ASTRONOMY: ON THE SUNSPOT CYCLE
Minor physical changes in the Sun often lead to extreme solar magnetic activity
that can affect the Earth, e.g., by disrupting radio communications and
influencing the weather. Although the sunspot cycle is of considerable interest,
we are far from understanding its origin and dynamics. ***FULL
TEXT BELOW***
Related Background:
PRECISION
MEASUREMENTS OF BRIGHT RINGS AROUND SUNSPOTS
ASTROPHYSICS:
THE PHYSICS OF THE SUN AND TERRESTRIAL CLIMATE
Text Notes:
Zeeman effect:
Main-sequence star:
ASTRONOMY: ON
THE SUNSPOT CYCLE
The bright surface layer of the Sun is called the
"photosphere", a region a few hundred kilometers thick at a
temperature that ranges from 5770 Kelvin's at its innermost part to 4400 Kelvin's
at its outermost part, the latter the Sun's temperature minimum. The term
"sunspot" refers to a dark area on the photosphere that is cooler than
its surroundings and associated with strong magnetic fields (on the order of 0.4
tesla). Sunspots generally appear in pairs or groups, the leading and following
spots with opposite magnetic polarities. Sunspot sizes vary from small pores
approximately 300 kilometers in diameter to groups of sunspots spanning more
than 100,000 kilometers. The largest sunspots usually last the longest, up to 6
months; small spots may last for less than an hour. For the most part, sunspots
are confined to belts above and below the solar equator. Since the Sun is not a
solid, different parts at the surface rotate at different rates. The term
"solar dynamo" refers to the action within the Sun whereby the kinetic
energy of the hot and highly ionized gas of the solar interior is converted into
the magnetic field that gives rise to solar activity. The consensus model, due
to H.W. Babcock, is that magnetic field lines under the photosphere run from
pole to pole (the "poloidal field") and are twisted parallel to the
solar equator (the "toroidal field") by the differential rotation of
polar and equatorial regions. The so-called "sunspot cycle" (solar
cycle) is a variation in the number of sunspots and other forms of solar
activity with an average period of approximately 11 years. In each successive
cycle, the north and south magnetic polarities of the Sun are reversed,
producing a 22-year magnetic cycle. The 11-year periodicity of the sunspot cycle
is believed to arise through the action of the solar dynamo. Douglas Gough
(University of Cambridge, UK) presents a commentary on current research on the
sunspot cycle, the author making the following points:
1) The author points out that minor physical
changes in the Sun often lead to extreme solar magnetic activity that can affect
the Earth, e.g., by disrupting radio communications and influencing the weather.
Although the sunspot cycle is of considerable interest, we are far from
understanding its origin and dynamics.
2) The author points out that we are currently
experiencing a peak in the solar cycle and therefore in the number of sunspots.
It is generally believed that the underlying cause of the sunspot cycle is the
interaction between the rotation of the Sun and the "dynamo"
responsible for the Sun's magnetism. In this model, the dynamo effect creates a
magnetic field from the electric currents caused by convection and large-scale
shearing motions within the Sun. But the outer regions of the Sun tend to rotate
faster near the equator and slower near the poles, and this results in strong
magnetic fields bursting through the photosphere. These magnetic fields inhibit
the convective transport of heat, permitting material to cool at the surface,
and producing the visibly darker regions called "sunspots".
3) The author points out that none of the
current dynamo models explain the small observed variation in the luminosity of
the Sun that also follows the sunspot cycle. These small changes in luminosity
(no larger than 0.1 percent) apparently derive from the release of stored energy
somewhere within the Sun. Thus, by studying the small changes in the radius of
the Sun, we might learn something about the source of this extra energy, and
also ultimately learn something about the process that causes the luminosity
change.
4) Recent work (M. Emilio et al: Astrophys. J.
543:1007 2000) based on sensitive satellite observations demonstrates evidence
that the energy responsible for variations in the Sun's radius and luminosity
does not come from the inner depths of the
Sun but rather from the outer layers. The author (Gough) states: "This
observation is certainly not the first claimed detection of a small variation in
the Sun's radius, but it may be the first to survive the test of time."
-----------
Douglas Gough: Sizing up the Sun.
(Nature 15 Mar 01 410:313)
QY: Douglas Gough: douglas@ast.cam.ac.uk
PRECISION
MEASUREMENTS OF BRIGHT RINGS AROUND SUNSPOTS
A "sunspot" is a dark area of the solar surface.
The center of the spot, called the "umbra", is darker than the outer
border, which is called the "penumbra". The average sunspot is
approximately twice the diameter of the Earth and may last for several weeks.
