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SOLAR PHYSICS AND TERRESTRIAL EFFECTS Chapter 3
Chapter 3
Studying the Sun
We see the Sun because of the radiation that leaves it and arrives at Earth after about 8 minutes of travel through space.
Until the 1940s, we had only seen the Sun in the visible range, which gives us a look at the photosphere and low
chromosphere where most of the visible light is produced. But as we began to look at the Sun in other wavelengths, by
devising appropriate sensing devices like x-ray telescopes, a new and vastly more detailed picture of the Sun emerged.
Each type of radiation—radio, infrared, visible, ultraviolet, x-ray, and gamma—originates predominantly from a
different part of the Sun. The different layers, from the photosphere up into the corona, can be seen by looking in
different specific wavelengths; each layer reveals different secrets of the complex and turbulent Sun.
Magnetograms —pictures of the magnetic field regions of the sun—give us another view of the Sun, which suggests
that the driving mechanism in the solar atmosphere is the magnetic field. The contorted, dynamic magnetic fields
emerging from the photosphere and chromosphere have a seemingly infinite complexity. Today it is clear that all of the
processes occurring on the Sun are beyond our present knowledge. Developing a model for the Sun is truly a new
frontier.
Section 1.—White Light: The Photosphere
The light that illuminates our world and enables us to see things with our eyes comes from the photosphere of the Sun.
To most of us, the photosphere is the Sun. Most of the energy that we receive from the Sun is the visible, or white light
that radiates from the thin, relatively cool photosphere. The photosphere is the region of the Sun where emitted photons
are able to escape into space, rather than being scattered or absorbed as they are in layers. At a mere 6000 K, it is one of
the coolest parts of the Sun; it has a thickness of only about 100 km, or about 0.1% of the solar radius.
The visible radiation produced in the photosphere is characteristic of matter at a few thousand kelvins, undergoing
atomic-energy-level transitions. The matter in the photosphere is a plasma, with a high degree of dissociation of
electrons from nuclei, resulting in charged particles. But the plasma of the photosphere is cool enough that it is largely
in an atomic state , much like the matter we are used to on Earth. This means that nuclei have electrons in orbit, although
outer electrons are missing, and the orbiting electrons are making transitions down to lower energy levels, producing
photons of light. When broken apart with a spectroscope, the white light from the photosphere makes a continuous
spectrum of wavelengths, interrupted by absorption lines. The spectral line signature of virtually every element has
been detected in this white light, but by far the most abundant elements in the solar atmosphere are hydrogen (92%),
and helium (7.8%). The Sun seems to follow the cosmic recipe found all over the universe of 10 parts hydrogen to 1 part
helium with a pinch of every other element. The most common trace elements are oxygen, carbon, nitrogen, iron and
magnesium.
Using proper filters the photosphere can be seen to have a granular appearance with a mixture of brighter and darker
spots. These regions are the tops of convective cells, with bright areas resulting from hotter plasma bubbling upward
and the darker regions caused by cooler plasma sinking into the interior. The large scale convective patterns within the
Sun are thought to be closely tied to the magnetic effects seen in the photosphere, and in the associated sunspot regions.
Section 2.—Sunspots
After the dazzling brightness of the Sun, its next most obvious feature is the appearance of sunspots on the photosphere.
Sunspots have been reported for more than 2000 years, and were probably seen in early times when the solar disk was
Chapter 3 SOLAR PHYSICS AND TERRESTRIAL EFFECTS
darkened near sunrise or sunset, or by the smoke of a volcanic eruption. Throughout most of history the Sun has been a
symbol of purity. Within this context, Galileo’s report of sunspots, observed with an early telescope around 1610, was
not taken well by the Church and other protectors of the status quo. There was much speculation about what these
blemishes were, including that they were planets or other objects passing in front of the Sun.
As telescopes improved, Galileo and others found that sunspots have a dark central region, called the umbra (shadow),
surrounded by a lighter region, the penumbra. Observations over many days soon revealed that the spots move across
the Sun as the Sun rotates and that the equatorial region moves faster than the higher latitude regions. Sunspots were
also observed to grow in clusters over several days or weeks and then gradually disappear. For reasons not yet
understood, there were very few sunspots from about 1645 to 1715. This Maunder Minimum , as it is now called,
coincided roughly with a very cool period in Europe, and this has raised the possibility of a connection between sunspot
activity and climate on Earth.
