Another major scientific advance came in 1817 with Fraunhofer's discovery of 'dark lines' in the spectrum of light from the Sun. Coupled with the understanding of atomic energy levels, studies of the solar spectrum gave us a scientific method to learn the atomic composition of the sun and the temperatures in its outer layers. Solar spectroscopy advanced rapidly late in the 19th century to the development of specialized instruments showing the existence of magnetic fields on the Sun at Mt. Wilson Observatory in 1908.
Solar astronomers continued fruitful studies of the Sun from the ground until the space age dawned in 1946 with rocket observations and then accelerated in 1957 with the launch of Sputnik. Rocket instruments and orbiting satellites allowed observation of the ultraviolet and x-ray regions of the solar spectrum for the first time (since these wavelengths are normally absorbed by the earth's atmosphere and do not reach the ground) and thus opened the way for high temperature astrophysics to be applied to the solar atmosphere where temperature can exceed 1 million degrees. Although our understanding of solar-type stars required ultraviolet radiation from the Sun, such short wavelength emission cannot be observed from the ground because of absorption by the Earth's atmosphere.
It is this instrumental heritage to record the radiation from the Sun
that gives us the tools we use today in the continuing study of the Sun
and its variation with time.
ca. 200 BC
The distance to the Sun
1543
The Sun moves to center stage
1609
The Sun in focus
1610
First telescopic observations of sunspots
1644
The Sun as a star
1645-1715
The Maunder minimum
1687 The mass
of the Sun
1774-1801
The physical nature of sunspots
1817
Solar spectroscopy is born
1843
The sunspot cycle
1852
The sunspot cycle is linked to geomagnetic activity
1859
First observation of a solar flare
1860
First observations of a coronal mass ejection
1908-1919
The magnetic nature of sunspots
Above the photosphere are two additional layers, the chromosphere and
corona, which were first identified at eclipses of the Sun by the Moon.
The chromosphere is an inhomogeneous layer extending 10,000 km above the
photosphere. It is best thought of as the transition from the
photosphere to the corona. The very outer extent of the Sun proper is
the tenuous corona which can extend several million kilometers into the
interplanetary medium. Such extensions of the solar atmosphere produce
the striking images seen at the times of solar eclipses.
It is the variation of the visible solar atmosphere that affects the
Earth's atmosphere directly as a source of heat as well as a modifier of
the its chemistry
. If we are to
anticipate the effects of solar change on short times of minutes to
decades, we must have a clear understanding of how the Sun's atmosphere
produces radiation at all wavelength from x-rays to the infrared. Since
the solar surface is mottled by darks spots and bright vein-like
features called faculae, their combination determines the net
variability in the radiation reaching the earth.
The outer solar atmosphere also expels solar gas and magnetism into the
interplanetary medium as a steady solar wind as well as transients
called coronal mass ejections. This flow of mass from the sun results
in variation in the amount of material trapped inside the dipolar
magnetic field of the earth. Therefore, today we study the total solar
output of radiation and material with the view to understanding its
sources inside the Sun as well as their variation and visibility in the
outer solar atmosphere. The parallel research to understand the
detailed effects of this variability in changing the Earth's atmosphere
gives the research strategy behind the larger study of coupling of the
outer atmosphere of the Sun with that of the Earth.
The sunspot cycle (HAO slideset, #17)
Hale's sunspot polarity law (HAO slideset, #19)
Two more coronal mass ejections (HAO slideset, #14)
Some great moments in the history of solar physics:
Does the Sun have a surface?
Does the brightness of the Sun change over time?
How does the Sun work (what goes on inside)?
What are the key areas of solar research today?
What is a sunspot?
What is a coronal mass ejection?
Return to HAO homepage.
-Written by orw@ucar.edu.
-Revised 3 March 2000 by cmwucar.edu.
Copyright 2000, NCAR. - Approved by Paul Charbonneau -