In the radiation zone of a localized source, the leading electromagnetic fields decay as , are transverse to the observation direction, and satisfy . Terms decaying faster than do not contribute to the limiting radiated power.
An isotropic source of average power has intensityat distance . For monochromatic vacuum radiation, its electric-field amplitude is .
For a localized source whose size is small compared with its radiation wavelength, the radiation fields of its electric dipole moment areTheir total instantaneous power is
For a source of characteristic size and angular frequency , the electric-dipole approximation requiresso the source is much smaller than its radiation wavelength. Observing the pure radiation field additionally requires the radiation zone, and .
If is the angle between the observation direction and , thenIntegrating over solid angle uses and gives the total electric dipole radiation power.
A charge undergoing simple harmonic motion of angular frequency and slowly varying amplitude radiates the averaged powerIts mechanical energy therefore has exponential decay rate and half-life
If a rigid body's inclined magnetic dipole rotates at angular speed , its radiated power is . Conservation of rotational energy then gives
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The radiation zone is a region within a star, specifically in stars like the Sun, where energy produced in the core is transported outward primarily through radiation. This zone is found between the inner core, where nuclear fusion occurs, and the outer convective zone. In the radiation zone, the high temperature and pressure conditions cause photons (light particles) to be absorbed and re-emitted by particles in the plasma, leading to a slow transfer of energy outwards.