As I explained here, I keep seeing >>smudges<< in oxygen O I 7772. Smudges, ie regions that are darker than expected based solely on the location of flares and plages, yet seem to be related to whatever is happening in extreme UV. Here’s this day’s smudge.
2026-03-21-1244 solar disk, oxygen divided by continuum (and some other math), showing a smudge
2026-03-21-1244 smudge marked
WIP/DRAFT
The first light of my spectroheliograph was in the summer of 2024. It was a stock Sol’Ex, with a modified setup, using full aperture bandpass filters as an energy rejection method, instead of the recommended full spectrum ND filter or Herschel prism, flooding the instrument with the right kind of wavelength, while also managing the thermal load and keeping the zero order and any internal spectral fog at bay. I quickly left the comfort of the Fraunhofer lines, and stumbled into prominences in „unknown” wavelengths[1], a frustrating experience that lead me towards literature, and also propelled me to create the atlas of spectroheliograms[2].
In the meanwhile, almost as a side project, dedicating telescope time to this in an on and off manner, I started to hunt down prominences in various spectral regions and lines, some known and expected, some not so much. Thus today I know that the ultraviolet „unknown” wavelengths showing prominences were most likely the magnesium triplet around 383nm, that Paschen hydrogens are dwarfed by not just Balmer hydrogens but also by the helium at 1083nm, which shows coronal holes, just like the yellow and the deep red helium do. I’ve seen the hydrogen ladder almost up till the Balmer break, and seen that neutral calcium is something worthy to at least check for prominences, that the infrared oxygen triplet can do very funny stuff, with obvious prominences, and that ionized metals show dark plages and often times prominences that are easier to see than some helium lines.
During my campaign to observe the neutral oxygen triplet in the infrared, through full disk spectroheliograms, I noticed a few things:
Smudges are areas that stand out as darker than expected, after taking into account spots, flares, and active regions. After contrasting the oxygen against a nearby continuum, either by subtraction or by division, smudges are revealed to be darker in oxygen, when compared to nearby active regions (which are themselves darkened) and thus a bit surprising. Smudges are stable across several minutes/hours, or even a day or two, while the flares I recorded are often times darker and short lived, on the order of minutes mostly. These smudges seem related to active regions, but don’t stand out in GONG Hα, where they can be a mere brighter spot like many others that don’t show up as smudges, nor AIA 1700 where the underlying area usually looks like any other active area that doesn’t show a smudge in oxygen. Better correlation can be seen with say SUVI 0304 or SUVI 0094 as small bright features in EUV, but it isn’t one-to-one either. Smudges stand out when they are somewhat towards the disk center, inside say 2/3 solar radii.
Below I compiled some scans with a nearby SUVI and AIA disk for comparison. I included smudges (marked with S), small and bright flares (marked with F), for context.
Instruments: 62/400 doublet, EO 775/25 OD4 narrow band filter, OD 0.6 neutral density filter, modified Sol’Ex, with 1800 ln/mm grating, modified/cooled ASI 678MM, JSol’Ex. Variable to good skies.