Sunspots tend to form in pairs or groups, and a large group may contain up to
100 spots and may last as long as 2 months. Sunspots appear dark because they
are cooler than the photosphere (the visible surface of the Sun or a star). The
temperature at the center of a typical sunspot is approximately 4240 Kelvin's,
while the solar photosphere is at approximately 6000 Kelvin's. Temperatures of
the order of 4000 Kelvin's, however, are significant: a sunspot emits enough
radiation so that a single sunspot on its own in the absence of the remainder of
the Sun would glow a brilliant orange-red and would be brighter that the full
Moon. Analysis of the *Zeeman effect in sunspots
indicates that the magnetic field in a typical sunspot is approximately 1000
times stronger than the average magnetic field of the Sun, and one theory is
that this powerful localized magnetic field inhibits gas motion below the
photosphere, with the result that rising gas cannot deliver its heat to the
surface. Thus, the area cools and a sunspot is the result. Infrared observations
of sunspots have suggested that the heat that does not emerge through the
sunspot is deflected and produces a slight increase in the temperature of the
photosphere around the sunspot, but so far these measurements have not been
precise and the slight increase has not been confirmed. The other major theory
of sunspots proposes that the removal of energy from the sunspot location is the
result of enhanced hydromagnetic wave radiation associated with so-called *plage
fields. Of the two theories, the first theory is currently favored.
M.P. Rast et al (6 authors at 2 installations, US) now report
high-photometric-precision observations of bright rings around 8 sunspots. The
authors report the rings are approximately 10 Kelvin's warmer than the
surrounding photosphere and extend at least one sunspot radius out from the
penumbra. Approximately 10 percent of the radiative energy missing from the
sunspots is apparently emitted through these bright rings. The authors conclude:
"Thus, isolated sunspots are seen to be commonly surrounded by a ring of
enhanced radiation, the origin of which is probably not bright vertical magnetic
elements (plage field), but the re-emergence of heat blocked by magnetic
inhibition of convective transport in the spot itself."
-----------
M.P. Rast et al: Bright rings around sunspots.
(Nature 14 Oct 99 401:678)
QY: M.P. Rast: mprast@ucar.edu
ASTROPHYSICS:
THE PHYSICS OF THE SUN AND TERRESTRIAL CLIMATE
The Sun, a *main-sequence
star 1.4 million kilometers in diameter, is composed predominantly of hydrogen
and helium (approximately 70 percent hydrogen by mass, 28 percent helium by
mass, and 2 percent heavier elements by mass) and it generates its energy via
nuclear fusion processes, particularly via the *proton-proton
chain reaction. As a result, the Sun is losing
mass at a rate of approximately 4 million metric tons per second.
The generation of energy occurs in
the "central core", which has a temperature of approximately 15
million Kelvin's, is approximately 400,000 kilometers in diameter, and contains
approximately 60 percent of the mass of the Sun in 2 percent of its volume.
Outside the core is the "radiative zone", an
envelope of unevolved material through which energy from the core is diffusively
transported by successive absorption and emission of radiation in collisions
between atomic particles. It has been estimated that it may take from 1 million
years to as long as 10 to 20 million years for the energy generated in the core
to reach the surface.
The radiative zone extends to within 200,000 kilometers
of the surface. In the surface layer (the "convective zone"), where
the temperature is only 1 million Kelvin's, convection is the most important
mode of energy transport.
Eugene N. Parker (University of Chicago, US) presents a
review of the physics of the Sun, the author making the following
points:
1) The Sun is essentially a thermonuclear core
enclosed in an opaque shroud that insulates the high temperature of the core
from the cold Universe outside. The core is brighter than 10 supernovas at
maximum light, but the enclosing shroud turns back
all but one part in 2 x 10^(11) of the thermal radiation. The outward journey of
the energy from the core takes approximately 1
million years, which illustrates the immense opacity and thermal capacity of the
shroud.
2) Approximately 10^(-5) of the outflowing
energy from the core of the Sun is diverted into magnetic fields that produce a
variety of exotic effects, including *coronal
mass ejection, *solar flares, the million degree
corona, the *solar wind, and x-ray emission. These
phenomena are of interest to the physicist because they represent unanticipated
manifestations of classical physics, extrapolations to astronomical scales of
basic principles traditionally studied in terrestrial laboratories.
3) The total luminosity of the Sun varies with
time, and systematic monitoring of several Sun-type stars during the past 4
decades reveals magnetic activity cycles comparable to that of the Sun. The
luminosities of some of those stars have been monitored for approximately 15
years, and the data show approximately the same variation as the magnetic
activity.
4) The Earth contains a great deal of
information about past solar activity. The rate of production of carbon-14
depends directly on the intensity of *cosmic rays,
and such rays are partially excluded from the Solar System by the outward sweep
of magnetic fields in the solar wind. Thus the cosmic ray intensity and
carbon-14 production vary oppositely to the general level of solar activity.
