to give clearcut determina-
tions of the general duration of ice ages, nor does
it seem to give very precise evidence for the time
at which the present ice age began. I, at least,
have been unable to find any precise determina-
tions of these quantities in my somewhat cursory
examination of the literature. Thus, this picture
for the production of ice ages would certainly be
in trouble if it were found that the present ice age
had extended for much longer than 3 or 4 million
years. I have seen a report in the popular press
that recent drilling in the Antarctic ice sheet has
indicated an age much greater than this, perhaps
of the order of 20 million years; until details of
this should appear in the scientific literature, it is
not possible to judge the validity of such reports.
If it should be decided that one wishes to pre-
serve this mechanism for accounting for Earth's
ice ages and also to accommodate longer dura-
tions of these ice ages, then there is one possible
way in which this might be done. If the hypothet-
ical mixing mechanism has a longer time period
associated with it than 6 million years, so that the
excess 3He is driven toward the center of the Sun
on this longer time scale, then the duration of the
luminosity dip in the Sun could be extended. How-
ever, the amplitude of the luminosity dip would
be correspondingly decreased. Under these cir-
cumstances, it would no longer be possible to
reduce the solar neutrino flux down to the limit
indicated by Davis's experiment, and the entire
motivation for this suggestion would disappear.
At the present time, I am rather pessimistic
about the possibility that this suggested media-;,
nism will solve the solar neutrino problem and
provide an explanation of the ice ages. The lack
of a suitable mixing mechanism despite the inter-
est generated by this suggestion is one cause for
such pessimism. The sharply limited duration pos-
sible for such ice ages is another. Nevertheless, I
think it is well worthwhile to carry out additional
work-on this suggestion, particularly with regard
to calculations of general worldwide climatic con-
ditions under conditions of a higher than normal
solar luminosity and additional investigations of
the dynamics of rotating fluids. Unfortunately,
astronomical evidence for such major luminosity
variations is unlikely to be found, because the
temperature and luminosity of the Sun change in
such a way as to drive the Sun straight down the
main sequence, so that other stars undergoing
these changes would simply now appear to be of
lower than normal mass but otherwise normal in
all respects. Meanwhile, if some other explanation
of the solar neutrino puzzle should prove to be
successful, then we would no longer have a
motivation for belief in the present suggested
mechanism.
146 RELATIONSHIPS BETWEEN SOLAR ACTIVITY AND METEOROLOGICAL PHENOMENA
REFERENCES
Cameron, A. G. W., 1973, "Major Variations in Solar
Luminosity," Rev. Geophys. Space Phys., 11, p. 505.
Ezer, D., and A. G. W. Cameron, 1972, "Effects of Sud-
den Mixing in the Solar Core on Solar Neutrinos and
Ice Ages," Nature, 240, p. 180.
DISCUSSION
RASOOL: The luminosity of the Sun has changed
over billions of years. Can you give the present thinking
of how this evolution has taken place?
CAMERON: The standard kind of solar models would
make the solar luminosity increase from the time when
the Sun was on the zero edge main sequence to now by,
I think, it is something like 35 or 50 percent, of that
order, a gradual increase. If you believe in time varia-
tion of G, you can actually make the solar luminosity
gradually decrease over all of that period of time. If you
believe in the Brans-Dicke theory, you can do anything
you want. If not, then the solar luminosity has increased
by an order of 40 or 50 percent since the time the Sun
was formed.
BOOK: Is it possible that there are neutrino absorbers
somewhere in the Sun that are far more effective because
there is far more mass in the Sun than in Davis' experi-
ment? How does one know that there is not a lot of
chlorine or some other neutrino absorber somewhere in
the Sun, since not very much is known about its consti-
tution?
CAMERON: There is nothing special about chlorine
except that it happened to lead to a convenient rare-gas
radioactivity at the detector. The neutrino cross sections
are pretty well calculated and they are known in some
cases experimentally, at least at the higher energies. The
standard calculations say that the mean free path for
absorption of typical solar neutrinos is something like 80
light years of ordinary lead. That is a measure of how
transparent matter ordinarily is to the passage of such
neutrino fluxes. This is why Ray Davis can have 100,000
gallons of cleaning fluid down in the mine and only
detect a few atoms per month. The stuff is really terribly
transparent.
