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I observed an interesting terminator irregularity or projection last night September 7 around 3:00UT, just preceding Mare Sirenum.
Visible in all wavelengths, images to follow.I assume this is due to the dust storm that was reported in late August. There are images on the Mars Section site showing projections from August 18 and 24 at the kind of latitude I saw it at, but what I saw was more sharply defined than the humps shown there.I urge other observers to look carefully at the terminator over subsequent nights.
All the images in this set showed the interesting terminator irregularity in the S hemisphere. It is clearest in the first image, R from 02:38, and I have given an enlargement of that one. Comparing with other published images from the same night, I believe this is due to straight N-S ranges of cloud over the Tharsis Montes casting shadows. The irregularity seen here is a combination of a bright line of cloud catching the light, and its shadow preceding, with another high bank of cloud to S just going over the terminator.
I have written about these at the Mars Section 2020 blog page and am about to post an update which includes some further comments on the images by David and others. These projections are always interesting to see, and I think that with the martian atmosphere having been rather free from the effects of major dust storms for several months now that there has also been the very unusual chance to observe shadows of high clouds very close to the terminator: the projections themselves are not so rare, but observations of their shadows are. Because there is always some residual dustiness in the martian air, any details of the terminator must inevitably faded by absorption and scattering of light under oblique illumination.
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