When sunspot activity returned in about 1715, Sun observers began to keep records of sunspot numbers. In 1843, an
amateur astronomer named Heinrich Schwabe studied these records and noticed that sunspot numbers reached a
maximum every 10 to 12 years, and nearly disappeared in between these periods. This sunspot cycle is now well
documented over the last 200 years. Solar cycles from 1900 to 2015 (Figure 3-1) illustrate the variances from one
cycle to another, not only in sunspot numbers, but also reveal a double peak of activity in recent years.
Figure 3–1.— Sunspot Cycles from 1900 to 2015. One periodic increase and decrease of sunspots defines a
cycle. Cycle 2 3 began in 1996, peaked in 2000 and ended in about 2008.
Beginning with the minimum that occurred around 1755, sunspot cycles have been numbered; for example solar
cycle 2 4 began with the 2008 minimum and had two peaks, the first in about 2011 and the second in early 2014
(Figure 3–2). This sunspot behavior reveals how scientists still have many questions about sunspots—their origin,
their behavior, and their relation to flares. Most flares originate in the active regions which usually surround
sunspots, but as yet it is still very difficult to predict when a flare will erupt.
Sunspots are cooler than the surrounding photosphere by about 1800 K. At about 4200 K, they are the coolest part of the
Sun. This lower temperature is thought to be due to a lack of convection which brings hotter plasma to the surface. Seen
from the side as they appear or disappear around the limb of the Sun, it is clear that sunspots are depressions in the
photosphere.
In 1908, George Hale discovered that sunspots had strong magnetic fields associated with them. He concluded this
when he observed the newly discovered Zeemann effect in the spectral lines of light emitted from sunspots. The
Zeemann effect is a splitting of spectral lines that occurs when the emitting or absorbing atoms are immersed in a
magnetic field. The Zeemann effect is still used today to make magnetic pictures, or magnetograms, of the Sun. The
Chapter 3 SOLAR PHYSICS AND TERRESTRIAL EFFECTS
(a)
(b)
Figure 3–3.— (a) A pair of bar magnets submerged below the photosphere would produce fields at the surface
resembling those of an active region near sunspots. (b) More realistically, flexible magnetic tubes, or flux tubes,
probably give rise to the magnetic fields that we see.
of the sunspot maximum, spots are most likely to form at latitudes of 10 to 15. As the cycle subsides toward a
minimum, the spots get smaller and appear closer to the equatorial region. There is an overlap of the end of one cycle
and the beginning of the next, as new spots from the next cycle form at high latitudes while spots from the present cycle
are still present near the equator. Recently, it has been discovered that the high-latitude spots of a new cycle have a
predecessor, known as ephemeral regions, which form at very high latitudes near the time of the maximum of the
previous cycle. This general drifting of sunspot appearance from high latitudes toward the equator was first discovered
by Edward Maunder in 1904, when he plotted the latitude of sunspots over many cycles (Figure 3–4). It is not yet
known why sunspots migrate in this way, but we suspect that a number of different interior convective and rotational
processes determine where magnetic flux emerges and how it becomes organized into sunspots.
A strong correlation has been established between sunspots and solar flare activity, with more numerous and
energetic flares found near the larger and more complex types of sunspot groups. Because flares can have such an
adverse effect on our technical systems on Earth, there is great interest in predicting when flares will occur and how
large they will be. Sunspot observations provide one of the best tools for flare prediction and there have been many
attempts to classify sunspots according to their likelihood of producing flare activity. The earliest such classification
was devised by Cortie in 1901. This system was modified by Waldmeier, in 1947, into what is referred to as the
Zurich system of sunspot classification. The Zurich system was found in the 1950s and 60s to still be too simple for
effective flare prediction. Highly experienced sunspot watchers, including Patrick McIntosh at the NOAA Space
Environment Lab in Boulder, began to notice structural and dynamic aspects of sunspot groups that correlated
well with flares but were not a part of the Zurich classification. Some of these missing parameters were
incorporated into a revision of the Zurich system and introduced by McIntosh in 1966. The McIntosh Classification
system has 60 types of sunspot groups and has been widely used ever since. Additionally, the Mount Wilson Sunspot
Magnetic Classification system is used to assess and categorize the magnetic complexity of sunspot groups. The
McIntosh classification system is used in conjunction with the Mount Wilson Sunspot Magnetic classification system
in order for space weather forecasters to better codify and track changes in sunspot group complexity. A relationship
often exists between sunspot regions' classifications to flare intensity and potential. These classification schemes
provide a means for forecasters to track regions' changes and aid in the ability to predict flares, especially when used
in conjunction with other available information sources like x-ray activity, radio emission levels, and magnetograms.