It would be far more upsetting to physics to say that
there was some sort of neutrino absorber in the Sun than
to assume that the Sun behaves in the way I suggested.
So it is a matter of choosing which field you want to do
drastic things in.
I. should have mentioned that the idea that we are
now in an ice age on Earth.has been picked up by Carl
Sagan and; some of his colleagues who say that Mars is
also in an ice age. One of the other things that he sug-
gested, however, I would like to lay to rest: that is that
when the sun changes in this way, the distribution of
stars (which are also doing this) on the main sequence
that one can measure for a cluster or something like
that is broadened.
j^. When we look at, in fact, how the temperature and
radius of the Sun change together, it turns out that the
Sun, when it decreased in luminosity, moved exactly
down the main sequence. Therefore, this does not pro-
duce any broadening of the main sequence, so this is
not an effect that one can look for astronomically.
QUESTION: How fast do you think the solar lumi-
nosity changes?
CAMERON: The time scale for a luminosity decrease
occurred in just a little less than 1 million years, and
most of the recovery occurred in about a 2-million year
period.
QUESTION: Yes, but that would be the rate of
change for this particular process. How fast do you
think it could change if you just perturbed it in some
way? What would be the lower limit for changing solar
luminosity due to maybe other forces? How fast can a
big thing like that change?
CAMERON: If you make any major perturbation in
the structure, the relaxation time is basically the Kelvin-
Helmholtz relaxation time. When one is dealing with
the core, it is just like 5 or 6 million years. If one is
dealing with the outer envelope of the Sun, it is rather
longer, maybe 50 million years; so you can get the fast-
est response if you just deal with the core. In terms of
the neutrino problem, just doing something to the
envelope is not going to help you.
ARKING: Can we have an explanation of why you
have to have such a drastic change in luminosity if you
were to, say, alter the rate at which you are producing
energy in the center of the Sun? Or another way of
looking at it, if you suddenly turn off the energy-
producing reactions in the center of the Sun, would not
the Sun continue to be luminous at approximately the
same solar constant for millions of years before the
effect would be seen on the surface?
CAMERON: That is correct. If you turned off all the
nuclear reactions in the Sun, the Sun would keep shining,
it would keep contracting, and the luminosity would, in
fact, follow pretty much the horizontal branch; that is,
it would stay level as the Sun shrunk and as the surface
temperature increased.
ARKING: So why do you need a 30-percent change
in luminosity?
CAMERON: The whole question is what do you have
to do to the Sun to shut off the neutrinos enough not to
violate the Davis experiment. The argument is that you
have to cause the center to expand, and, therefore, you
have to dump energy into it, and it is a natural conse-
quence of the response of the Sun to dumping that
energy into the core that decreases the luminosity.
QUESTION: Would a strong magnetic field of the
crder of millions of gauss, in the interior of the Sun
have any effect?
CAMERON: Such a field would help a little bit. It
would not help nearly as much as you need if you
wanted to try to cure the neutrino problem strictly with
such a field.
DAVIS: I am curious as to where you got your 20
million year figure for the Antarctic Icecap because, as
SOLAR MODELS IN RELATION TO TERRESTRIAL CLIMATIC VARIATIONS 147
I recall, the ice at the bottom of the core at Byrd Sta-
tion has a radiocarbon date of about 40 000 to 50 000
yr, which would probably fit your theory better.
CAMERON: It would be fitted very much better. All
I remember is that sometime this summer I read an
interview with somebody who had done a measurement,
and it was quoted as 20 million years. I have not seen
it in the literature, all I have seen it in is a popular
report; therefore, I don't know how good that number
is. Other people have tried to look at ocean tempera-
tures and have said that they seem to have been steadily
decreasing over the last 50 million years, for example,
and I do not know how good those numbers are. If one
can say that the duration is longer than about 6 million
years, the basic point I am trying to make is that one is
in trouble with this explanation no matter what you do
because, even if you make the Sun behave this way, it
will not cure the neutrino problem. Maybe there is some
other explanation for the neutrino problem, and the
Sun still behaves this way, but we still do not know of
a driving mechanism that would make it behave this
way, another very fundamental weakness of this theory.


Solar models in relation to terrestrial climatic variations

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