Why take the square root? An assessment of interstellar magnetic field strength estimation methods

Vol. 656
6. Interstellar and circumstellar matter

Why take the square root? An assessment of interstellar magnetic field strength estimation methods

Magnetic fields play a crucial role in the evolution of the interstellar gas, but quantifying their strength represents an observational challenge. Direct measurements using the Zeeman effect are technically demanding, and observers often rely on indirect methods, such as the one proposed in the 1950s by Davis, Chandrasekhar, and Fermi (DCF). This method uses the dispersion of the dust polarization angles, which can be estimated relatively easily from optical or radio observations, and it prescribes that the magnetic field strength should be inversely proportional to the dispersion value. The DCF method, however, assumes incompressible magnetohydrodynamic fluctuations, which are inconsistent with the compressible nature of the interstellar medium turbulence.

Skalidis and collaborators have critically revised the now venerable DCF method by relaxing the incompressibility assumption and developing a model that predicts a magnetic field strength inversely proportional to the square root of the polarization angle dispersion. To compare this new method with that of DCF, these authors have run a set of 26 magnetized, isothermal, ideal, magnetohydrodynamic (MHD) numerical simulations without self-gravity and with different types of forcing. They find that the new method has an accuracy of better than 50% over the range of Alfven Mach numbers explored by their simulations (between 0.1 and 2.0), while the DCF method only performs adequately over a narrow range of Alfven Mach numbers.