Lecture 4 Electromagnetic radiation : Interaction with matter and atmosphere
Interaction of electromagnetic energy What happens when solar irradiance leaves the space and enters the Earth’s atmosphere? • Microscopic or atomic/molecular level • Megascopic level
Atmospheric Layers and Constituents Very thin atmosphere Temp rises again; 1200 C at the top Very thin atmosphere [O] > > O2, NO Very cold (Temp falls to -100o C);N2, O, N, NO; Most meteors burn up Temp rises due to absorption of UV rays by Ozone (close to 0o C) TERRANE SURFACE O3 + hυ (0.22-0.33 µm) = O2 + O Temp goes down with height (-60o C at the Tropopause) N2, O2, CO2, H2O
+ + - - - + Interaction of electromagnetic energy – atomic level hυ1= En=3 - En=1 En=1 En=3 Excite state E=hυ2 E=hυ1 Ground state Ground state • Electrons are the -ively charged particles that revolve around the +ively charged nucleus of an atom in specific orbits denoting specific energy classes or levels in the ground state. • For an electron to be boosted to an orbital with a higher energy, it must overcome the difference in energy between the orbital it is in, and the orbital to which is going. This means that it must absorb a photon that contains precisely that amount of energy, or take exactly that amount of energy from another particle in a collision. This is called excited state. • After a short time, the electron falls back to the original energy level or ground state and gives off radiation. The wavelength of radiation given off is a function of the quantum of energy it absorbed to cause the electron to be moved to the higher orbit, and is responsible for the color of the element.
Interaction of electromagnetic energy – atomic level LYMAN SERIES (UV) Absorption BALMER SERIES (VISIBLE) Emission Ionization PASCHEN SERIES (IR) HYDROGEN ATOM The single electron in n = 1 can be transferred into higher energy states (n = 2 through 6). On the transition back from any of these levels back to n = 1, discrete energies are released for each level transition, according to the Planck equation. The first group of excited states, starting from n = 1, comprises the Lyman series. The energy change for each gives rise to spectra that fall within the UV region. Or the electron may be placed in n = 2 or 3 and then jump to higher states. The results are two more series, the Balmer series (Visible) and the Paschen series (IR). Each transition has a specific wavelength representing the energy involved in the level changes.
Interaction of electromagnetic energy at atomic level : color After being energized by several thousand volts of electricity, the outermost electron in each energized atom of sodium vapor climbs to a higher orbit or energy level and then returns back to a lower energy level. The difference in these energy levels is 2.1 eV. This corresponds to yellow light. Jensen 2005
Interaction of EM energy with atmosphere and terrestrial objects: Megascopic level Ө1 Ө2 Ө1= Ө2 Ө1 Ө1> Ө2 Ө2 • Solar irradiation arrives at Earth at wavelengths determined by the photospheric temperature (0.2 and 3.4 µm). Transmission • Some objects transmit the light through without significant diminution • Some materials absorb light (and in part re-emit at longer wavelengths) • Or, the light is reflected at the same angle as it formed on approach. • Or an object's surface roughness may cause scattering in all directions. Reflection Absorption • As solar rays arrive at the Earth, the atmosphere absorbs or scatters a fraction of them and transmits the remainder. Scattering • Upon striking the terrestrial surface the solar irradiance partitions into 3 modes of energy-interaction response: • Transmittance - Part of the radiation penetrates into certain surface materials (e.g., water) and if the material is transparent and thin in one dimension, passes through, generally with some diminution. • Absorption - Some radiation is absorbed through electron or molecular reactions - a portion of this energy is then re-emitted, usually at longer wavelengths, and some of it remains and heats the target. • Reflectance - some radiation reflects (moves away from the target) at specific angles and/or scatters away from the target at various angles, depending on the surface roughness and the angle of incidence of the rays..
Interaction of EM energy with atmosphere • Once electromagnetic radiation is generated, it is propagated through the space and later through the earth's atmosphere almost at the speed of light in a vacuum. • Unlike a vacuum in which nothing happens, however, the atmosphere may affect not only the speed of radiation but also its wavelength, intensity, spatial distribution, and/or direction. • These effects are results of: • Scattering, or • Absorption
Incident sunlight Scattered light Atmospheric particles Atmospheric Scattering Scattering is the process by which small particles suspended in a medium of a different index of refraction diffuse a portion of the incident radiation in all directions. Scatter differs from reflection in that the direction associated with scattering is unpredictable, whereas the direction of reflection is predictable. With scattering, there is no energy transformation, but a change in the spatial distribution of the energy.
