Noctilucent cloud

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Noctilucent cloud, 2009-06-17, Edinburgh.
Noctilucent cloud, 2009-06-17, Edinburgh.

Contents

Noctilucent clouds (NLC) are clouds of water ice at the top of the mesosphere at an altitude of 80 to 85 km. At such height these clouds can be in sunlight after the Sun has set - or before it has risen - on the lower tropospheric clouds. Scotland lies in the range of geographic latitude where these clouds can be seen; at low latitudes the mesosphere is too warm to form water ice, at high latitudes the summer nights are too bright. The clouds themselves can form only during the season when the mesopause is coldest, i.e. the summer months. In the northern hemisphere the NLC season runs from late May to late August.

Geometry

The geometry of illumination of NLC.
The geometry of illumination of NLC.

The graphic shows a cut through the Earth with the observer on the left looking in the direction where the Sun is below her horizon. The observer is looking upwards into the sky. Near the observer the line of sight is in the Earth's shadow and any cloud will probably appear dark against the sky. But further away from the observer the line of sight crosses into the sunlit part of the atmosphere and any cloud there may appear bright against the background.

Consider a cloud at height z above the surface of the Earth and along the surface a distance α away from the observer. If we were looking north the angle α would be the difference in geographic latitude of the cloud and the observer. We count α positive if the cloud is toward the Sun and negative if the observer sees the cloud at the opposite azimuth to the Sun. Given z, the altitude h seen by the observer and the distance angle α are related by

h = atan2[(R+z) cos(α) - R, (R+z) sin(α)]
α = acos[R cos(h) / (R+z)] - h

Note that the "altitude" h can be larger than 90°. This will happen when α is negative: Imagine the observer looking towards the horizon where the Sun is, then raising her head to look at the cloud. If h > 90° (if α < 0) she must bend over backwards to look beyond the zenith to see the cloud.

But the main question is whether the cloud is in sunlight. This depends on the angle hSun, which is a negative angle telling how far below the horizon the Sun is for the observer. We look for the intersection point of the layer with height z with the border of the shadow. The ground distance and observer's altitude of that point are

αsh = -hSun - acos[R/(R+z)]
hsh = atan2[(R+z) cos(αsh) - R, (R+z) sin(αsh)]

(Note that the maths is this simple only for the case of NLC in the same direction as the Sun or in the opposite direction of the Sun. The equations cannot be applied to NLC at azimuth significantly to the left or right of the Sun.)

NLC on 2006-07-12/13 from Edinburgh, Scotland.
NLC on 2006-07-12/13 from Edinburgh, Scotland.

At sunset the light/shadow border is at hsh = 180°. This is on the horizon opposite the Sun, i.e. the whole sky is still sunlit (whatever height z we are considering). When the Sun has dropped 4° below the horizon the z = 15 km level is illuminated up to only hsh ~ 35°. Around this time normal cloud is passing into the shadow and will appear dark against the sky. When the Sun is 6° below the horizon (civil twilight) the 15 km level is illuminated only to 2° above the horizon. At the same time the 85 km level is still in sunlight to hsh ~ 168°, virtually all the sky is still sunlit for clouds at that level. This is why it is recommended to start NLC observations when the Sun has dropped 4 or 6° below the horizon.

When the Sun is 9° below, the shadow at the 85 km level is moving quickly across the observer's sky, and at 12° below NLC would be illuminated only if below an altitude of 12° and toward the Sun. The recommendation is to observe until the Sun is 16° below, because by then NLC would be confined to only 2° on the sunward horizon.

Automatic camera

I have set up an automatic camera in Edinburgh to take images of the northern sky every 5 min during the night. These are then inspected and reports made to the BAA Aurora Section and to the NLC observers' home page. Reports of "no NLC" are as important as positive reports of seeing NLC. Of course, when there is tropospheric cloud or fog in the way, no report can be made.

Thanks are due to the Royal Observatory Edinburgh, where the camera is located. They also provide part of the equipment, an old Linux laptop, network bandwidth, and a nice view to the north across the city. Thanks also to Michele Cirasuolo of the ROE, whose office view is somewhat impaired by the equipment. The camera is an old digital SLR set to 200 ISO and f/3.5. The exposure time (shutter speed) depends on the altitude of the Sun and is set prior to exposure by gphoto2 on the laptop. The best exposure in seconds for a given altitude of the Sun in degrees seems to be

t = exp[-(hSun+7.3°)/1.2°] s

Remember that hSun is a negative number. Images are taken whenever hSun < -4°. The resulting exposure times range from 1/8 s to 8, 16 or 30 s, depending on how dark it gets at midnight. These values are good for the sky background and reasonably faint NLC. For bright NLC much shorter exposures are necessary, as short as 2 s for very bright NLC.

Results

Precursor webcam

The dSLR has been in operation since the start of June. During the second half of May I tried to use a networked webcam, but the images were no match for a visual observer or a proper camera.

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