ON THE THEORY OF THE SEASONAL FREQUENCY OF NOCTILUCENT CLOUDS OCCURRENCE

(A historical review)

Wilfried Schröder

Summary:

The seasonal frequency of noctilucent clouds has been discussed on the basis of visual data from the Northern Hemisphere. It is shown that there is a strong dependence on the atmospheric constitution at the mesospheric level and the seasonal transition in the mesopause. Before spring transition and after autumn transition of the mesosphere at the 80 km level no NLC are observed on the Northern hemisphere.

Conversely we can conclude the same distribution for the Southern hemisphere and the strong relation between transition periods and occurrence of NLC.

Keywords:

Noctilucent clouds, Transition periods of Mesosphere, PMC

1. Formulation of the problem

Any successful theory of noctilucent clouds (NLC) must first of all explain their monthly appearance and  then  the special effects which have been noted in  past observations; that is the sporadic appearance of NLC, their absence during the months of October-April in the Northern hemisphere, the nearly constant height of about 82 km as well as the morphological features.

The fact that an  explanation is only partly successful, may be due, on the one hand, to insufficient knowledge the physical composition of of the various layers of altitude and, on the other hand, to the scarcity of mathematical-physical estimates. The hypothetical character of past  evaluations inclines one to replace these speculations by a more general treatment, which  will still be subject to certain limitations. In doing this we will not obtain a clearly proven theorem but an explanation which may eliminate many of the uncertainties.

2. Past attempts at explaining the appearance

In the literature we meet the expressions "Dust"- and "Ice hypothesis". By that it is meant that NLC are either the product of cosmic particles or are formed by the addition of a new influence,namely lower tempertaure.

During recent years NLC  have been described in many publications. (Chapman and Kendall,(1965) Hesstvedt,( 1967), Soberman (1966), and Webb.(1965). Haurwitz (1964) also made a contribution to the morphology of NLC (wave characteristics). Regarding the meteoric comopnents, Soberman made some interesting comments on the formation of NLC:

"Basically, the new theory is that a small updraft between 50 and 80 km keeps meteoric dust from drifting to Earth. Over a period of days or weeks, a large concentration of the dust accumulation in this altitude region. An updraft lasting long enough for such an accumulation to take place,  could occur only certain conditions. These conditions, summer and high latitude, are exactly those under which NLC occur.

... Since this altitude change involves a corresponding temperature drop from a few degrees above zero (centigrade) to - 100 degress C, any water vapor present would freze on the condenstaion nuclei provided by the meteoritic dust particles... These particles, illuminated by the Sun's ray coming from far below the observer's horizon, would be seen from ground as NLC as they pass through the twilight region", Soberman,1966,  p. 101-102)

Figure 1 graphically present Soberman's ideas.    Chapman and Kendall,(1965) raise the following points:

"we suggest the occurrence of noctilucent clouds may require the following conditions to be satisfied simultaneously:

(1) The mesopause temperature minimum Tm sinks to a sufficiently low value.

(2) The turbopause descends to about the level of the mesopause.

(3) Convection carries an adequate amount of water vapour up to the turbopause, and turbulence creates a dust ledge there" (page 130).

The views of Webb regarding an "upward movement of captured meteoric particles are, in a sense, analogous to those  given byHoffmeister and Schröder earlier (see Schröder, 1966).  Webb outlined also a system of circulation which is responsible for the movement of these particles into the main region of the NLC.

A cosmic origin is found only in paper (Hoffmeister, 1960  ) That paper also outlines a "system of circulation " for the movement of cosmic dust particles.

The  aforementioned papers give the impression taht in order to explain individual question certain assumptions must be made. This complicates the explanation,  and,  in addition, involves assumptions which are largely questionable. However, these papers point to a lesser or greater dependency on cosmic particles.