Flares are likely to erupt in large sunspot regions that are growing rapidly and rotating like hurricanes. Flares can also
arise in areas far from sunspots, and sometimes large sunspot areas produce very little flare activity. At present, we are
SOLAR PHYSICS AND TERRESTRIAL EFFECTS Chapter 3
Figure 3–4.— A schematic illustration of the migration of sunspots from higher latitudes toward the equator dur-
ing each cycle (shown schematically below) is seen in the characteristic “butterfly” pattern. The overlap of the
end of one cycle and the beginning of the next can also be seen.
Years
Solar Latitude
Sunspot number
fairly good at predicting where flares will erupt, but not very good at predicting when they will occur. A deeper
understanding of sunspots is certainly required before our flare prediction can be accurate.
Section 3.—Hydrogen-Alpha: The Low Chromosphere
By the early 1800s, scientists had begun to explore the nature of light by breaking it up into its component spectrum of
wavelengths. The spectral analysis of light from different flames revealed that each element produced a unique set of
wavelengths, or spectral lines. The explanation for this would not come for another 100 years, with the Bohr model of
the atom and the idea of transitions between atomic energy levels. Nonetheless, early spectroscopists could identify
elements from the light they emitted, and astronomers began to look at the light of the Sun with spectroscopes. The
spectral lines seen in sunlight were similar to those seen in the light from laboratory sources on Earth, and so the
chemical composition of the Sun could be surmised. Around 1814, Joseph Fraunhofer noticed that there were dark
lines in certain places on the bright emission spectrum from the Sun (Figure 3–5). He did not understand what caused
these dark lines, but he mapped the most prominent ones and labeled them simply A, B, C, and so on. By 1859, Gustav
Kirchhoff had discovered in his experiments that these dark lines, now called Fraunhofer lines , were caused by the
absorption of light as it passed through a vapor of atoms. He suggested that the white light being emitted from the
photosphere must be passing through a cooler layer that was absorbing particular wavelengths characteristic of the
elements in that cooler layer.
In 1885, Balmer completed a detailed study of the spectrum of visible light produced by hydrogen. The hydrogen
spectrum had a very distinctive pattern of lines crowding closer and closer together toward shorter wavelengths.
Balmer was able to find an equation which accurately gave the wavelengths of these visible lines, now called the
Balmer Series. The longest wavelength line in the Balmer series—the line—is red with a wavelength of 656
nanometers, and this line is seen prominently in the solar spectrum. In 1913 Niels Bohr was able to explain the Balmer
SOLAR PHYSICS AND TERRESTRIAL EFFECTS Chapter 3
Figure 3–6.— Two views of the Sun, one photograph taken in white light, one taken in H
Figure 3–7.— A close view of the low
chromosphere taken in the H depicting
the fibril structure around a solar region
The bright areas of plage lie above the
sunspots on the photosphere. Photo
courtesy of Big Bear Solar Observatory.
of the Sun reveal that filaments form along the boundaries between regions of positive and negative magnetic polarity
on the Sun. These boundaries, called neutral lines, run all over the solar surface. It is not known why filaments form on
some neutral lines but not others, but filaments are generally of interest because they are a common source of eruptions.
Solar physicists have constructed maps of the solar surface showing the locations of active regions, neutral lines,
filaments, and coronal holes for each month during the last 20 years. Looking at how these features appear, move, and
disappear will undoubtedly give us more understanding of the physics of the Sun.
