Basic Origin of Solar Energy and Atmospheric Influence

Date : 27 April 1997

Solar energy* (S) is an extensive topic (like many) though, which can be discussed weekly during an entire year. I mention that any link in this article with * is from the definition list, so you don't always access them, but know they are defined. This week I describe its origination to basic influence of our atmosphere, next week specific atmospheric affects (particularly clouds), and the subsequent week how this resource is used. So I suppose this will be solar energy month !

Our Sun

Sol (our sun) is a rather average star in our Milky Way Galaxy. It is almost the exclusive energy-provider for Earth, and ultimately causes our weather. Its diameter is 1391900 km, 109 × Earth's (distances, etc. are best estimates). Average Sol to Earth distance (not precisely) is called an astronomical unit* (AU), and is 149570000 km. (Planetary spacing in our solar system indicates that distance from Sol to Mercury might more appropriately be called such.) Earth's orbit around Sol is quasi-elliptical, so distance between them varies 3.3% during a year. A planetary orbit inscribes equal areas during equal times, so that Earth's orbit is slowest at aphelion* and fastest at perihelion*. Other geologic time-scale motions of Earth are supposed. Milutin Milankovitch studied those in an attempt to better understand their relationships with long term climate changes, particularly glaciations.

Thermonuclear fusion reactions of light elements cause the enormous heat produced in Sol. Most are proton fusions of H to He. For an idea of the number of molecules in our sun, occurrence of those reactions averages 14000000000 years. With a total mass of 1.991 x 1030 kg though, a person can easily imagine a sufficient number occur so Sol continues burning. Because 10000000 °K is required for proton fusions (so collisional energy can overcome electrical repulsion), a significant portion of the solar core must be that temperature or greater. Extending outward from the core region a radiation-convection zone is supposed, where cooling and expansion against gravity occurs until an upper granular layer and photosphere (solar 'surface') is reached. On the photosphere are sunspots and various other features, sunspots being locally cool regions, typically 4000 °K, compared to the average 5800 °K photospheric temperature. Sunspots appear dark on the photosphere (contrast), and can cover significant portions of the solar surface. They do not decrease emitted S as much as expected because hot, bright faculae surround them. S flux can decrease fractions of a % during vigorous sunspot activity though. (Such is much less variation than thickness change of a cloud blocking our sun usually causes on Earth.) An approximately 22 year sunspot cycle occurs, during which 2 activity maxima and minima occur, polarity of associated magnetic fields reversing during those. From photospheric features, a rotation period varying from approximately 25 days along our sun's equator to 34 at its poles is evident. Above our sun's photosphere are the chromosphere, solar flares and prominences, and a corona. Hot gases which scatter S the photosphere emits may mainly cause the corona. Its temperature is 500000 - 2000000 °K, but it is so diffuse that it emits much less energy than the photosphere does. The solar magnetic field keeps the corona is mainly within a few solar radii of Sol. Some parts of the corona overcome the pull of that field, and a solar wind of plasma continually flows outward from Sun. The terrestrial magnetic field diverts the solar wind around our earth, protecting us from harmful particles, often causing magnificent auroras ! The corona's great temperature causes far ultraviolet (UV) and X-ray electromagnetic (EM) radiation* emission. Such is absorbed by N2, O2, N, O, H, and He at altitudes above 50 km in our atmosphere. O3 (ozone) at altitudes of 12-50 km and near our earth's surface protects us from harmful photospheric near UV EM radiation. Only near UV, visible (VIS), and near infrared (IR) S significantly remains, entering the troposphere. Photospheric temperature causes a .47 µm peak S "wavelength" emission, and very nearly all is emitted between .25 and 4 µm (1 cm = 10 000 µm).

Extraterrestrial Solar Energy Flux

The solar constant is mean S flux* perpendicular with the solar beam in outer space, at mean distance from Earth's center to Sol. It is 1370 W/m2. Although it is not a constant flux, I feel it is appropriately named; being extremely reliable and consistent. Earth's orbit causes varying extraterrestrial S flux* between approximately 1329 and 1421 W/m2.