3. Observations

For methodological  reasons we shall consider in his study only the results obtained by British, German and earlier Soviet observers before 1970. At this time we  had a good regular watch with well qualified observers from different sites in these conuntries, and   the data are mostly valid reliable.

a. On the reliability of data

For the following deliberations it is particularly important to assess the reliability of the visual observations. In Britain, observations of NLC have been made by Paton and colleagues since 1939. In addition, observations were made both at regular meteorological stations and by many volunteer observers. The observational procedures for both groups were the same. Since the observations also include a watch for auroras they extended over the entire year.

In Germany, in the years after 1885, there existed under the direction of Jesse, a working group in the  Commission of astronomy and cosmical physics (VAP). However, for many  years later only sporadic data were recorded. It should also be pointed out here that the " Catalogue of German Observations of NLC 1885-1956" (Zeitschrift für Meteorologie, 1967) needs to be revised. Well documented were the long-term observations by Hoffmeister and colleagues in their programme of increased airglow which also included NLC. As to the Roennebeck data (Northern Germany) , the following selection criteria should be observed: meteorological and geomagnetic numbers, position of the sun, morphology Of clouds, as well as knowledge gained from the medical and psychophysical sciences (e.g. the adaption of the eye by normal sight (10/10) etc.

b. Seasonal frequency of NLC

Table 1 records the observations.

Table 1. Seasonal frequency of NLC

Country                   Months of NLC                                         Authors                    occurrence

______________________________________________________

England                    May-August                                               Paton

Germany                   May-August                                            Schröder

USSR/Russia             May-August                                   

__________________________________________________________

The following comments complement Table 1: For Britain, Paton remarks: "Our observations of the occurrence of noctilucent clouds, using data from Britain, Scandinavia and Denmark, show that the clouds are never seen before May 16th and seldom after August 3rd". Only in 1966 were NLC still seen in mid-August. For Germany we find the same picture, with some differences in dates for the the beginning and the end of period. Most NLC were  observed in the 10-day interval June 21-30. From the earlier USSR there were repeated reports of NLC  observations during the autumn months , but these are questionable and very rare.We have disregarded them.

Of special importance is the question of the appearance of NLC in winter of the Northern hemisphere. It should again be pointed out that in Britain and Germany observations are made throughout the entire year. In Germany (Roennebeck, 53.2N) in addition to NLC, auroras as well as the appearance of increased airglow were recorded.  Reference should also be made here to the statistics of increased airglow (see  Hoffmeister, 1960) which covers a period of over 30 years and which was  published in full.When we examine the results of these three undertakings, all independent from each other, we can say that during winter months no NLC appear. The possibility that, for one reason or another, NLC were "overlooked" may also be excluded. If NLC did really occur in winter at any time  one of these groups would have surely  detected them

4.The meteor phenomenon and impetus to NLC research

An impetus to NLC research was given by the rocket launchings in Sweden in the year 1962 and later (cf. Hemenway et. al. 1964).  The electron microscope analyses that were made at that time showed only weakly defined structures around the particles. Only after chromium vapor was applied did a few particles show a halo. This pattern is supposed to be the result of a tendency to change of a volatile material (cosmic particles). The particles showed radii of about 0.08 to 0,8, only a small number showed a radius smaller than 0.05. The original chemical analysis has in the meantime been given, in revised form, by Skrivanek.  The halo particles are said to contain a relatively large amount of sulphur. Otherwise, only silicon  and a small component of esther  iron or calcium were identified. But we know from meteor physics that sulphur is not a regular component of meteorites. From the halo-shaped structures on the nitrocellulose film it was deduced that some of the particles were ice-coated. In addition it was found that the number of particles was 1000 times greater in the presence of NLC than in their absence (see Hemenway et. al. 1964)

These results were widely reported in the sixties and later. However,  the results of 1962  have so far not been  confirmed by later studies. This is a all the more remarkable since further ascents took place in Sweden in 1964.