Section 4.—Ultraviolet: The High Chromosphere
Pictures of the Sun at ultraviolet wavelengths were not possible until space-based instruments could be used, since our
atmosphere blocks most of the ultraviolet radiation. During 1973 and early 1974, the three Skylab missions made
extensive observations of the Sun in ultraviolet light and x-rays, which brought an avalanche of new knowledge about
the Sun (see Figure 3–8). Skylab was a low-orbiting space station that could hold a crew of three astronauts.
Skylab was equipped with a variety of instruments for studying the Sun in ultraviolet, x-ray, white light, and H light.
The three missions, which lasted 28 days, 59 days, and 84 days, were among the most scientifically significant
Chapter 3 SOLAR PHYSICS AND TERRESTRIAL EFFECTS
Figure 3–8.— An x-ray image of the Sun taken during the Skylab mission on June 30, 1973. The right photo
shows a coronal hole outlined in white. The large boot-shaped coronal hole stretches from the north pole into the
southern latitudes. The left photo shows the lines of reversal in polarity of magnetic fields. The coronal hole lies
in the middle of a large unipolar area of positive polarity.
accomplishments of our space program to date. The vast amount of data collected by Skylab brought about a huge leap
in our understanding of the Sun. Ironically, Skylab itself was a victim of solar activity. Its orbit decayed rapidly as
Earth’s atmosphere heated and expanded because of high activity levels on the Sun; Skylab fell to Earth in 1979. Since
that time, we have had limited capability to collect ultraviolet data because government priorities have shifted away
from the launching of research satellites.
The ultraviolet radiation that the Sun emits comes from the upper chromosphere, which is at a temperature of around
70,000 K. To some extent, the type of radiation produced by each region of the Sun may be thought of as corresponding
to the temperature of that region. A plasma emits radiation as a blackbody , producing a broad spectrum of wavelengths
(Figure 3–9). However, any blackbody has a peak at a certain wavelength that is determined by the temperature, and it
will produce radiation predominantly in this wavelength. Although each region of the Sun emits virtually all types of
200 400 600 800 1000 1200 1400 1600 1800 2000
6000 K
4500 K
3000 K
Figure 3–9.— Schematic black-
body radiation curves at three
temperatures. The wavelength of
maximum intensity varies in-
versely with the absolute tem-
perature as in the formula
m
C
T
Relative Inensity
(nm)
Chapter 3 SOLAR PHYSICS AND TERRESTRIAL EFFECTS
flares. X-rays produced by a rapid deceleration are referred to as Bremsstrahlung (German for “braking rays”) x-rays.
X-ray images of the Sun (Figure 3–10) reveal the structure and behavior of the corona. The x-ray emission is brighter
where the coronal plasma is hotter and more dense. Flares are usually first detected here on Earth from the rise in x-ray
flux, and for this reason the monitoring of the Sun at x-ray wavelengths has a high priority. At the present time, the Sun
is continually monitored in the x-ray wavelengths by NOAA weather satellites called Geostationary Operational
Environmental Satellites ( GOES ).
Figure 3–10.— An x-ray image of the Sun
taken in 2 016 , from the GOES-13 solar x-
ray imager (SXI). Various exposure
settings are captured throughout a
sequence of images taken each minute.
This particular image (Level-1) allows a
better look at coronal structure; as well
as some examination of active regions
and flares.
There are two main geosynchronous orbital locations occupied by GOES satellites in order to cover the
meteorological needs of North America, to include the east Pacific and west Atlantic oceans; they are GOES west
(about 135W-137W) and GOES east (about 75W). Space weather packages on these platforms monitor the Sun and
the near-Earth space environment with x-ray and energetic particle sensors, and magnetometers. Unfortunately,
satellites such as these have limited lifetimes as their orbits drift. Some become unusable over time as sensors go bad
or their location wanders too much from the proper "stationary" location. Early in their lifetime, orbital corrections
can be made by on-board thrusters; but eventually the fuel for these runs low and they need to be replaced. The
GOES satellites comprise a "constellation" and the latest platforms are expected have an operational lifetime through
the year 2036. The GOES satellites' raw data are downlinked to antennas in Boulder, CO and processed at SWPC.
