4.2 Dynamics of noctilucent clouds (NLC)

The main results of the study of the dynamics of noctilucent clouds (NLC) are presented. Estimates of the heights of the silver clouds, the direction and speed of their movement are given.

The main results of the dynamics noctilucent clouds (NLC)

As we already mentioned, in order to process the received observational data, we created the software package. These programs not only significantly accelerated the processing, but also allowed (using an original processing technique) obtaining (for a relatively short time) the following results:

— Estimates of the geometric characteristics of NLCs (NLC field and dimensions of their individual parts, and their formation heights);

— Estimates of NLC dynamic characteristics (the front velocity and their movement direction, existence of the billow patterns and identifying the important parameters);

— NLC field location relatively to the base point (in our case it is Petropavlovsk), or in other words — their geographical distribution in the region.

From morphological analysis, we can conclude the following:

The NLC front velocity mostly high — from 250 km/h to 350 km/h

A direction of the front movement is mainly southwestern. However, there was to the North (1 case), to the South (3 cases) or to the North-West (4 cases).

NLC fields had mostly very complex structure, which is a superposition of some billow motions — the large- and small-scale motions, usually propagating on different directions.

Small-scale structure is billows with wavelengths (distances between crests) from 5 to 15 km. The crest lengths are mostly small. In all the cases, the billows were practically located across the bands, and, their spreading velocity (~ 100 km/h) was different from the speed of the NLC front. This structure, as a rule, had a short lifetime, but there were cases (rarely) when it existed for a long time, slightly changing its direction and shape. As for the large billows, their crests were located at distances of 35-40 km from each other. The crest lengths reached hundreds of kilometers.

In almost every case there were long (80 — 200 km), nearly parallel bands that stretched along the direction of the front movement. It is interesting to note that the vortices observed in them often were born either in frontage part of NLC field, or on its opposite border.

Despite a relatively small number of our statistical material (22 nights with good NLC images), we noticed a relationship between a complexity of the NLC structure and its front velocity: NLCs of a rather simple structure moved more slowly (100 — 200 km/h, 5 cases), and vice versa. However, there also observed NLC of a complex structure, moving with a velocity 150 km/h.

NLC altitudes that we measured from observations are within 75 km —
90 km, for all this, we registered some cases when individual NLC portions changed their height from ~ 80 km to 75 km, up to their disappearance.

NLC parts’ sizes:

— The band lengths are 80-300 km,

— The knot areas are up to 230 km2,

— The vortices curvature radii are ~ 40 km

— The billow crest lengths are from a few kilometers to hundreds of kilometers.

NLCs observed mostly moved to the North-West or to the North from Petropavlovsk, there were NLCs moving to the North-East. Their distance from Petropavlovsk was of 650 — 800 km (7 cases) and 400 — 600 km (12 cases). It is possible that the revealed asymmetry in the NLC azimuth distribution is a result of differences in directions of lighting them by the sun in the evening and in the morning.

The area occupied by NLCs changed (increased or decreased) depending on the movement direction and the time of the day. The area values, according to our estimates, range from 170 000 to 400 000 km2 (12 cases), and from 20 000 to 110 000 km2 (6 cases), that implies that their distribution areas are very large.

On some nights, it was difficult to determine the parameters mentioned above.

In summary, we can conclude the following:

— NLCs form at 90 — 80 km, while at an altitude 75 km the ice particles that may be in their composition, evaporate. Moreover, the individual NLC layers (small-scale and large-scale billows) perhaps are located at different levels and have different velocities and movement directions: transverse billows travel at ~ 100 km/h on the background of the main NLC front movement at velocities over 300 km/h.

— Wavelengths of a small billow structure are 5-15 km, of a large one — 30-40 km.

— NLCs occupy huge areas: hundreds of thousands km2.

— A direction of air mass movement at the level of NLC formation (mesosphere) is mainly southwestern at a speed 300 km/h.

— NLC structure is predominantly complex. It, perhaps, indicates a very complex dynamics of the upper atmosphere.

— Rather simple NLC structure (no turbulence eddies, billows, etc.), are observed rarely enough and may indicate a more placid dynamics of the mesospheric layers.

Some additional results of our investigations one can find in [19-27].

Now let us speak about NLC spectral characteristics and possible NLC luminescence.



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