Since then much rocket data has been collected from mesospheric levels and there have been many attempts to link NLC formation to different particles. But  no clear explanation of the formation of clouds has been given, up to now. It is of interest that many proposed factors for NLC growth have been noted, but a simple physical explanation is still  not available.

 It is therefore  questionable whether the cited halo particles are really present in NLC and, if they  are, it is still unproven that we are dealing with traces of cosmic dust. Moreover, it should be borne in mind that there is no dust cloud such as was repeatedly assumed in earlier works. The number of particles which fall into the earth's atmosphere is much smaller than was heretofore assumed. This explains the difficulty of finding a mechanism which cumulates the particles at the 80-84 km level. The fall-rate of extraterrestrial dust is almost identical with the dust density in planetary space, as demonstrated by the zodiacal light phenomenon For further discussion on the problem of cosmic dust see also Grjebine (1967  ).

Various calculations of the processes of micro- meteorites in the Earth's atmosphere have been done, e.g. by Oleak (1956/57) Regarding the intensity of vaporization he writes: " It is evident that the largest loss of mass occurs at heights proportionately greater the smaller the particles and the greater the initial velocities." (Oleak, p 140).

Regarding the intensified nocturnal air glow we find that the corresponding layers of altitude (90-180 km) will be penetrated in a few minutes. But this cannot be reconciled with the observed fact that the luminous streaks are not only visible for many hours but remain unchanged. His study shows that " even 'small' particles... are not directly resonsible for the appearance of the luminous streaks since they are virtually completely vaporized at great altitudes. "(Oleak, p. 143)).

 This conclusion can also be applied to NLC because, for example, they can often be observed for several hours. If we compare these facts we see no basis for the participation of cosmic particles in the direct formation of NLC. It is also of interest that the meteor streams of comets (e.g. Perseids) cannot make an active contribution to the intensified nocturnal airglow (see , Hoffmeister, 1960, Schröder, 1966).

Two further results argue against cosmic components. In a study of the "Quadrantids" current, which occurs in January, Glöde ,1966 reaches the following conclusion:

"...the additional amount of material falling into the Earth's atmosphere during the shower must also be smaller than that from the sporadic meteors. Then we could not expect that the "Quadrantids" would have secondary geophysical effects which could be noted in addition to the always present effects of sporadic meteors. Nor could we generally use the effects which might or might not have been produced as criteria for the action of the meteor dust in the higher atmosphere". Glöde, p. 163)).

Past research  (Kohl, 1968)    also shows that depolarisation can be interpreted as an additional primary scatter effect of cosmic dust particles. In general it is surprising that no direct relation has been  found between the meteor stream activity and the development of NLC. In summer different streams are active but they show no direct correlation with   the frequency of NLC. If we remember that  NLC displays are observable for some hours, and very often show abrupt changes in the different forms, it seems  that a "layer" must be present in the NLC zone. This layer will not be persistent and is very dependent on  the wind streaming at these height which very often and suddenly destroy the NLC-layers and produce the different various cloud forms. So, there is a accumulation layer of material at 82km  which comes from meteors or as a  result a part of pollution, e.g. such as that observed after the great Krakatoa eruption in 1883.

 It required two years for the volcanic material  to reach the 82km layer and create the conditions for the formation of the NLC which were first detected  in June 1885.Because N LC are only observed during a very limited time-span it can be see that the particles alone are not enough for the formation of the clouds.  Humidity and low temperatures are also required, conditions not appreciated in the time of O. Jesse, so that he was unable to construct a more detailed theory of NLC.

The idea that meteor particles play an important role was not new. Various researchers since 1885 had discussed this aspect of  the theory, but it was not enough to explain all the details shown by the visual and photographic observations (cf. Schröder, 1966). A very difficult point is the abrupt variation in forms which can be observed during a few minutes. This can  only be  explained by turbulence in the wind streaming at the NLC layer which suddenly destroys the homogeneous layers.

