Archived Pages from 20th Century!!
Introduction: some basic solar facts and terminology
Nearly 1,392,000 km in diameter and kg
in mass, the Sun is the geometric and gravitational center of the planetary
system. The Earth, third planet 149,600,000 kilometers away from center,
remains to date the only planet in the solar system known to host life
forms. The energy radiated by the Sun ultimately is the source of all life
on Earth, a fact intuitively grasped by the majority of early civilizations,
most of which accordingly granting the Sun a place of prominence in their
respective religious practices. The
Watt of energy radiated by the Sun originate in its deep interior, where
thermonuclear fusion reactions combine hydrogen nuclei into helium nuclei.
The energy released by these nuclear reactions is carried outward in about
yr,
first by radiation from the center to about 70 percent of the Sun's radius,
then to the surface primarily via large-scale convective motions. This
latter region, comprising the outer 30 percent in radius of the Sun, is
known as the convection zone. Temperatures in the center of the
Sun approach 15 million degrees Kelvin, falling to a mere
degrees (!) at its surface. Because of these high temperatures, solar material
is in a state called plasma, which refers to a gas of ionized atoms.
In the case of the Sun, however, densities become high enough (particularly
in the deeper solar interior) that the solar plasma behaves more like a
fluid than a conventional gas. When speaking of the solar surface,
one is then not referring to a solid surface such as provided by the Earth's
crust, but rather to the photosphere, a fictitious spherical surface
from which the bulk of solar radiation originates. The solar atmosphere
refers to the region extending upward from and including the photosphere.
Sunlight passing through a glass prism or diffraction grating is decomposed
into its constituent colors, which make up the solar spectrum. At
first glance the solar spectrum appears characteristic of a body heated
to a temperature of 5800 degrees Kelvin (5530 degrees Celcius). The portion
of the spectrum visible to the human eye consists of the continuum of colors
violet---blue---green---yellow---red, mapping onto the wavelength range
4000---7000? (?
cm)
collectively referred to as white light. The Sun also emits radiation
at shorter (ultraviolet, X-ray) and longer (infrared, etc.) wavelengths,
but the solar radiative output is most intense in the visible portion of
the spectrum. Further scrutiny of the solar spectrum reveals the existence
of narrow, dark bands where the intensity of sunlight is greatly reduced.
These are known as spectral lines, and are associated with allowed
electronic transitions in atoms present in the solar atmosphere; sunlight,
traversing the atmosphere from below, is preferentially absorbed and scattered
away from the line of sight at these wavelengths, leading to reduced intensities
in the net outgoing spectrum. Because each chemical species is characterized
by a different set of allowed electronic transitions, identification of
spectral absorption lines in the solar spectrum allows the determination
of the chemical composition of the Sun's atmosphere. Photographs of the
Sun taken through narrow filters centered on strong spectral lines, because
of the greatly reduced background brightness, often reveal details lost
in white light. Other spectral lines are formed through collisional processes
(rather than by radiative excitation, as in the case of absorption lines)
processes, and so can be used as indicators of non-radiative effects in
the solar atmosphere. This slide set includes many such images, taken with
filters centered on the K line of neutral Calcium (
?)
and of the first Balmer line of Hydrogen (H
,
?).
The apparent daily path of the Sun in the Earth's sky is from East to
West. In reality, of course, this motion of the Sun is only apparent and
is due to the Earth spinning on its axis in the opposite direction. This
direction of rotation ---counterclockwise for an observer in space looking
down on the North pole--- is the same as the direction of the Earth's orbit
around the Sun, and of the spin of the Sun around its axis. With the direction
of rotation defined as East to West by convention, solar rotation produces
a displacement of features on the solar disk that proceeds from left to
right as seen from the Earth and with the solar North pole oriented towards
the top of the field of view (which is the case for all slides in this
set). Following the motion of features on the solar surface is complicated
by the fact that the Sun does not rotate as a solid body; a fluid parcel
located in the equatorial regions completes a revolution in
days, while in polar regions a revolution requires some 30 days.
Measurements of the solar rotation rate at high heliospheric latitudes
are arduous at best, so that the derived polar values consequently carry
significant uncertainties; that the solar poles rotate at least
%
slower than the equator is nevertheless well established. This pattern
is known as differential rotation, and persists throughout the solar
convection zone. Finally, one must realize that because the Earth moves
through
of its orbital
path in 25 days, the equatorial solar rotation period as seen from the
Earth (the synodic period) is longer than its true (sidereal) period by
about two days. We are now ready to turn to the slides.