Knowra Dispersion measure Dispersion measure Dispersion measure is the integrated free-electron density along the path from a radio source to an observer. It is inferred from the frequency-dependent delay of radio pulses traveling through ionized matter.
Plasma dispersion relation : The relation connecting a wave’s frequency and wavenumber in a plasma, where propagation depends on electron density. Its frequency dependence produces the arrival-time delays used to infer dispersion measure.
Pulsar timing : The measurement of pulsar pulse arrival times to study rotation, orbital motion, gravity, and intervening matter. Tracking dispersion-measure changes prevents interstellar delays from masquerading as pulsar timing variations.
Free electron : An electron not bound to an atom or molecule and able to move through an ionized medium. Dispersion measure counts these particles along the radio signal’s path.
Galactic electron-density model : A model of how free-electron density varies through the Milky Way. Such models assign Galactic distances to observed dispersion measures, but their errors limit accuracy.
Dispersion delay : The frequency-dependent difference in arrival time caused by a signal propagating through a dispersive medium. Radio pulse arrival times at different frequencies reveal the integrated electron density.
Fast radio burst : A brief, bright radio transient whose dispersion can encode the electron column between its source and Earth. Its large dispersion measure helps constrain the burst’s distance and intervening ionized material.
Electron column density : The number of free electrons integrated per unit area along a line of sight. It is the physical quantity represented by dispersion measure, commonly expressed in parsecs per cubic centimetre.
Scattering measure : A measure of small-scale electron-density fluctuations that scatter radio waves along a line of sight. Dispersion and scattering trace different properties of plasma, complicating interpretation of pulse propagation.
Cold plasma : An ionized gas whose particle thermal energies are small compared with the energies governing its collective electromagnetic response. The standard dispersion-measure relation assumes radio waves traverse a cold, unmagnetized plasma.
Interstellar medium : The gas, dust, and radiation occupying the space between stars in a galaxy. Its free electrons contribute to the Galactic portion of a measured dispersion measure.
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