Extreme Negative Polarization (peer reviewed Astrophysical Journal)

[Julian_M]

https://iopscience.iop.org/article/10.3847/2041-8213/ae0c08

https://iopscience.iop.org/article/10.3847/2041-8213/ae0c08/pdf

Above is peer reviewed article from The Astrophysical Journal Letters concerning "Extreme Negative Polarization of New Interstellar Comet 3I/ATLAS"

What is really the meaning of extreme negative polarization using the polarized sunglasses as analogy? Ever since last year I couldn't understand the meaning of extreme negative polarization about 3i/Atlas.

from the peer reviewed article above:

"We present the first polarimetric observations of the third discovered interstellar object, 3I/ATLAS (C/2025 N1), obtained pre-perihelion with FORS2/VLT, ALFOSC/NOT, and FoReRo2/RCC, over a phase angle range of 7.7-22.4°. This marks the second ever polarimetric study of an interstellar object, the first distinguishing 2I/Borisov from most Solar System comets by its higher positive polarisation. Our polarimetric measurements as a function of phase angle reveal that 3I is characterised by an deep and narrow negative polarisation branch, reaching a minimum value of -2.7% at phase angle 7°, and an inversion angle of 17° -- a combination unprecedented among asteroids and comets, including 2I/Borisov."

I googled a lot about it but managed to find a few illustrations only

To aid in understanding. About the paper description "reaching a minimum value of -2.7% at phase angle 7°, and an inversion angle of 17°". What is the minimum value, phase angle and inversion angle of a pair polarized sunglasses? I only know 90 degrees vertical and horizontal polarization description.

Also polarized sunglasses are dimmer because the other 90 degree is filtered. In the case of 3i/Atlas. Does it mean it looks dimmer because some polarization is shifted in angle very steep 2.7% like the 90 degree horizontal polarization being filtered in sunglasses?

 

[A.T.]

What is really the meaning of extreme negative polarization ...

It's explained in the paper you linked:

The degree of linear polarization Pr of sunlight scattered by cometary dust particles is measured as the difference between the fluxes perpendicular and parallel to the scattering plane (Sun–comet–observer plane), normalized by their sum, and is highly sensitive to the solar phase angle α (Sun–comet–observer angle). Because of this definition, polarization is positive when the polarization direction is oriented perpendicular to the scattering plane, and negative when parallel.

... using the polarized sunglasses as analogy?

Polarized sunglasses do not generate polarization by scattering, so you cannot apply the above defintion to them.

 

[Ibix]

Light reflecting off any surface is usually partially polarised, either parallel or perpendicular to the scattering plane (the flat plane defined by the sun, the comet, and the camera). Negative polarisation means it's parallel to the plane, positive means perpendicular. The underlying cause is that how strong a reflection is depends on the angle of incidence of the light, but it depends differently for the polarisation parallel and perpendicular to the surface. So you get a changing mix of polarisations as the angle of the reflection varies.

So all the graph is showing you is how much difference in brightness you see when you put a polarising sheet in front of your camera parallel and perpendicular to the scattering plane, and how that varies as the phase angle (which I believe is the angle between the direction of travel of the incident and reflected light) varies. "Inversion angle" is the angle at which the polarisation sign switches. It's also very weakly polarised - less than 3% difference in the parallel and perpendicular fluxes.

The result tells you something about the surface roughness (for example, Titan does not have the negative polarisation dip that all other moons have because it has a thick atmosphere that reflects light, rather than a rocky surface), but you'd need to know more about polarimetry than I do to say anything helpful.