5) The carbon-14 record indicates that over the
last 70 centuries the Sun has been without normal activity for 10 centuries and
hyperactive for 8 centuries. The other 52 centuries were variable but more or
less normal. The most recent quiescent period was from 1645 to 1715, the period
called the *"Maunder Minimum". The 12th
century "Medieval Maximum" is the most recent epoch of hyperactivity.
The empirical relation between the total luminosity and magnetic activity, based
on many Sun-type stars, suggests that the Sun was fainter during the Maunder
Minimum by 0.4 +- 0.2 percent, and perhaps brighter by a comparable amount
during the Medieval Maximum. The mean annual temperature in the northern
temperate zone was lower than normal by 1 to 2 degrees centigrade during the
Maunder Minimum and higher by 1 to 2 degrees centigrade during the Medieval
Maximum. The fractional change in temperature is comparable to the fractional
change in solar brightness, with the implication that the Sun is the driver of
the climate. The consequences for agriculture were severe during both periods,
the Maunder Minimum being disastrous in northern
Europe and China, and the Medieval Maximum disastrous in the semi-arid regions.
These periods of abnormal activity of the Sun are without explanation, as are
the variations within the so-called "normal centuries".
6) The general level of solar activity doubled
or tripled from 1900 to 1950, an estimate based on sunspot numbers and on the
intensity of geomagnetic activity. This increase suggests an increase in solar
luminosity by perhaps one part in 2000, and the author suggests it is
interesting to note that the mean temperature in the northern temperate zone, as
well as the surface sea water temperatures, rose during the same period. "Warmer
seas, of course, reduce the rate at which atmospheric carbon dioxide is absorbed
into the oceans. It appears that the global warming since 1950 is in part a
consequence of the continuing increase in solar brightness, seriously aggravated
by the extravagant burning of fossil fuel. So the mystery of the variations in
the total luminosity of the Sun is part of the complicated picture of global
warming."
-----------
Eugene N. Parker: The physics of the Sun and the gateway to the
stars.
(Physics Today June 2000)
QY: Eugene N. Parker, University of Chicago 312-702-9808.
Text Notes:
Zeeman effect: (Zeeman splitting)
The splitting
of a spectral line due to a magnetic field. Named after Peter Zeeman
(1865-1943). The effect is widely used for the determination of magnetic fields
in astronomical objects, especially concerning the Sun and sunspots. In general,
the Zeeman effect occurs when atoms emit or absorb radiation in the presence of
a magnetic field: the field modifies the energy configuration of the atom with
the result that a spectral line is split into 2, 3, or more closely spaced
components. The spacing of the components is a measure of the magnetic field
strength.
Plage fields: A "plage"
is a brighter, hotter patch in the *chromosphere of the Sun, and a region of
particularly strong magnetic field.
Chromosphere: The region of the
Sun's atmosphere directly above its photosphere. Visible only immediately before
or after a total solar eclipse.
The Main Sequence is a region on the *Hertzsprung-Russell
diagram where most stars lie, including our own Sun. The evolution of a star can
be diagrammed as a movement along the Main Sequence and an eventual branching
off the Main Sequence to regions associated with various types of old stars.
Hertzsprung-Russell diagram:
The Hertzsprung-Russell diagram is a plot of stellar absolute magnitude against
spectral type, and is perhaps the most useful diagrammatic aid in astrophysics.
It allows the portrayal of the evolution of a star as occurring along various
paths in the diagram.
Proton-proton chain reaction: A
chain of nuclear reactions inside a star that converts hydrogen to helium, with
the associated release of energy. In the reaction, 4 hydrogen nuclei (protons)
fuse to form one nucleus of helium, with the production of a number of
intermediate nuclei such as deuterium and isotopes of lithium, beryllium, and
boron. The proton-proton reaction is the most important stellar reaction at
temperatures below 18 million Kelvin's, and thus operates chiefly in stars of
less than 2 solar masses.
Coronal mass ejection:
The corona is the Sun's faint outer atmosphere, where the temperature is 2
million degrees Kelvin or more, the corona consisting of a low-density hot gas
that glows with a pale white color.
Solar flares: A solar flare is a
sudden release of energy in the corona of the Sun, the phenomenon usually
lasting up to several hours (in rare cases, up to more than a day).
Solar wind: The solar wind is the
steady flow of charged particles, consisting primarily of protons and electrons,
from the solar corona into interplanetary space. The solar-wind particles have
energies high enough to enable the particles to escape the Sun's gravitational
field, but the wind is influenced by the Sun's magnetic field, and the particles
can be trapped by planetary magnetic fields.
Cosmic rays: Highly energetic
particles moving at close to the speed of light and continuously bombarding the
Earth's atmosphere from all directions. The energies of the particles are
enormous and range from 10^(8) to over 10^(19) electron volts.
Maunder Minimum: Named after
the astronomer Edward W. Maunder (1851-1928), who first noted the absence of
reports of
sunspots in the period 1645 to 1715.