184
13 Solar Astronomy
dropped to 5,500 K, and the energy created in the core is reemitted as optical and
ultraviolet light. The typical time taken for a photon to escape the Sun via this process
is of order a million years.
The atmosphere of the Sun above the photosphere consists of several layers hosting
many of the events you will observe. The coolest layer is that immediately above
the photosphere. It is at about 4,000 K, which is cool enough for some molecules to
exist. It is this region that generates many of the molecular spectral lines we see in
the solar spectrum (see Chap. 14). Above this cool layer is the hot region known as
the chromosphere. This region is the source of many emission lines, including the
important hydrogen alpha line. Above this lies the transition zone, a region of very
rapid temperature rise, which is largely visible in the ultraviolet. The transition zone
is, in turn, enveloped by the corona, an extremely hot region, up to 20 million K,
which can be considered the extended atmosphere of the Sun. The corona is not
normally visible to ground-based observations except during a solar eclipse.
13.2 Solar Features
In the practical section of this chapter, you will be asked to observe a number of solar
features. These can be placed into two groups of features: photospheric and chromospheric. How you observe them depends on their location and their temperature.
13.2.1 Sunspots
The Sun has a very powerful magnetic field that drives many of the processes that we
see on the photosphere. It is believed that the source of this field is the tachocline, the
boundary between the radiative and convective zones. The radiative zone and core
rotate as a solid body, while the convective zone and the boundaries above rotate
differentially. This differential rotation drives a magnetic dynamo that is the source
of the Sun’s magnetic field.
Magnetic field lines escape the photosphere and loop high into the Sun’s atmosphere before descending and reentering the photosphere. These field lines suppress
convection, preventing the flow of hot plasma from below. The reduced convection
results in a temperature drop at the photosphere of order 500 K. Although the photosphere is still hot, about 5,750 K, the 500 K temperature drop is enough for the region
of suppressed convection to look dark against the bright photosphere. This dark spot
is a sunspot. Sunspots always appear in pairs (although that is not always noticeable
to an observer), reflecting the north and south poles of the magnetic field lines. The
order in which the sunspots appear—for example, a north spot may be in front of a
south spot in the direction of rotation—is reversed between solar hemispheres. The
Sun goes through a period of increased magnetic activity, characterised by an increasing number of sunspots, every 11 years, a period known as the sunspot cycle. After
13 Solar Astronomy
dropped to 5,500 K, and the energy created in the core is reemitted as optical and
ultraviolet light. The typical time taken for a photon to escape the Sun via this process
is of order a million years.
The atmosphere of the Sun above the photosphere consists of several layers hosting
many of the events you will observe. The coolest layer is that immediately above
the photosphere. It is at about 4,000 K, which is cool enough for some molecules to
exist. It is this region that generates many of the molecular spectral lines we see in
the solar spectrum (see Chap. 14). Above this cool layer is the hot region known as
the chromosphere. This region is the source of many emission lines, including the
important hydrogen alpha line. Above this lies the transition zone, a region of very
rapid temperature rise, which is largely visible in the ultraviolet. The transition zone
is, in turn, enveloped by the corona, an extremely hot region, up to 20 million K,
which can be considered the extended atmosphere of the Sun. The corona is not
normally visible to ground-based observations except during a solar eclipse.
13.2 Solar Features
In the practical section of this chapter, you will be asked to observe a number of solar
features. These can be placed into two groups of features: photospheric and chromospheric. How you observe them depends on their location and their temperature.
13.2.1 Sunspots
The Sun has a very powerful magnetic field that drives many of the processes that we
see on the photosphere. It is believed that the source of this field is the tachocline, the
boundary between the radiative and convective zones. The radiative zone and core
rotate as a solid body, while the convective zone and the boundaries above rotate
differentially. This differential rotation drives a magnetic dynamo that is the source
of the Sun’s magnetic field.
Magnetic field lines escape the photosphere and loop high into the Sun’s atmosphere before descending and reentering the photosphere. These field lines suppress
convection, preventing the flow of hot plasma from below. The reduced convection
results in a temperature drop at the photosphere of order 500 K. Although the photosphere is still hot, about 5,750 K, the 500 K temperature drop is enough for the region
of suppressed convection to look dark against the bright photosphere. This dark spot
is a sunspot. Sunspots always appear in pairs (although that is not always noticeable
to an observer), reflecting the north and south poles of the magnetic field lines. The
order in which the sunspots appear—for example, a north spot may be in front of a
south spot in the direction of rotation—is reversed between solar hemispheres. The
Sun goes through a period of increased magnetic activity, characterised by an increasing number of sunspots, every 11 years, a period known as the sunspot cycle. After