Please enable JavaScript.
Coggle requires JavaScript to display documents.
Lecture 6- Propagation Effects (Material effects (Reflection (at a…
Lecture 6- Propagation Effects
Material effects
Reflection (at a surface)
Refraction (from gradients in a refractive index)
Diffraction (at an edge)
From abrupt changes or gradients in the properties of the intervening medium, like optic in visible band
Absorption (interaction with solids, liquids or quantised rotations of gas molecules with permenent dipole moments
emission (if material absorbs energy it also has to re emit it in equilibrium the rate of emission = the rate of absorption)
scattering (small scale variations in refractive index, like water vapour and rain drops) - small in radio
Polarisation (reflection or anisotropy in propagation condition for orthogonal states of polarisation e.g. magneized interstellar plasm)
Ionosphere
At low radio frequencies reflection can occur in ionosphere at frequency depending on electron density and time of day
At higher frequencies the waves are refracted
depends on the angle of incidence
Troposphere
significant refraction at radio frequencies
when observing close to horizon
caused by oxygen and water vapour
Radio horizon extended because of refraction
Absorption and re-emission in semi-transparent media
radiation from background source passes through medium
absorption decreases specific intensity along path
emission increase specific intensity along path
Neither means specific intensity constant along path
Radiative transfer
absorption coefficient
fraction of incident power lost passing through unit distance
Emission coefficient
specific intensity added per unit distance through medium
Maths
Optical depth is a summary parameter telling us how opaque a region is to radiation passing through it
Decibels dB vs nepers
Source function measure of how photons are removed and replaced by new photons
Special case
Uniform medium in thermodynamic equilibrium (or LTE) with radiation field at a single physical temperature BB conditons
Rayleigh Jeans Regime
all specific intensities can be summarised by a brightness temperature
either an actual physical temperature of a black body or an equivalent bb temp
allows simplification of equation
values of Tau
0 - perfectly transparent to source
0.1 - source dominated, cloud in absorption
1 - cloud dominates, cloud in emission
infinite - cloud is perfectly opaque, only see its surface
Broadness of atmospheric lines comes from pressure broadening
Observations further from zenith