New Concept: Rotation Measure

[msdemlei]

Name the new concept Rotation Measure

Describe the concept The integrated Faraday rotation polarised radiation propagating through an ionised medium with a magnetic field experiences.

Describe where the concept fits within the existing hierarchy I suppose for the time being it should be narrower than radio-astronomy; but I've not researched if rotation measures are obtained elsewhere, too. If they are, I suppose it would have to sit below observational-astronomy.

It should probably be related to spectropolarimetry (which is how RMs are measured).

Provide 1-3 supporting articles

There's an upcoming standard for this kind of data that could very well use such a term, https://github.com/CIRADA-Tools/RMTable.

Otherwise, the use of rotation measures is well established in both extragalactic (e.g., 2002ARA&A..40..319C) and galactic (e.g., 2012ApJ...757...14J) astronomy.

[BartlettAstro]

@ebortey Could you please find a definition from an authoritative source?

[BartlettAstro]

@msdemlei Would Faraday Rotation Measure be a better name for this concept? Could you please recommend a useful definition?

[msdemlei]

On Tue, May 28, 2024 at 02:59:56PM -0700, Jennifer Lynn Bartlett wrote:

@msdemlei Would Faraday Rotation Measure be a better name for this concept? Could you please recommend a useful definition?

As to a good label, I would like to defer to radio astronomers @.***, can you help out?); if other rotation measures are in use, then we should qualify. Me, I only ever have heard the term as defined by this:

The plane of polarized radio emission is rotated in the interstellar medium. The rotation measure, depending on the electron densities and the magnetic fields along the line of sight, is defined as the rotation angle as a function of wavelength divided by the square of the wavelength.

[ebortey]

I found this explanation online:

"the RM can be observationally derived through several different methods, such as by measuring or fitting the relationship between polarization angle and wavelengthsquared (e.g., Brown et al. 2003). Other methods, such as RM synthesis (Burn 1966; Brentjens & de Bruyn 2005), characterize the polarized emission in the space of all possible Faraday depths (the Faraday dispersion function, FDF), and define the RM as some characteristic Faraday depth associated with that function (e.g., the value of Faraday depth at which polarized intensity is found to be maximum)."

• https://iopscience.iop.org/article/10.3847/1538-4365/acda24/pdf

[msdemlei]

On Wed, May 29, 2024 at 12:49:13AM -0700, ebortey wrote:

I found this explanation online:

"the RM can be observationally derived through several different methods, such as by measuring or fitting the relationship between polarization angle and wavelengthsquared (e.g., Brown et al. 2003). Other methods, such as RM synthesis (Burn 1966; Brentjens & de Bruyn 2005), characterize the polarized emission in the space of all possible Faraday depths (the Faraday dispersion function, FDF), and define the RM as some characteristic Faraday depth associated with that function (e.g., the value of Faraday depth at which polarized intensity is found to be maximum)."

Hmyes... that's certainly true, but in the interest of compact definitions, I'd generally avoid getting into technicalities (such as how to obtain some observable) and define, as concisely and precisely as possible, what something is. In that sense I'm not even sure I like the first sentence of my definition. In a way, perhaps

The Rotation Measure is β(λ)/λ² with the Faraday rotation angle towards the source β as a function of the wavelength λ.

would probably be ideal from where I stand. But then I stand on top of the ivory tower we've built on Nerd Island, which may not be the best place to author definitions for a resource intended to be accessible beyond core science. Someone else be the judge...