  No increased meteor streams or enhancement occur in late June/early July .The normal flux of meteoric material at those times  also plays a part in the structure of the upper atmosphere in their composition with different particles.Because the studies by Oleak et. al showed no direct increase with  micrometeoric influence it is open to question whether meteors contribute to NLC formation.

We conclude therefore that, there is no definite proof of cosmic influence in the formation of NLC.

5. On the structure of the mesosphere

Haurwitz (1964)   pointed out in his study the inadequate knowledge at that time in the field of mesospheric physics. Hesstvedt (1962, 1967) makes a similiar observation. In fact this inadequate knowledge means that proof of the various views cannot be carried out with  precision. There have  many interesting measurements in the past, e.g. by the Kühlungsborn group and other scientific institutes. But, in general our knowledge of the mesospheric processes is insufficient to account for the behaviour observed in NLC   This comment also applies to  polar mesospheric clouds. In many cases one must depend on assumptions which may quickly be proven false by measurements in the future.  

a. Temperature condition

Figure  2  shows the  of temperature at  various height levels of the atmosphere. This  average course of temperature variations shows a steep decline during summer in the region of the mesopause. A few measurements of the temperature profile of NLC werde recorded during the 1962 rocket ascents. A value of Tmin= 133K was obtained when NLC were present and one of Tmin=140K when they were absent (see Witt et. al., 1965). Further measurements were obtained in that years in Barrow (71N) As to the significance of temperature values in the formation of NLC the authors state: " Thus the coldest temperatures did not necessarily produce noctilucent clouds, but the clouds were always accompanied by mesospause temperatures less than 150K" (Theon et. al. 1967, p. 419). Even though in paper (31) other new factors were introduced into this problem only one fact really matters: the appearance of NLC is tied to low temperatures, a result which has been confirmed during the recent years. These, however, are present in the Northern hemisphere only in summer and solely in the region of the mesopause. This  clear relationship is also shown by recent data .

b. Humidity Data

Researchers had available only scant material on the H20 mixing ratio in earlier years. Hesstvedt then pointed out that there are no objections to the theory of formation of sublimation (see Hesstvedt 1967, see figure 3).

Various discussions have been published on the use of different mixing ratios and relations to the freezing point (see Gadsden and Schröder, 1989) We quote here a few comments made by Hesstvedt since they best describe the general situation in his time:

"...that the more photochemical model presents highly misleading  results for H2 and for water vapor between 70 and100 km...the water vapor still takes up 60% of the available hydrogen. At 95km, 22% of the hydrogen will still be in the form of water vapor" (Hesstvedt 1967, p. 5)

From the formula for the growth rate of particles we find that the initial radius of a particle increases from 0.05 within of two hours .Recent studies gives a more detailed picture of the processes but do not succeed in explaining all the problems.

c. The model of mesospheric circulation

Concerning the reaction that arose at the time of the finding of a temperature decrease in the mesopause Faust remarks: " This finding, which at the time caused much surprise and led numerous arguments, can be easily understood by means of the cited model of circulation" , (Faust 1967, p. 6). Since the model by Faust  and Schröder occupies a special position in understanding NLC, we shall describe it in more detail. Figure 4 shows that in summer, above 60km altitude, upward currents set in as soonas the transition of summer circulation is completed. These rising currents produce a cooling off at altitudes greater than 60km. Theoretically the spring transition of winds begins in high latitudes and spreads to lower ones.When the summer circulation is in full swing rising movements at higher latitudes are matched by downward movements at lower latitudes. Of special interest,  is the fact that the spring transition of winds starts at a different time each year over a four to six week period. During the winter there are downward currents at 80km altitude in high latitudes, an occurrence which is automatically reflected in the model by Faust and Schröder. In the meantime, a generally accepted explanation of this occurrence has been given by Kellogg (1961). In  Faust (1967) the following statement is made:

" Above the approximate 60km level downward air currents must predominate during winter in high latitudes, a conclusion that can also be arrived at by assuming a reversal of the large scale vertical movement in the 60km wind maximum on the basis of the "zero layer concept". In this downward movement, air particles, which had been stimulated in the ionosphere and which have again released their stimulated energy in the mesosphere, are carried downward" (p. 155).

