Noctilucent clouds and a blue phenomenon on Jun 21, 2019

An extreme outbreak of Noctilucent clouds (NLCs) has been captured with several cameras on Jun 21, 2019 . After the NLC's disappeared at horizon a dim blue phenomenon was observed.

NLC's are are Earth's highest clouds and occur in altitudes of about 80 km to 85 km. These clouds consists in tiny ice crystals of up to 100nm. NLC's are too faint to be seen in daylight and they are only visible if the sun is between 6° and 16° below horizon. Then the NLC's are illuminated by sunlight while lower layer of the atmosphere are in shadow and sky becomes dark.

The moving filaments that can be seen in the video are caused by gravity waves, the same type of waves which occur if you drop a stone into a pool af water.

Stills and video from photo camera pointing to north

High resolution images of the NLC display has been taken with a digital photo camera with fish-eye lens. The begin of the event has not been captured with this camera and the last photos where disturbed by scattered light (orange lightening at the corners) caused by dew on the lens. Below is a video made from the photos and a high resolution still image. At the end of the video sequence the blue phenomenon becomes visible.

Click on image or use the buttons to start a video or display a hi-resolution image.

Sky surveillance camera pointing to north

The entire display was recorded with a sky surveillance camera pointing to north.

Click on image or use the buttons to start a video or display a hi-resolution image.

Sky surveillance camera pointing to south

For a short time NLC's have been visible even far to the south.

Click on image or use the buttons to start the video.

Sky surveillance camera pointing to west

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Blue phenomenon

After the normal NLC's disappeared at horizon an dim blue phenomenon has been observed with both cameras pointing to north. This unknown object is also visible in the video made with the photo camera.

Below are time-averaged images. Timezone is CEST (= UTC+2h).

The bright bar on horizon are the normal NLC's. They look reddish due to Rayleigh light scattering of the atmosphere.

Photo camera

Jun 21, 23:47 to 23:49	Jun 21, 23:49 to 23:51
Jun 21, 23:51 to 23:53	Jun 21, 23:53 to 23:56
Jun 21, 23:56 to 23:58	Jun 21, 23:58 to 00:00
Jun 22, 00:00 to 00:02	Jun 22, 00:02 to 00:04
Jun 22, 00:04 to 00:06	Jun 22, 00:06 to 00:08
Jun 22, 00:08 to 00:11	Jun 22, 00:11 to 00:13
Jun 22, 00:13 to 00:15	Jun 22, 00:15 to 00:17
Jun 22, 00:17 to 00:19	Play/Pause: 5fps Play/Pause: 2fps Stop

Sky surveillance cameras

Jun 21, 23:50 to 00:00	Jun 22, 00:00 to 00:10
Jun 22, 00:10 to 00:20	Jun 22, 00:20 to 00:30
Jun 22, 00:30 to 00:40	Jun 22, 00:40 to 00:50

Measurements and Calculations

Some data could be derived from the images which may be help to identify this phenomenon. At 00:15 CEST (=22:15 UTC) the maximum apparent altitude of the phenomenon was 25° above horizon and the sun altitude was -14.5° at this time. If the altitude of the object was 82.5 km above ground (like NLC's) the source of light must have been 11.8° below horizon. If the object was illuminated by sun it's altitude must have been 135km above ground.

Perhaps this are NLC which are illuminated by more northern NLC rather then directly by the sun, i.e. the bluish light is double scattered light from the sun. This would also explain why the structure seems to be similar to that of normal NLC's.

Observation data:

Date:	Jun 21/22, 2019 between 21:50 and 00:50 CEST (=UTC+2h)
Location:	Pulsnitz, Germany
Instrument:	APS-C digital photo camera with fish-eye lens and sky surveillance cameras