Continuing from last time, I discuss our sun with greater detail and then briefly mention radiation exchange, which is largely a consequence of that and electromagnetic radiation previously discussed, and largely determines earth and atmospheric temperature.

Solar interior to surface

Solar composition was previously mentioned, regarding abundance of elements in our sun. Additionally, the solar interior consists of many free particles - some products of nuclear fusion, some a consequence of molecular collisions in a high temperature environment. A mixture of charged particles is called a plasma, which exists inside and outside the sun (assuming a boundary is specified).

Sunspots

The solar surface more clearly affects weather. Perhaps the most well-known solar features are sunspots, dark magnetic regions in the solar photosphere. If a person guessed that sunspots are dark because they are cool, they'd be correct - sort of. Their typical temperature being about 4000-5000 °K, they are hot, and would appear quite bright if they could be isolated in space; but because of contrast, are quite dark in the 5800 °K photosphere. If you are very picky regarding words like me, you may be thinking 'in the photosphere...?' - yes. These relatively cool areas are dense, and thus form a depression in the photosphere (which can be seen on the solar limb); lowest area being a dark umbra, which a less dark penumbra surrounds. Why don't hotter photospheric gases fill the depression? Sunspots' magnetic fields evidently prevent such a flow, shielding its base & periphery. Hot gases are forced around them, perhaps causing the bright faculae there (though faculae are also isolated and rather abundant near the solar poles, where sunspots seldom occur). Sunspots are greatly magnetized regions, a magnetic couplet occurring with each sunspot. The situation is opposite for each solar hemisphere; - magnetic regions (dark) often associated with sunspots or those forming) leading + magnetic regions (bright) in one hemisphere, + magnetic regions leading - in the other. You may be familiar with the sunspot cycle - that number of sunspots varies quite significantly with an approximately 11 year period, as illustrated. An extended period occurred (~ 1645-1715) though, during which almost no sunspots were observed, known as the Maunder Minimum. That was not long after Galileo's original observation of sunspots during 1610 and those which followed. Why did that happen? I know of 2 possibilities - the reasonable one being existence of a much longer cycle of several hundred or perhaps 1000 years; the paranoid one being that once extraterrestrials knew we knew about sunspots, they decided to play a little game with us, making them disappear for several decades No matter what reason exists, energy emitted from our sun evidently increases during times of large sunspot activity, as correspondence of recent satellite measurements of the solar constant (definition) and annual number of sunspots indicates. This is mainly because of many other bright areas such as faculae during active solar periods. Solar energy flux does decrease slightly when large sunspot groups pass near the sun's center. You can see that the sunspot cycle has been very consistent during the past few centuries. A magnetic dynamo because of Sun's differential rotation is thought as being responsible for the sunspot cycle, causing sunspot formation at greatest solar latitudes at the beginning of a new cycle (which can be seen on current solar images), developing toward the solar equator at the end of a sunspot cycle, as a butterfly diagram indicates. The dynamo causes a magnetic pole reversal in the sun, such that the next cycle occurs with polarity reversed (+ & - regions mentioned above reversed). Thus, many people consider the sunspot cycle a 22 year cycle.

Solar atmosphere

Above the photosphere are the chromosphere and the corona, often considered the solar atmosphere (particularly the chromosphere). The chromosphere is an eventful area, with hot gases often above sunspots, and magnetic loops and prominences. Solar flares are often ejected from the solar surface, becoming part of its atmosphere. The solar corona is perhaps the most brilliant solar feature, a diffuse portion of the outer solar atmosphere with temperatures of as much as 2000000 °K from which X-rays are emitted. For much time it was only observable during solar eclipses because the intense solar beam & great scattering near the solar disc mask it. Invention of the coronagraph, which blocks the solar disc and eliminates stray light near it using 2 lenses, enabled study of the corona. Now very sophisticated coronagraphs are flow on satellites, for more detailed observation. Coronal mass ejections (called the hurricanes of solar weather) occur, though processes causing them are not extremely well-known.

Solar wind & magnetosphere

A constant flow of plasma is emitted from our sun, called the solar wind. It travels toward earth and all other directions with speeds of several hundred km/sec, extending to the outer extent of our solar system is currently measured using satellites. Its interaction with comets (image description), dirty iceballs, causes the familiar tail, which points directly from the sun whichever direction the comet travels. Interaction of the solar wind with Earth's magnetic field causes radiation belts, ring currents, and other features in and below the magnetosphere. Strong solar flare events and other solar magnetic disturbances greatly augment the solar wind, with affects including aurorae and modification of earth's magnetic field in the upper atmosphere near Earth's polar regions, where magnetic field cusps exist. Some geomagnetic storms are sufficiently strong to cause electric currents to flow across satellites, power lines, and other electrical objects on or near Earth, damaging some and disabling others. Space weather forecasts (main site) are often issued for aiding preparation for such events.

