Energy Sources and Radiation Principles GEO 420 Dr. Garver

Electromagnetic Sensors Operate from airborne & spaceborne platforms. Acquire data on the way Earth’s features emit and reflect energy.

Basic wave theory C = vl (1.1) All forms of energy are similar & radiate in accordance to wave theory Light travels as c l = distance between peaks V = cycles per second past a fixed point Photons move at the speed of light Move as waves

Particle Theory (1.2, 1.3) EMR composed of discrete units photon - fundamental unit of EM radiation. Underlying basis for r.s. is measuring the varying energy levels. Variations in photon energies are tied to wavelength or its inverse, frequency. EM radiation varies from high to low energy levels, comprises electromagnetic spectrum.

EMR extends over wide range of wavelengths. Radiation from specific parts of EMS contain photons of different wavelengths. EMR extends over wide range of wavelengths. Photon energy is measured at detectors electromagnetic (EM) spectrum - continuum of all radiant energies Other wave types require a carrier (water) Photon waves can transmit through a vacuum (space).

Images made from data acquired as electronic signals, rather than recorded on film. Produced by sensors operating in the visible and near-IR. Some radar and thermal sensors.

EMS intervals and descriptive names: visible region - 0.4 and 0.7 microns infrared region: 0.7 to 100 microns (1) reflected IR: 0.7 to 3.0 microns (2 ) thermal bands: 3 to 100 microns 3 to 5 microns, and 8 to 14 microns. microwave region - 0.1 to 100 cm, includes interval used by radar systems.

Energy and Radiation The dividing line between reflected and emitted IR wavelengths is 3 m. below 3 m = reflected energy above 3 m = emitted

Primary source of energy is the Sun Solar irradiation arrives at Earth-at wavelengths determined by temperature of sun (~6000° K). As solar rays arrive at Earth, atmosphere absorbs or reflects (backscatters) a fraction and transmits remainder.

Visible = 0.4-0.7 mm = Sun Thermal IR = 10 mm = Earth

2 Important Laws: Stefan Boltzman Law = the hotter the object the more energy it emits Wein’s Law = the hotter the object the shorter the wavelengths emitted

SB Law M = sT4 T = temperature of emitting body s = SB constant M = total energy emitted Energy emitted increases rapidly with inc. T.

Wein’s Law lm = A/T lm = maximum wavelength A = constant T = temperature, K

Kelvin = Celsius + 273.15 Celsius = 5/9 x (Fahrenheit -32) Fahrenheit =(Celsius/(5/9))+32

Solar Constant - Insolation at top of atm. = 1372 Wm-2 Peak in the blue region. Insolation - Solar radiation that reaches a horizontal plane at Earth

Global Net Radiation

Land, ocean , and atmosphere - incoming radiation partitioned into: Transmission Absorption Reflection Scattering

Transmission, Absorption, Scattering, and Reflection. photons passing through medium (usually air) experience one or more reactions:

Energy gained/lost by Earth/Atm. Transmission - passage of energy Reflection - energy not absorbed, no change in wavelength, the angle of reflection of a light ray is the same as the angle of incidence.

Energy gained/lost by Earth/Atm. Absorption - conversion of radiant energy to heat energy In atmosphere – Ozone, carbon dioxide and water vapor absorb at various wavelengths. Ozone – UV CO2, water vapor trap heat for Earth See atmospheric windows figure

Stopped lecture here Thursday 5/5/16 Quiz 1 will be this lecture up to this point, plus any matching material from the online text Section 1 pages 5 – 22.

Atmospheric windows – white areas

Scattering Some particles and molecules found in the atmosphere have the ability to scatter solar radiation in all directions. Different from reflection (where radiation is deflected in one direction) 3 Types

A target is a Rayleigh scatterer if D<<l Rayleigh scattering - Caused by constituents (O2, N2 CO2 and water vapor) that are much smaller than the radiation wavelengths. Increases with shorter wavelengths (blue sky effect). A target is a Rayleigh scatterer if D<<l

Longer wavelengths pass straight through atm. WHY IS THE SKY BLUE? Rayleigh scattering. Longer wavelengths pass straight through atm. Not much red, orange and yellow light is affected. Shorter wavelengths scattered by gas molecules in different directions (blue).

D ~= l, then the target is a Mie scatterer. 2. Mie scattering - atmospheric constituents (i.e., smoke, dust, water vapor) whose dimensions are of the order of the radiation wavelengths. D ~= l, then the target is a Mie scatterer. where D is the diameter of the target.

3. Non-selective Scattering If D >> l, then the target is a non-selective scatterer. Water droplets, large particles. All l scattered equally (fog, clouds)

Atmospheric scatter can be 80 to 90% of signal observed by a sensor. Makes an image hazy, low contrast.

69% absorbed + 31 reflected = 100% 21% + 3% absorbed by atm. 45% absorbed by surface

Earth-atmosphere energy balance Follow 100 units of solar input: 31% reflected to space (albedo) 21% absorbed by clouds, dust, gases 3% absorbed by O3 in stratosphere 45% absorbed by surface 100% 69% re-radiated to space

Global Net Radiation

Albedo = % energy reflected Blacktop or snow? Avg. albedo of Earth = 30% Clouds and volcanoes

Earth’s Avg. 31%

From previous graph Calculate % decrease in energy between solar constant and Earth’s surface in visible peak. Radiance and spectral radiance are radiometric measures that describe the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle in a specified direction. They are used to characterize both emission from diffuse sources and reflection from diffuse surfaces. The SI unit of radiance is watts per steradian per square metre (W·sr-1·m-2). Irradiance, radiant emittance, and radiant exitance are radiometry terms for the power of electromagnetic radiation at a surface, per unit area. "Irradiance" is used when the electromagnetic radiation is incident on the surface. "Radiant exitance" or "radiant emittance" is used when the radiation is emerging from the surface. The SI units for all of these quantities are watts per square metre (W·m−2).

Exercise #1 question #3: Peak at ______ nm of irradiance curve for sunlight as it reaches the outer atmosphere. Spectral irradiance reads ~_____ W/m-2/nm Sea level irradiance curve at the same peak position = ~_____ W/m-2/nm

The atmosphere messes things up: Most r.s. is conducted above Earth within or above atmosphere. Gases in atmosphere interact with incoming solar energy and outgoing infrared from the Earth's surface. The atmosphere itself is excited by EMR, becomes another source of released photons.

2 Energy Sources Used in R. S. R. S. is limited to Atmospheric Windows Common Sensors

Blue zones - minimal passage incoming and/or outgoing radiation White areas - atmospheric windows Most r.s. instruments operate in windows by detectors tuned to wavelengths that pass through atmosphere. Some sensors, meteorological satellites, directly measure absorption phenomena - CO2.

Opacity - measure of impenetrability to electromagnetic radiation, especially visible light. An opaque substance transmits very little light, and therefore reflects, scatters, or absorbs most of it.

Remote sensing of the Earth Reflected energy in vis, near and mid IR Most r. s. systems designed to collect reflected radiation. Emitted energy in thermal IR and microwave Signals analyzed numerically - image variations represent different intensities of photons multispectral remote sensing: gathering of continuous or discontinuous ranges of wavelengths.

Images made from varying wavelength/intensity signals astronomical body viewed through telescopes equipped with different multispectral sensing devices. four views of Crab Nebula, now in a state of chaotic expansion after a supernova explosion first sighted in 1054 A.D. by Chinese astronomers.

Multispectral Remote Sensing IR Radio X-ray Visible