Dispersion measure: Confusion, Constants & Clarity

Understand

The dispersion measure (DM) is one of the key attributes of radio pulsars and Fast Radio Bursts (FRBs).

• There is a mistaken view that the DM is an accurate measure of the column density of electrons between the observer and the source.
• To start with, the DM, unlike a true column density, is not a Lorentz invariant.
• Next, the DM also includes contribution from ions and is sensitive to the temperature of the plasma in the intervening clouds.