Discussions on a former observation from the Pic du Midi observatory (France) on 2003, March 29th

 

        During a coronagraphic mission of one week at the Pic du Midi observatory (altitude: 2,877 m), Marie-France Balestat and Frédéric Guinel had the opportunity to observe a conjunction of atmospheric optical phenomena – see Figure 1.

 

   

Figure 1. All of these two pictures show, behind the cables, a coloured glory (which is not complete, because of the shadow of the tower and of the cupola of the coronagraphs) and over, at the same azimuth, a white anthelic pillar in a cloud above the horizon. © Marie-France Balestat

 

         A preliminary remark: while the pillar is due to oriented ice crystals in the air, the glory (accordingly to its rotational symmetry) is due to tiny water droplets. Crystals and droplets could coexist in the same cloud, but only transitorily: the crystals pumps water of the droplets (a phenomenon known as the Bergeron effect, described in 1933), because the tension of vapour in equilibrium with the corresponding solid is lower than with the corresponding supercooled liquid at the same temperature.

However, to explain the conjunction shown by Figure 1, the luck to photography just during such a transitory moment isn’t necessary, because it is clear on the pictures that the two clouds, respectively producing the glory and the pillar, are obviously distinct – the first cloud is near and slightly below the observer, while the second cloud is much farther and above the observer.

 

        Let us now focus on the white pillar, also visible on another picture – see Figure 2. 

 

 

Figure 2. The same anthelic pillar (and, on the right, the Pic du Midi cable car station)
© Marie-France Balestat

 

 

Being on the opposite half of the sky with respect to the Sun, this cannot be an ordinary reflected Sun pillar. Two main possibilities remain open:

a)      the foot of the Y produced with a low Sun by a diffuse-B arc (a refractive halo, presenting apparent achromatism);

b)   Mikkilä’s soul – a retrodiffractive halo having the form of an anthelic diffraction pillar, discovered in 2012 by Marko Mikkilä, and explained later by Nicolas Lefaudeux.

 

Let us compare the features of these two haloes a) and b), and illustrate them by photographs already published in the Halo Vault.

 

a)      Diffuse-B arc [1]	b)     Anthelic diffraction pillar – “Mikkilä’s soul”
On this photo above, due to Marko Riikonen (2008, November 6th), a careful examination reveals the presence of the 2 arms characteristic, for a Sun height between 2° and 8°, of the Y given by a diffuse-B arc – see the last row of [1]. On Figures 1-2 we do not guess any Y, but this could be due to the uneven present cloud (on these Figures, with a too weak apparent vertical extent of the ice cloud). No irization is perceptible, despite the dispersion due to refraction; this is coherent with the apparent achromatism – also presented, among the family of anthelic arcs [1], by the Tricker and diffuse-A arcs, but not by the Wegener arc (see Figure 3).	. Neither on the original image above, due to Minna Kinnunen (2021, November 8th) nor with the treatment (below, due to Nicolas Lefaudeux) we see any Y of a low-Sun diffuse-B arc.  The treated image reveals a bright central fringe, accompanied by lateral fainter fringes, as usual in diffraction. A notable fact is the absence of visible diffraction irizations (comparatively with coronae); it could be due to the heterogeneity of the crystals size, coherent with the low-contrast and small number of the diffraction fringes.
The radiance maximum seems to be on the anthelic point – which is coherent with its nature of double point for all the anthelic arcs [1].	The extrapolated radiance maximum could be on the subanthelic (aka antisolar) point – which is coherent with the retrodiffractive nature of Mikkilä’s soul. This isn’t obvious on Figures 1-2, but it could be due, once more, to the uneven cloud visible on these Figures.

 

As a temporary conclusion, according to the criteria and the remarks above, until further information or image treatments it is not possible to decide whether the white pillar shown by Figures 1 and 2 is a fragment of diffuse-B arc, or of Mikkilä’s soul partially blurred because of a notable heterogeneity of the crystal sizes; it could also be a combination of these two haloes.

 

Appendix: on the effect, in the family of anthelic arcs, of ice / air chromatic dispersion

In hexagonal columnar ice crystals, the sequence of two refractions and one reflection producing the Wegener arc does not yield the property of apparent achromatism, contrarily to the sequences (with more reflections) producing the Tricker and diffuse-A and B arcs respectively [1]. This is coherent with the fact that, on the superior part of the Figure 3 that shows a Wegener arc, a red irization is clearly visible on the lower side of this arc – the Sun being at the height of the parhelic circle seen on the inferior part of this Figure.

Figure 3. From top to bottom: a Wegener arc (with a reddish lower rim) and a parhelic circle with a paranthelic point (also known as 120° parhelion) – which both present, accordingly to the theory, the property of apparent achromatism. The anthelic point is outside the frame, on its right, accordingly to the inclination of Wegener’s arc. © Marko Mikkilä (2007, September 9^th)

 

Reference

[1] R. G. Greenler & E. Tränkle, « Anthelic arcs from airborne ice crystals », Nature 311, 339-343 (1984) – see Figure 1.

Posted by Luc Dettwiller