Double (possibly triple) CZA in Zhangye, China

Finally, double/triple CZA, a.k.a the Jensen arcs ( https://thehalovault.blogspot.com/2020/04/new-halo-triple-cza-jensen-arcs-in.html ), made a debut in China. On Sep 27, LING Xiaoyun caught them with her iPhone in Zhangye, Gansu province.

Captured at 09:20:14 UT, when the secondary CZA below the main one was most prominent. Sun elevation 20.0°.

Captured at 09:21:16 UT. The secondary CZA had somewhat faded. Sun elevation 19.8°.

The secondary CZA is also visible through the ultra wide lens.

Catpured at 09:19:33 UT. Sun elevation 20.2°.

She even managed to grab a super close-up shot placing her iPhone behind a pair of 10x50 binoculars.

Captured at 09:20:44 UT. Sun elevation 19.9°.

When enhanced, a third CZA pops up above the main CZA in some of the close up shots. There's also the slightest hint of a fourth CZA below the secondary CZA.

The 09:20:14 UT shot processed. USM above, USM + BGR below.

It should be noted though modern smartphone data are prone to artifacts introduced by HDR and other algorithms, so processed results should be taken with a grain of salt.

Assuming the third CZA above the main one is real, it can be explained by some new models proposed after Jensen arcs' discovery.

Diffraction model:

Nicolas Lefaudeux suggests that the Jensen arcs would be caused by diffraction through prism faces of thin plates. For this to happen, the crystals need to be both thin and have homogeneous thickness. It is the same physics as diffraction through a slit and diffraction corona around the sun.

Because of the combined effect of the dependency of the diffraction with wavelength (longer wavelength diffracts farther) and color spread of CZA, it creates clear arcs below the CZA.

Above CZA, depending on the crystal size, it can be quite a white glow or very washed out arcs. About arc distance below CZA, it is directly linked to crystal thickness.

Below is a simulation made by Nicolas with ZEMAX software (applying some diffraction properties to the prism faces of the crystals).

Thick plates with flat pyramidal tops model:

JI Yun proposes that thick plates with flat pyramidal tops could create extra CZAs above and below the main one. Below is his simulation with HaloPoint. An apex angle of 177.2° is adopted for the pyramidal top.

As far as we know there're also other new models in the works. Now that we have four sightings of Jensen arcs within two years, it probably won't take long for us to have enough cases to verify which theory wins out.

Jia Hao