Opposition surge of Saturn ring in 2021
(Heiligenschein phenomenon)


Fig. 1 Saturn images before & after opposition in 2021

i : Phase angle of Saturn at taken time

Fig. 2 Comparison of horizontal luminance distribution

Acquired average counts in yellow rectangle with Makali'i 2.1


Photography condition of images used for measurement:
Optical: Kasai 20cm(7.9") F6 Newtonian (NERO-200DX),
with Televue Powermate 5× (Syn. F35.8)
Auto-guided with TAKAHASHI EM-200 Equatorial
Camera: ZWO ASI 183MC with IDAS IR-cut filter
Location: Hitachi city, Ibaraki pref.

Camera Settings: 16 flames/sec.×90sec., Exp....1/16sec
1200 flames stacked with AutoStakkert3 & wavelet process with Registax6



Fig. 3 Relation of phase angle & luminance of B-ring
  The Saturn in 2021 had the opposition in the Capricornus on Aug 2. It is known that the Saturn's ring looks brighter than the body of Saturn for a few days before and after the opposition. The phenomenon is called "Opposition surge" or "Heiligenschein" ("Aureole" in German).
  Currently the following two effects have been thought to be influential as the causes of opposition surge.
  1. The effect of shadow hiding of ice blocks that form the ring during a few days before and after the opposition
  2. The interference effect of entered and reflected sunlight by fine unevenness on the surface of ice block (coherent backscatter)
  However, the contribution rate of these two effects has not been investigated in detail even now.
  Fig. 1 shows a set of Saturn images taken from Jul 20 until Aug 6, 2021. "i" in images represents the phase angle meaning the elongation of the Sun and the Earth seen from the center of Saturn. Smaller i means that the Saturn is shone by sunlight from closer to the front. You can recognize that the widest B-ring looks obviously brighter than the body of Saturn on Aug 1, the day before the opposition with i = 0.11°.
  Fig. 2 shows a comparison of horizontal distribution of luminance of individual Saturn images extracted from a yellow rectangle indicated on an upper example image. The luminance value has been normalized to that of the body of Saturn. The luminance of B-ring on Jul 20 (white) and Aug 6 (cyan) has almost the same with the body of Saturn. In contrary, it was observed that the luminance increased on dates with smaller - i (orange, yellow and magenta).
  Fig. 3 plotted the left and right average of maximum luminance of B-ring as a function of i. The vertical axis indicates relative luminance on the basis of that on Jul 19. The opposition surge of Saturn ring can be observed during a short period with i less than 0.5°. From this observation, it was confirmed that the ring becomes brighter by 30% or more from normal period.



Saturn images in season of 2021
Saturn images in season of 2020

Copyright(c) 2021 by Naoyuki Kurita, All rights reserved.
To top page To Moon&Planets index