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NLC 2009-05-13/14

Pseudo NLC on 2009-05-13/14
Pseudo NLC on 2009-05-13/14.

Non-NLC on 2009-05-19/20
Clearly not NLC on 2009-05-19/20.

This is the first test night, and the images are overexposed for the brightest part of the twilight sky. This night has resulted in some intriguing images. They show what appear to be whispy, whitish, slow-moving clouds at high altitude above the northern horizon. The initial response from Ken Kennedy of the BAA Aurora Section was that it is unlikely that NLC would occur so early in the season at such high altitude. His inspection of data from the AIM satellite confirm that this is most likely not NLC.

One kind of "pseudo NLC" is when a pattern of bright background and dark tropospheric clouds appears like the opposite - bright NLC on a dark sky background (NLC test from Norway, in NLC observers forum). Here there must be another kind of "pseudo NLC" at play. A few days later at very similar time of night I notice that the thin edges of clouds can appear grey against a darker sky even when the thicker centre of the cloud is dark or reflecting street lights. It would then also be possible for a thin low cloud to appear like NLC.

The first image shows the pseudo NLC in the top-left quadrant of the image. The second image shows a dark (and hence tropospheric) cloud in the top-right quadrant that nonetheless has a bright edge. On its own this might appear as pseudo NLC similar to the first image.