What atmospheric effect causes scintillation?

Scintillation is primarily caused by variations in the atmospheric path that result in refraction. As radio waves or light pass through different layers of the atmosphere, changes in temperature, pressure, and humidity can create inhomogeneities. These variations can cause the waves to bend or change direction, leading to fluctuations in signal strength or brightness, which is perceived as scintillation.

The influence of these atmospheric conditions creates a dynamic environment in which the path of the propagating waves is not uniform. This results in a phenomenon where the signal appears to twinkle or fluctuate rapidly, much like the way stars appear to flicker due to atmospheric disturbances.

Other options, such as scattering particles either much larger or much smaller than the wavelength, pertain to different forms of signal distortion or degradation, like fading or attenuation, rather than scintillation specifically. Changes in temperature of a target could affect detection but do not directly relate to the atmospheric effects responsible for scintillation. Thus, variations in the atmospheric path are the key factor behind this phenomenon.