The primary x-ray wavelength monitored for flare activity is 1 to 8 Angstroms. When a solar flare occurs, the x-ray
level (flux) rises dramatically. When the x-ray flux rises above a certain threshold, alerts are immediately
transmitted by SWPC forecasters to customers all over the world. The x-rays leaving the site of a flare travel at the
speed of light and take approximately 8 minutes to reach Earth. Larger flares from favorable locations may on rare
occasions accelerate quantities of energetic particles from the Sun that can arrive to Earth within 15 minutes to some
hours later. Proton flux is monitored for these particles and when it appears that specific thresholds are expected to
be reached, warnings are sent out.
Additional satellites have been placed into orbits that allow monitoring of the Sun and the space environment to
include: DSCOVR (Deep Space Climate Observatory), a NOAA operational satellite placed in an orbital area that
keeps it directly between the Sun and the Earth, about 1 million miles from Earth known as L1. This platform
measures the solar wind environment (speed, density, temperature, and interplanetary magnetic field (IMF)
parameters) and acts as a kind of observational "space bouy" for conditions soon to arrive at Earth. DSCOVR is
SOLAR PHYSICS AND TERRESTRIAL EFFECTS Chapter 3
critical to SWPC forecasters ability to monitor solar wind conditions real-time and warn of impending geomagnetic
storms. SOHO (Solar & Optical Heliospheric Observatory) is a NASA research satellite also located at L1. The
SOHO LASCO instrument (Large Angle & Spectrometric Coronagraph) takes periodic images of the Sun's corona
using an occulting disk that allows a visual look at the outer corona at different distances from the Sun. The two most
used by forecasters are the C2 (1.5 to 6 solar radii) and C3 (3.5 to 30 solar radii) coronagraphs. The LASCO
instrument is crucial for identifying large expulsions of plasma from the Sun known as coronal mass ejections
( CME ); often associated with solar flares or filament eruptions. Forecasters examine LASCO imagery for indications
of CMEs and when detected, analyze them using computer tools and models to help determine if they may be Earth-
directed, and if so, predict timing of arrival and issue geomagnetic storm watches up to three days in advance.
Section 6.—Radio Emission: Solar Flares
Radio astronomers have been monitoring the Sun in the radio wavelength range since the 1940s. The Sun is a very
“noisy” radio source, meaning that it produces a wide range of radio emissions on a steady basis. Unlike the ultraviolet
and x-ray emissions, radio waves generally do penetrate Earth’s atmosphere, so they can be picked up with ground
based instruments. These radio telescopes have large parabolic collecting areas for concentrating weak signals. The
radio range is extremely broad, from long waves that have wavelengths of thousands of kilometers to microwaves with
wavelengths of a few millimeters. Much of the study of the Sun in the radio range has been done at a wavelength of 10.
cm, but today there is extensive monitoring down to wavelengths of a few millimeters. It is believed that these short
radio waves (or microwaves) are produced by charges in circular motion during a flare. Radiation produced by charges
in circular motion is often referred to as synchrotron radiation. When a flare occurs, electric fields arise which
accelerate the particles of the plasma. Electrons are accelerated in the opposite direction from protons or positive ions.
When these charges interact with the magnetic fields that are present, they accelerate in circular or spiral-shaped paths
around the field lines. Electrons, with their small mass, have a high acceleration and produce synchrotron radiation
with a wavelength on the order of 10 cm.
The level of 10.7-cm emission seems to parallel the level of solar activity. In fact, radio emissions follow the 11-year
sunspot cycle, reaching maximum intensity during times of sunspot maxima (Figure 3–11). It is hoped that the 10.7-cm
radio emission will provide another clue that will help us forecast flares.
0
30
60
90
120
150
180
210
240
1964 1974 1984
Figure 3–11.— Radio
emission (10.7 cm flux)
follows the sunspot
cycles fairly well.
* Flux Units are 10
-
W / (m
2
Hz)
Sunspot cycle
Radio Emissions
Year
Sunspot numbers and Flux Units*
