Mie scattering
Mie scattering (also known as Lorenz-Mie scattering) is a type of elastic scattering of electromagnetic radiation (including visible light) by spherical objects whose diameter is approximately equal to the wavelength of the radiation. It is similar to Rayleigh scattering. However, the particle size here is much smaller, typically less than 1/10 of the wavelength.
Mie scattering Characteristics
- Particle Size: The size of the scattering particles is comparable to or larger than the wavelength of the incident radiation. Examples include water droplets in clouds, dust particles, pollen, or smoke.
- Wavelength Dependence: The wavelength dependence of the scattering is much less than that of Rayleigh scattering. All wavelengths of visible light are scattered approximately equally.
- Scattering Angle Distribution: The scattering is highly dependent on the scattering angle and typically shows strong forward scattering. The scattering patterns can be complex and often exhibit pronounced maxima and minima, which depend on the size and shape of the particles.
- Polarisation: The polarisation of the scattered radiation is also complex and depends on the particle size, refractive index, and scattering angle.
Examples: The white colour of clouds (water droplets scatter all wavelengths of light approximately equally), the halo around the sun or moon (diffraction and Mie scattering by water droplets or ice crystals).
The decisive factor that determines whether it is Rayleigh or Mie scattering is the relative size of the scattering particle to the wavelength of light. If the particles are much smaller than the wavelength, Rayleigh scattering dominates with its strong wavelength dependence. If the particles are comparable to or larger than the wavelength, Mie scattering occurs, which has less wavelength dependence and stronger forward scattering.
Impact and Applications of Mie Scattering in Remote Sensing
Mie scattering plays a significant role in satellite remote sensing and influences the way we interpret and use data from Earth observation satellites. Sometimes Mie scattering needs to be corrected, and sometimes it can also be used to gain information.
1. Atmospheric Correction
- Signal Disturbance: Mie scattering by aerosols (such as dust, smoke, pollen, pollution) and larger particles (such as water droplets in clouds) in the atmosphere alters the light reflected or emitted from the Earth's surface before it reaches the satellite sensor. This leads to distortion of the measured spectral signatures and can significantly affect the accuracy of remote sensing data.
- Need for Correction: To obtain accurate information about the Earth's surface, it is essential to correct the effects of Mie scattering. This is done through atmospheric correction algorithms that attempt to remove or reduce the influence of the atmosphere on the measured data.
- Complexity of Correction: Correcting Mie scattering is more complex than correcting Rayleigh scattering because Mie scattering is highly dependent on the size, shape, and composition of aerosols, which can vary greatly in space and time.
2. Information about the Atmosphere
- Aerosol Characterisation: Although Mie scattering is often seen as a disturbance for Earth observation, it can also be used to gain information about the atmosphere itself. Satellite instruments that measure at certain wavelengths and under specific observation angles can utilise Mie scattering to determine the concentration, size, and type of aerosols in the atmosphere.
- Cloud Studies: Mie scattering is the dominant scattering mechanism in clouds, as the water droplets and ice crystals in clouds are comparable in size to the wavelengths of visible and infrared light. Analysing the radiation scattered by clouds provides important information about the physical properties of clouds, such as their optical thickness, particle size, and cloud phase.
- Air Quality Monitoring: Changes in Mie scattering can indicate an increased concentration of air pollutants. Therefore, satellites can contribute to air quality monitoring by capturing the scattering properties of the atmosphere.
3. Impact on Image Quality
- Reduced Contrast and "Haze": Mie scattering can lead to a reduction in image contrast, especially in regions with high aerosol loads. The scattered portion of the light overlays the light reflected from the surface, creating a "haze" effect that can obscure details in the image.
- Colour Distortions: Since Mie scattering has less wavelength dependence than Rayleigh scattering, it can alter the colour composition of the light received by the satellite, making the interpretation of colour images more difficult.