Although the model is known in its broad outlines, information is lacking on individual variations. First of all it must be understood that the model is valid only under average conditions. This is why it is understandable that the zero layer circulation may at times be stronger along one circle of longitude than along another. Maybe this explains the single "out-time" observation also of NLC in spring and autumn.To draw reliable conclusions from the situation would necessitate repeated rocket measuremnts from the polar areas. These were not available in the sixties and earlier seventies. At that time only the stations in Fort Churchill and Fort Greeley were in operation.Now, we have many more observational points and a various satellite and rocket programme by many research groups. They have developed an interesting insight into the formation of the mesosphere, but it has not been possible to present a generally accepted theory of the NLC and later PMC.

It appeared desirable to apply the conditions of Faust's and Schröder's  model to NLC. Past results (see  Schröder, 1968) showed that the continuous observations of NLC in the Northern hemisphere occurred only after the spring wind transition, mainly 4-6 weks later (Schröder 1968). It was also noted that NLC disappeared almost exactly at the time of the autumn wind transition (cf. Theon et. al. 1969) in mid-August. From the available material it can be concluded that a temperature effect must be present, particularly so since the connection between the start of the East stream and the sudden appearance of NLC is unmistakable. These views are also supported by the knowledge that winter temperatures of the mesopause in the Northern hemisphere are some 60 degree higher: this results from the downward movement shown in Faust's  and Schröder's model.

It is also of interest to examine the effectiveness of Faust's and Schröder's  ideas on the basis of NLC data.We should therefore attempt to stress more closely the formation of average conditions. Past views of NLC observations lead to the assumption of a frequency zone (NLC-belt) in the vicinity of 50-70N/S. Maximum devations of this range occur between about 45N or 75N. The research of Astapovic (see Schröder, 1966)  has also shown that no NLC occur south of 45N: this result is also confirmed by American data. All of the studies in the sixties show that outside this region no NLC were observed in the mesopause region.

6. Transition periods of the mesophere and NLC

It has been demonstrated from earlier data (Schröder,1968) that there exists a strong relationship between the periods of transition in the mesophere - in spring and autumn - and the appearance or disappearance of NLC. NLC have never been observed before the spring transition was complete. The delay between transition and the apperance of NLC shows a variation of some weeks (3-6). The frequency of clouds varied extremely in spring from year to year, so that there is no definite day of first appearance.

A rather different result has been found for autumn: If the transition period in the 80km region is complete, NLC vanish and cannot be observed. Throughout  the last 100 years no positive observations after the autumn transition have been noted for the Northern hemisphere. This is a key factor in the relationship between the changes in the mesosphere and the appearance of NLC.

The data (1962-1968 and 1991-1996) shows   a variability in May over for 4-6 weeks. The NLC period is nearly constant in August in the first 20 days. After August 19 no NLC have been noted showing  that the transition  was then complete for the mesopause region and the physical conditions for the formation of NLC no longer existed.

From this it follows that  NLC can only be oberved visually during the full mesospheric circulation times which is limited by the transition times in spring and autumn.This result is valid for both hemispheres, e.g. we have  NLC in summer in the Northern hemisphere, and during  the Austral summer in the Southern hemisphere (cf. Schröder,1968). As we shown there seems to be a little deviation from this rule altough an occassional NLC was observed at an earlier or later time.  This can be explained by the duration and complex exchange of the mesospheric layers in the  the spring and autumn transitions.

Acknowledgment:

I am grateful to Prof. N. J. Skinner for translation help and Prof. J. Verö for his assistance.

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