Radiation exchange at Earth

Though geomagnetic storms are a major concern, our sun obviously affects Earth more so because it provides heat for it and its atmosphere. Quite often the term 'radiation balance' is used, though such a term implies equal amounts entering and leaving, which is not necessarily so. Thus, I like the term radiation exchange - regarding solar electromagnetic (EM) radiation and terrestrial EM radiation. EM radiation emission increases as temperature does (proportional with its 4^th power for a theoretical blackbody), thus solar EM radiation (solar energy) is mainly ultraviolet, visible, and infrared and terrestrial EM radiation (terrestrial energy) is mainly infrared. An equilibrium temperature can be supposed, a consequence of solar and terrestrial energy balance. I plan to provide such calculations later, but at this point recognition that increasing solar energy increases such an equilibrium temperature is sufficient (more solar energy implies warmer Earth).

Long-term climate & glaciations

I obviously cannot completely discuss these topics here, but mention the very basics, at least for now. Because of radiation exchange, possible connection among climate and solar parameters is often considered. Geology and paleoclimatology has revealed likelihood of many abnormally warm and cold periods during Earth's existence, corresponding with long and short term glacial (ice age) and interglacial periods. What causes such changes? Many theories exist, but considering long-term changes, solar heating changes because of orbital changes are often considered most relevant; and geologic data supports such a theory. Johannes Kepler calculated that planetary orbits (around Sun) are elliptical, a consequence of 3 Laws governing orbital motion. This idea was quite controversial during his days - a time when religion ruled and the creation of God was commonly considered perfect and unblemished. Even the idea of spots on a supposedly perfect sun was ridiculed. Now we live during a different time, rather opposite, when a scientific explanation is required if practically any theory regarding physical occurrences is taken seriously. Another troublemaker, Issac Newton, used Kepler's discoveries for development of the Universal Law of Gravitation, which adequately described all particle motion. Because of Earth's elliptical orbit :

most solar energy reaches Earth when closest to Sun, presently about 3 January.

During the early 1900's, Militun Milankovitch worked many years developing a mathematical theory relating Earth motion with geologic evidence of past glaciations, emphasizing Earth's solar energy collection. Detailed astronomical measurements indicate basic changes of 3 aspects of Earth motion (and other very small ones - some chaotic!) - eccentricity :

tilt :

and equinoctial precession:

The primary eccentricity cycle (main site) is about 100000 years, and tilt cycle about 41000 years. Precession is a combination of 2 factors - wobbling of earth's rotation axis (like a top), and movement of earth's orbit (itself), which combine for about a 22000 year cycle (i.e., an extremely slow top!). How do these factors affect climate and thus glaciations? A question for that question is how do glaciers form and advance equatorward and recede poleward? Many possibilities exist, but seasonal growth and melting is considered the primary reason. If orbit changes so that Earth receives a more consistent solar energy amount during a year (especially near poles where glaciers are), summertime glacier melting is less effective, and winter weather not as cold (which probably increases snow amounts and thus glacier advancement). Using that as a basic premise, I consider influence of each of the orbital affects mentioned. If eccentricity became very large, such that perihelion (orbital point closest to sun) corresponded with Mercury's position, much if not all ice would melt during summer. That exaggerated example illustrates a circular orbit is most favorable for glaciations. According to Milankovitch theory, earth's orbit during the past several hundred thousand years has varied between near circular and about .06 eccentricity. Correspondence of the eccentricity cycle and supposed major glaciations is very good, though some researchers question whether such is because of near coincidence, other factors being responsible. Orbital eccentricity is presently .0167 and decreasing, favoring development of an ice age many thousands of years from now. If Earth's axis were tilted perpendicular with its orbital plane, polar regions would receive little sunlight and remain cold year-round, such that glaciers would probably extend much further south than presently. Earth's axis tilt varies between about 21.8° & 25.0°. It is presently 23.44°, and decreasing. According to theory, this also favors development of an ice age several thousands of years from now. Precession's affect is less certain, but a glance at our globe reveals that most land is in the Northern Hemisphere and most ocean in the Southern Hemisphere. Particularly, a large ocean expanse surrounds Antarctica, preventing great glacier movement there. Thus, glacier advancement is mainly a Northern Hemisphere phenomenon. Presently, Earth aphelion corresponds nearly with Summer Solstice, and perihelion nearly with Winter Solstice. Theory indicates that Summer Solstice is approaching perihelion, reaching it about 10000 years from now - least favorable for glacial conditions (most heating variation among seasons). We're not experiencing an ice age now though, only about 840 years after the most favorable situation. Rather than considering these factors separately, Milankovitch calculated combined effects regarding solar energy collection, and was confident that such changes corresponded with major glaciations.

Other factors are more relevant regarding our climate situation - a possible greenhouse effect, etc.

Text and embedded images are copyright of Joseph Bartlo, though may be used with proper crediting.