The graph below shows the amount of radiation produced by the sun
over the range of wavelength
from 10 Angstroms (1 Angstrom = 10 ** -10 metres; or 0.1 nanometres) up
to 50,000 Angstroms.
This range spans from the extreme ultraviolet, through the ultraviolet,
visible and into the infra-red. The vertical axis on the graph gives the
amount of radiation received from the sun above the Earth's atmosphere
and so is not affected by the varying transmission of the atmosphere. The
amount of radiation (fv) is expressed in units of erg/square cm/second/Angstrom
and is plotted as a logarithm (log fv) because it varies by many powers of ten over the range in the graph.
The distribution of radiation can be divided into two regions. Above
a wavelength of around 1000 Angstroms (solid curve) the radiation produced
by the sun is 'thermal' in origin - i.e. it arises because the sun is a hot object. The spectral distribution has a shape known as a black
body curve with the peak occurring at around 5000 Angstroms - in the middle
of the range of wavelengths which we are able to see. For higher, or lower, wavelengths the radiation produced by the sun decreases.
Within this range, the region from around 1000 Angstroms up to around
4000 Angstroms is the ultraviolet and this radiation is generally
absorbed in the atmosphere of the Earth by the Ozone layer. It is this
radiation which, if not absorbed by the atmosphere, is important in the production of skin cancer. The amount of
radiation reaching the ground depends on factors such as whether the sun is close
to overhead in the sky, and conditions in the atmosphere such as the concentration of Ozone.
The second significant region in the figure is shown as two dashed lines
below a wavelength of around 1000 Angstroms. There are two lines because,
unlike the constant thermal radiation, this component varies with the
solar cycle with a larger amount being produced by the sun during
the higher parts of the solar cycle than during the lower parts of the cycle. Normally, this component is absorbed
by the atmosphere - in fact in the ionosphere which it is responsible for creating.
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