Interplanetary scintillation

As interplanetary scintillation is caused by the solar wind, measurements of interplanetary scintillation can "be utilized as valuable and inexpensive probes of the solar wind."[11] As already noted, the observed information, the intensity fluctuations, is related to the desired information, the structure of the solar wind, through the phase change experienced by waves traveling through the solar wind. The root mean square (RMS) intensity fluctuations are often expressed relative to the mean intensity from the source, in a term called the scintillation index, which is written as

${\displaystyle m={\frac {\langle \Delta I^{2}\rangle ^{1/2}}{\langle I\rangle }}.}$

This can be related to the phase deviation caused by turbulence in the solar wind by considering the incident electromagnetic plane wave, and yields

${\displaystyle m\approx {\sqrt {2}}\Delta \phi .}$[12]

The next step, relating the phase change to the density structure of the solar wind, can be made more simple by assuming that the density of the plasma is highest towards the sun, which allows the "thin screen approximation." Doing so eventually gives an RMS deviation for the phase of

${\displaystyle \phi _{RMS}=\lambda r_{e}\left(aL\right)^{1/2}\left[\langle \delta N^{2}\rangle \right]^{1/2},}$[13]

where ${\displaystyle \lambda }$ is the wavelength of the incoming wave, ${\displaystyle r_{e}}$ is the classical electron radius, ${\displaystyle L}$ is the thickness of the "screen," or the length scale over which the majority of the scattering takes place, ${\displaystyle a}$ is the typical size scale of density irregularities, and ${\displaystyle \delta N^{2}}$ is the root mean squared variation of the electron density about the mean density. Thus interplanetary scintillation can be used as a probe of the density of the solar wind. Interplanetary scintillation measurements may also be used to infer the velocity of the solar wind.[14]

Stable features of the solar wind can be particularly well studied. At a given time, observers on Earth have a fixed line of sight through the solar wind, but as the Sun rotates over an approximately month-long period, the perspective on Earth changes. It is then possible to do "tomographic reconstruction of the distribution of the solar wind" for the features of the solar wind which remain static.[15]

The power spectrum that is observed from a source which has experienced interplanetary scintillation is dependent upon the angular size of the source.[16] Thus interplanetary scintillation measurements can be used to determine the size of compact radio sources, such as active galactic nuclei.[17]