Types of Atmospheric Scattering • Type of scattering is a function of: • the wavelength of the incident radiant energy, and • the size of the gas molecule, dust particle, and/or water vapor droplet encountered. Water droplets
0.7 Rayleigh Scattering Rayleigh scattering occurs when the diameter of the matter (usually air molecules) are many times smaller than the wavelength of the incident electromagnetic radiation. It is impossible to predict the direction in which a specific atom or molecule will emit a photon, hence scattering. The approximate amount of Rayleigh scattering in the atmosphere in optical wavelengths (0.4 – 0.7 mm) may be computed using the Rayleigh scattering cross-section algorithm: where n = refractive index, N = number of air molecules per unit volume, and λ = wavelength. The amount of scattering is inversely related to the fourth power of the radiation's wavelength. For example, blue light (0.4 m) is scattered 16 times more than near-infrared light (0.8 m).
Rayleigh Scattering - effects • Blue skies • Red sunsets • Haze in satellite imagery, which diminishes crispness or contrast of an image. • Images taken in shorter wavelengths are more strongly affected by Rayleigh scattering
Mie Scattering • Mie scattering takes place when there are essentially spherical particles present in the atmosphere with diameters approximately equal to the wavelength of radiation being considered. • For visible light, water vapor, dust, and other particles ranging from a few tenths of a micrometer (Visible) to several micrometers(NIR) in diameter are the main scattering agents. • The amount of scatter is greater than Rayleigh scatter and the wavelengths scattered are longer. • Leads to diffused images, especially in overcast conditions.
Non-selective Scattering • Non-selective scattering is produced by particles several times the diameter of the radiation being transmitted. • This type of scattering is non-selective, i.e. all wavelengths of light are scattered, not just blue, green, or red. • For example, water droplets, which make up clouds and fog banks, scatter all wavelengths of visible light with equal intensity. These objects therefore appear white. • Scattering can severely reduce the information content of remotely sensed data to the point that the imagery looses contrast and it is difficult to differentiate one object from another.
Atmospheric absorption • In contrast to scattering, atmospheric absorption results in the effective lossof energy to atmospheric constituents. • Absorption is the process by which radiant energy is absorbed and converted into other forms of energy. • An absorption band is a range of wavelengths (or frequencies) in the electromagnetic spectrum within which radiant energy is absorbed by substances such as water (H2O), carbon dioxide (CO2), oxygen (O2), ozone (O3), and nitrous oxide (N2O). • The cumulative effect of the absorption by the various constituents can cause the atmosphere to become opaque in certain regions of the spectrum. • Results in fewer available wavelengths for remote sensing.
Atmospheric absorption • In certain parts of the spectrum (e.g., visible region 0.4 - 0.7 m), the atmosphere transmits the incident energy effectively. • Parts of the spectrum that are transmitted effectively are called “atmospheric windows”. • Certain wavelengths of radiation are affected far more by absorption than by scattering. This is particularly true of infrared, and wavelengths shorter than the visible light. • Stratospheric O2 and O3 molecules absorb 97-99% of the sun's high frequency UV light (0.15 to 0.30 µm).
Atmospheric absorption Exosphere ~600 km High energy ultraviolet (< 0.1 µm) 1200 C Thermosphere [O] > > O2, NO Ultraviolet (0.2 - 0.3 µm) ~85 km -100 C Mesosphere Infrared, visible and ultraviolet (> 0.3 µm) ~50 km -2 C O3 + hυ (0.22-0.33 µm) = O2 + O Stratosphere ~10 km -60 C N2, O2, CO2, H2O, smoke and smog Troposphere 30 C TERRANE SURFACE
Energy sources, atmospheric windows and common remote sensing systems ASTER BANDS
Atmospheric windows The absorption of the Sun’s incident EM energy in the region from 0.1 to 30 µm by various atmospheric gases • The atmosphere essentially “closes down” in certain portions of the spectrum while “atmospheric windows” exist in other regions that transmit incident energy effectively to the ground. • The combined effects of atmospheric absorption and scattering reduce the amount of solar irradiance reaching the Earth’s surface at sea level. • Remote sensing systems must function within the available atmospheric windows mainly in the visible, NIR and TIR regions
Conclusion • The sensor to be used for the given remote sensing task cannot be selected arbitrarily. • One must consider: • the spectral sensitivity of the sensor available, • the presence or absence of atmospheric windows in the spectral range(s) one wishes to sense, • the source, magnitude, and spectral composition of the energy available in these ranges.