Interaction of Solar Energy with Our Earth's Atmosphere

When S enters our atmosphere, it is absorbed and scattered, and some is transmitted directly thru, unaltered. Considering all S reaching our earth's surface, 10% is near UV, 45% VIS, and 45% near IR. As no coincidence, our eyes sense EM radiation between .38 and .72 µm (1 µm = 1000 nm), the portion of the EM spectrum* for which S we receive is most intense. After atmospheric scattering, peak S wavelength is .55 µm, reasons for which are discussed below. Thus, an ex-professor of mine, Volker Mohnen states "Our eyes are electromagnetic sensors in the visible wavelength region.".

Atmospheric constituents can be categorized in 2 groups - molecular and aerosols. Molecular constituents are gases of dry air in our atmosphere and water vapor. Dry air is quite homogeneous up to an altitude of around 80 km, and its main volumetric components are : 

  Component      Symbol  % of Dry Air
  Nitrogen         N2       78.09
  Oxygen           O2       20.95
  Argon            A          .93
  Carbon Dioxide  CO2         .03
  Water Vapor     H2O      variable
  Others                    small

Water vapor concentration is quite variable, and can occupy as much as 5% of atmospheric volume. Aerosols are suspended particles in the molecular air, such as dust, smoke, and pollen. Even clouds can be correctly be considered as consisting of aerosols, because they are organized masses of water droplets and/or ice crystals. The main S absorbers in our atmosphere are aerosols, water vapor, ozone, carbon dioxide, and diatomic oxygen. All atmospheric constituents scatter some S, their size mainly determining scattering characteristics. Molecular scattering, for which S wavelength is significantly longer than molecule size, occurs rather equally to all directions (isotropic). Aerosol scattering tends to be predominately to a forward direction, aerosol size and shape determining distribution of scattered EM radiation. Molecular scattering tends to be inversely proportional to wavelength (l) to the 4th power :

Molecular scattering ~ 1/l4

S near the blue end of the EM spectrum is scattered preferentially, and only when optical air mass* is quite large is S near the red end of the EM spectrum abundantly scattered :

Aerosol scattering is similar, but not nearly as extensive, tending to be inversely proportional to wavelength :

Aerosol scattering ~ 1/l

Thus, when a person looks at a cloudless sky, they typically see a light blue background color (molecular scattering), with a white-yellow solar disc surrounded by a yellow-orange circumsolar* region (aerosol scattering). During cloudless days with a very clean atmosphere, a purplish tinge in the sky can sometimes be seen, suggesting that very little aerosol scattering is occurring. Those colors all shift to longer wavelengths as solar elevation* decreases, causing pink-red sunsets typically seen.

Solar Energy Components at Ground

Solar energy in our atmosphere consists of 2 components* - direct, the unscattered solar beam; and diffuse, scattered. An expression for global S, all S in our atmosphere is : 
Global S  =  Direct S  +  Diffuse S

Direct S flux, diffuse S flux, and global S flux are such fluxes incident to a specific orientation, which I plan discussion of later regarding S usage. When mentioning global S flux with no further specification, that incident to horizontal is usually assumed (a natural reference for it). More precisely, the term global horizontal S flux can be used. Same for direct and diffuse S flux. People often use the terms direct normal S flux and even global normal S flux to specify those incident to a surface directly facing our sun (normal to the solar beam). Diffuse S can be considered to consist of several components. They include downward-scattered sky diffuse solar energy, ground-reflected solar energy, and backscattered solar energy (after ground reflection and including multiple scattering). Cloud-transmitted and cloud-reflected solar energy can also be specified. Magnitudes of each can be estimated for various sky and terrain conditions.

Atmospheric solar energy is modeled using both detailed analysis of radiative transfer equations, using attenuation coefficient for specific dry air constituents and cloud droplet/crystal distributions, and also parameterizations using transittances as estimates of bulk scattering and absorption properties of basic dry air constituents and clouds. The former is theoretically fine, but often impractical. From the previous discussion regarding scattering, it should be clear that under cloudless skies, diffuse S tends to be blueish, leaving direct S a bit reddish. This has significant consequences regarding S usage, particularly solar cell performance. I plan discussion of such topics later.


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

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