SOLAR FEATURES
Sunspots
An area seen as a dark spot on the photosphere of the Sun; sunspots are concentrations of
magnetic flux, typically occurring in bipolar clusters or groups; they appear dark because they are cooler than the surrounding
photosphere.
In 1610, shortly after viewing the sun with his ‘new
telescope’, Galileo Galilei made the first European observations of sunspots. Daily observations were started at the
Zurich Observatory in 1749 and with the addition of other observatories continuous observations were obtained starting in
1849.
Early records of sunspots indicate that the Sun went
through a period of inactivity in the late 17th century. Very few sunspots were seen on the Sun from about 1645 to 1715. Although
the observations were not as extensive as in later years, the Sun was in fact well observed during this time and this lack
of sunspots is well documented. This period of solar inactivity also corresponds to a climatic period called the "Little Ice
Age" when rivers that are normally ice-free, froze and snow fields remained year-round at lower altitudes. There is evidence
that the Sun has had similar periods of inactivity in the more distant past. The connection between solar activity and terrestrial
climate is an area of on-going research.
The Royal Greenwich Observatory has obtained detailed observations of sunspots since 1874. These observations
include information on the sizes and positions of sunspots as well as their numbers. These data show that sunspots do not
appear at random over the surface of the sun but are concentrated in two latitude bands on either side of the equator. A butterfly
diagram (updated monthly) showing the positions of the spots for each rotation of the sun since May 1874 shows that these
bands first form at mid-latitudes, widen, and then move toward the equator as each cycle progresses. The cycles overlap at
the time of sunspot cycle minimum with old cycle spots near the equator and new cycle spots at high latitudes.
The photosphere is the visible surface of the Sun that
we can observe. Since the Sun is a ball of gas, this is not a solid surface but is actually a layer about 100 km thick (very,
very, thin compared to the 700,000 km radius of the Sun). When we look at the centre of the disk of the Sun we look ‘straight
in’ and see somewhat hotter and brighter regions. When we look at the limb, ‘or edge’ of the solar disk
we see light that has taken a slanting path through this layer and we only see through the upper, cooler and dimmer regions.
This explains the "limb darkening" that appears as a darkening of the solar disk near the limb.
The Sun rotates on its axis once in about 27 days. This
rotation was first detected by observing the motion of sunspots in the photosphere. The Sun's rotation axis is tilted by about
7.25 degrees from the axis of the Earth's orbit so we see more of the Sun's North Pole in September of each year and more
of its south pole in March.
A number of features can be observed in the photosphere
with a simple telescope (along with a good filter to reduce the intensity of sunlight to safely observable levels). These
features include the dark sunspots, the bright faculae, and granulation.
Sunspots
Sunspots appear as dark spots on the visible surface
of the Sun. Temperatures in the dark centres of sunspots drop to about 3700 K (compared to 5700 K for the surrounding photosphere).
They typically last for several days, although very large ones may live for several weeks. Sunspots are magnetic regions on
the Sun with magnetic field strengths thousands of times stronger than the Earth's magnetic field. Sunspots usually come in
groups with two sets of spots. One set will have positive or north magnetic field while the other set will have negative or
south magnetic field. The field is strongest in the darker parts of the sunspots - the umbra. The field is weaker and more
horizontal in the lighter part - the penumbra. The largest sunspot ever recorded was visible in March and April 1947 and covered
an area of over 7,000 million square miles; about a hundred Earths could be fitted into this area!
The sunspot number is calculated by first counting the
number of sunspot groups and then the number of individual sunspots. The "sunspot number" is then given by the sum of the
number of individual sunspots and ten times the number of groups. Since most sunspot groups have, on average, about ten spots,
this formula for counting sunspots gives reliable numbers even when the observing conditions are less than ideal and small
spots are hard to see. Monthly averages show that the number of sunspots visible on the sun waxes and wanes with an approximate
11-year cycle.
It is interesting to recall that it was Samuel Heinrich
Schwabe ( 1789– 1875), a German apothecary and amateur astronomer that discovered the 11-year cycle. In the hope of
discovering a new planet between Mercury and the Sun, he made daily observations and tallies of sunspots. In 1843, after 17
years of sunspot counts, he noted a periodicity of 10 or 11 years in their totals. Julius Schmidt and Rudolf Wolf supported
this periodicity while others remained sceptical. Only when other independent observations made by Wolf, Sabine, Gautier and
J. von Lamont supported the sunspot cycle and variations in the Earth's magnetic field with an 11-year frequency, did Schwabe
receive wide support. Before that time (1826 – 51) no such periodicity was even suspected. The RAS Library at Burlington
House, London, keeps 31 volumes with his observational notes covering the years 1825 to 1867.
Faculae
Faculae are bright areas that are usually most easily
seen near the limb, or edge, of the solar disk. These are also magnetic areas but the magnetic field is concentrated in much
smaller bundles than in sunspots. While the sunspots tend to make the Sun look darker, the faculae make it look brighter.
During a sunspot cycle the faculae actually win out over the sunspots and make the Sun appear slightly (about 0.1%) brighter
at sunspot maximum than at sunspot minimum.
Granulation
Granules are small (about 1000 km across) cellular features
that cover the entire Sun except for those areas covered by sunspots. These features are the tops of convection cells where
hot fluid rises up from the interior in the bright areas, spreads out across the surface, cools and then sinks inward along
the dark lanes. Individual granules last for only about 20 minutes. The granulation pattern is continually evolving as old
granules are pushed aside by newly emerging ones. The flow within the granules can reach supersonic speeds of more than
7 km/s (15,000 mph) and produce sonic "booms" and other noise that generates waves on the Sun's surface.
Pores
Solar pores are small sunspots that lack a penumbral
structure. Because they are part of the early stage of sunspot evolution, they are important in understanding the mechanism
of small-scale flux emergence on the surface of the Sun. The magnetic field lines in pores are almost vertical, however, the
lines of force in sunspot penumbrae are highly inclined. Studying the evolution of pores can lead to a better understanding
of the interaction between magnetic fields and the surrounding convective motions. In order to study the fine structures in
and around pores and sunspots, high spatial, temporal, and spectral resolution observations are needed. The disturbing effect
of the Earth’s atmosphere limits the resolution that we can obtain even when using large telescopes.
Super Granules
Super granules are much larger versions of granules
(about 35,000 km across) but are best seen in measurements of the "Doppler shift" where light from material moving toward
us is shifted to the blue while light from material moving away from us is shifted to the red. These features also cover the
entire Sun and are continually evolving. Individual super granules last for a day or two and have flow speeds of about
0.5 km/s (1000 mph). The fluid flows observed in super granules carry magnetic field bundles to the edges of the cells where
they produce the chromospheric network.
SOLAR FEATURES
Flares
Solar flares are tremendous explosions on the surface of the Sun. In a matter of just a few minutes they heat
material to many millions of degrees and release as much energy as a billion megatons of TNT. They occur near sunspots, usually
along the dividing line (neutral line) between areas of oppositely directed magnetic fields
Flares release energy in many forms - electro-magnetic (Gamma rays and X-rays), energetic particles (protons
and electrons), and mass flows. Flares are characterized by their brightness in X-rays (X-Ray flux). The biggest flares are
X-Class flares. M-Class flares have a tenth the energy and C-Class flares have a tenth of the X-ray flux seen in M-Class flares.
The National Oceanic and Atmospheric Administration (NOAA) monitor the X-Ray flux from the Sun with detectors on some of its satellites.
Solar flares are observed using filters to isolate the
light emitted by hydrogen atoms in the red region of the solar spectrum (the H-alpha spectral line). Most solar observatories
have H-alpha telescopes and some observatories monitor the Sun for solar flares by capturing images of the Sun every few seconds.
With the introduction in the last few years of H-alpha filters affordable by amateurs, many can now carry out this work. This
is very important as the energy sent out by the flares can not only cause great damage to the astronauts onboard the International
Space Station, (ISS), but also disrupt electrical and radio installations on Earth, as well as giving us nice displays of
aurora! These flares are normally referred to as Coronal Mass Ejections or CMEs.
SOLAR FEATURES
Layers
From
Core to Corona
Layers of the Sun
Image Credit: p. 110,125, Kaler
The Core
The innermost layer of the sun is the core. With a density of 160 g/cm^3,
10 times that of lead, the core might be expected to be solid. However, the core's temperature of 15 million kelvins (27 million
degrees Fahrenheit) keeps it in a gaseous state.
In the core, fusion reactions produce energy in the form of gamma rays and neutrinos. Gamma rays are photons
with high energy and high frequency. The gamma rays are absorbed and re-emitted by many atoms on their journey from the envelope
to the outside of the sun. When the gamma rays leave atoms, their average energy is reduced. However, the first law of thermodynamics
(which states that energy can neither be created nor be destroyed) plays a role and the number of photons
increases. Each high-energy gamma ray that leaves the solar envelope will eventually become a thousand low-energy photons.
The neutrinos are extremely nonreactive. To stop
a typical neutrino, one would have to send it through a light-year of lead! Several experiments are being performed to measure
the neutrino output from the sun. Chemicals containing elements with which neutrinos react are put in large pools in mines,
and the neutrinos' passage through the pools can be measured by the rare changes they cause in the nuclei in the pools. For
example, perchloroethane contains some isotopes of chlorine with 37 particles in the nucleus (17 protons, 20 neutrons). These
Cl-37 molecules can take in neutrinos and become radioactive Ar-37 (18 protons, 19 neutrons). From the amount of argon present,
the number of neutrinos can be calculated.
Solar Envelope
Outside of the core is the radiative envelope, which is surrounded by
the convective envelope. The temperature is 4 million kelvins (7 million degrees F). The density of the solar envelope is
much less than that of the core. The core contains 40 percent of the sun's mass in 10 percent of the volume, while the solar
envelope has 60 percent of the mass in 90 percent of the volume.
The solar envelope puts pressure on the core
and maintains the core's temperature.
The hotter a gas is, the more transparent it
is. The solar envelope is cooler and more opaque than the core. It becomes less efficient for energy to move by radiation,
and heat energy starts to build up at the outside of the radiative zone. The energy begins to move by convection, in huge
cells of circulating gas several hundred kilometers in diameter. Convection cells nearer to the outside are smaller than the
inner cells. The top of each cell is called a granule. Seen through a telescope, granules look like tiny specks of light.
Variations in the velocity of particles in granules cause slight wavelength changes in the spectra emitted by the sun.
Image Credit: p. 369, Chaisson
"Convective cells are arranged in tiers containing
cells of progressively smaller size as the surface is neared. This is still a highly simplified diagram, however. There are
many different cell sizes, and they are not so neatly arranged." (369, Chaisson)
Photosphere
The photosphere is the zone from which the sunlight we see is emitted.
The photosphere is a comparatively thin layer of low pressure gasses surrounding the envelope. It is only a few hundred kilometers
thick, with a temperature of 6000 K. The composition, temperature, and pressure of the photosphere are revealed by the spectrum
of sunlight. In fact, helium was discovered in 1896 by William Ramsey, when in analyzing the solar spectrum he found features
that did not belong to any gas known on earth. The newly-discovered gas was named helium in honor of Helios, the mythological
Greek god of the sun.
Chromosphere
In an eclipse, a red circle around the outside of the sun can sometimes
can be seen. This is the chromosphere. Its red coloring is caused by the abundance of hydrogen.
From the center of the sun to the chromosphere,
the temperature decreases proportionally as the distance from the core increases. The chromosphere's temperature, however,
is 7000 K, hotter than that of the photosphere. Temperatures continue to increase through the corona.
Sunspots
Sunspots are dark spots on the photosphere, typically
with the same diameter as the Earth. They have cooler temperatures than the photosphere. The center of a spot, the umbra,
looks dark gray if heavily filtered and is only 4500 K (as compared to the photosphere at 6000K). Around it is the penumbra,
which looks lighter gray (if filtered). Sunspots come in cycles, increasing sharply (in numbers) and then decreasing sharply.
The period of this solar cycle is about 11 years.
The sun has enormous organized magnetic fields
that reach from pole to pole. Loops of the magnetic field oppose convection in the convective envelope and stop the flow of
energy to the surface. This results in cool spots at the surface which produce less light than the warmer areas. These cool,
dark spots are the sunspots.
Corona
The outermost layer of the sun is the corona.
Only visible during eclipses, it is a low density cloud of plasma with higher transparency than the inner layers. The white
corona is a million times less bright than the inner layers of the sun, but is many times larger.
The corona is hotter than some of the inner layers.
Its average temperature is 1 million K (2 million degrees F) but in some places it can reach 3 million K (5 million degrees
F).
Temperatures steadily decrease as we move farther
away from the core, but after the photosphere they begin to rise again. There are several theories that explain this, but
none have been proven.
SOLAR FEATURES
Prominences
A strand of relatively cool gas in the solar corona which appears bright when seen at the
edge of the Sun against the blackness of space.
Prominences are dense clouds of material suspended above
the surface of the Sun by loops of magnetic field. Prominences and filaments are actually the same thing, except that prominences
are seen projecting out above the limb, or edge, of the Sun. Both filaments and prominences can remain in a quiet or quiescent
state for days or weeks. However, as the magnetic loops that support them slowly change, filaments and prominences can erupt
and rise off of the Sun over the course of a few minutes or hours. They are often visible during a total solar eclipse.
SOLAR FEATURES
Solar Wind
The solar corona is constantly losing particles.
Protons and electrons evaporate off the sun, and reach the earth at velocities of 500 km/s. Most of the mass of the sun is
held in by magnetic fields in the corona, but particles slip through occasional holes in the fields. Solar wind affects the
magnetic fields of all the planets in the solar system. When the solar wind hits the Earth's magnetic field, the wind compresses
the field and creates a shock wave called the Bow shock. Closer to the Earth are the Van Allen radiation belts where solar
particles are trapped due to magnetic forces. Still closer are huge rings of electric current around the poles, formed by
the influence of the solar wind on the magnetic field. Earth, Jupiter, Saturn, Uranus, and Neptune have magnetotails where
the wind extends their magnetic field.
The heliopause is the boundary where the sun's
solar wind hits the gasses of interstellar space. The sun's particles flow at least to Neptune, and probably farther. That
means that we're inside the sun!
