Phenomena of the high atmospere

The high atmosphere of the Earth is classified into regions according to the degree of ionization of the atoms of the atmospheric gases at the level concerned, that is, according to the electrical conductivity. In the lower atmosphere air is a very good electrical insulator, i.e. its conductivity is very low. In the high atmosphere, however, many of the atoms are ionized so that the air is electrically conducting. The classification into regions depends on the different degrees of conductivity and other electrical properties at the different levels. There are three regions of the following approximate heights: the D region, between 48 and 80 km up; the E region, between 80 and 130 km; and the F region above 130 km.

Long-distance radio transmission in the short-wave band (say in the frequency range from 3 to 30 megahertz) is normally achieved by reflection of the radiation from the F region, the main influence of the D and E regions being to absorb some of the energy of the radio waves. When the blast of ultraviolet light from a solar flare reaches the Earth it increases the ionization in the D region, however, and this increased ionization causes absorption of HF signals, reducing the signal strength considerably or even causing a complete fadeout of the received signals. Such a HF radio fadeout can occur only on the daylight hemisphere of the Earth; it usually lasts for about twenty minutes.

Lightning flashes in the lower atmosphere are a source of radio noise, detected over a wide range of frequencies as the crackles called atmospherics. Thunderstorms and lightning flashes are most numerous in the tropics, and transmission of the resultant radio noise around the Earth to higher latitudes is by means of reflection from the various conducting regions of the high atmosphere. For the low-frequency (LF) part of atmospherics (in the frequency range below 100 kilohertz) it is the D region which acts as the reflector. Following a solar flare and the consequent increased ionization in the D region, the reflection of LF atmospherics is suddenly improved and the noise level of atmospherics may rise to as much as double its normal value. The rise takes place in a few minutes and the level remains high for an hour or two.

When the material ejected from the sun during solar flares, and at other times, reaches the Earth, it affects the electrical properties of the F region in such a way that it ceases to act as a good reflector for HF radio. This effect is called an ionospheric storm. It is most severe in high latitudes because the material particles coming from the sun are electrically charged and are guided to the polar regions by the Earth's magnetic field. Since they are often connected with solar flares, ionospheric storms are more frequent during solar activity maxima, but this is not always the case. The HF radio blackout associated with a severe ionospheric storm may last for several days.

The Earth's magnetic field is almost entirely of internal origin, being most probably produced by electric currents flowing in the molten material of the core, but there is a small part of it (less than one hundredth) which can be attributed to electric currents flowing in the high atmosphere. It is believed that these currents flow mainly in the E region. The daily heating of the atmosphere by the sun causes the currents to go through a regular daily cycle of change. These regular changes are very small in most places and cannot be detected by an ordinary compass needle, the maximum change of direction of the needle during a day being about one-fifth of a degree of arc.

Quicker changes occur at the times of HF radio fadeouts, but they also are too small to be detected by a compass needle. They are known as magnetic crochets or as solar flare effects.

Much larger deviations can occur, however. They are the result of large, irregular changes in the E layer currents, called magnetic storms, which occur in conjunction with ionospheric storms and HF radio blackouts in high latitudes. Like the ionospheric storms of the F region these B region magnetic storms are caused by the arrival of the material particles ejected from the sun during solar flares and at other times.

If the magnetic storm is severe, the compass needle may be deflected continuously in one direction, to the extent of about half a degree, for some hours. In more intense storms the needle may oscillate one degree or more on either side of its normal position, and such oscillation may continue for as long as ten or twenty minutes before dying out. Further oscillation may occur after a period of quiescence, and deviations of 2º or more have been known, though these are rare. During the great magnetic storm of 25 January 1938 a deviation of 4º eastward was observed off the Portuguese coast.

On a clear starlit night, in the absence of normal or abnormal twilight, moonlight, lunar twilight, thin high cloud over the sky, auroral displays or artificial illumination from towns, etc., the sky background is not dark, but has a certain degree of luminosity. While some of this luminosity is due to the combined light of stars too faint to be seen individually without the aid of a telescope, the greater part is due to a faint glow known as the airglow. Older names for this were 'permanent aurora' and 'earthlight'.

The airglow is generally uniform over the sky except towards the horizon where it is usually somewhat brighter. The intensity is not always the same on different nights and there are exceptional nights when the sky background appears to be unusually light. There are no means of estimating the intensity of the glow by visual observation, so that the phenomenon is not one which can be usefully observed at sea.

Artificial Satellites – Launches and Decays

When seen, a satellite appears as a pinpoint of light (stellar appearance) moving slowly across the sky. These objects can readily be distinguished from meteors and fireballs because the latter take just 3 or 4 seconds to cross the sky, whereas bright artificial satellites take much longer, possibly up to 45 minutes. The lowest satellites, at heights of a few hundred kilometres, orbit the Earth in about 90 minutes and from a position on the surface of the Earth, they move at a rate of about 2º per second, that is about 4 moon diameters per second. The brightness of a satellite depends on many factors and the light may be constant. A tumbling rocket case (shaped approximately like a milk bottle) will present varying aspects to the Sun and the observer, and hence may appear to flash. A large flat surface on a satellite can produce exceedingly bright irregular flashes.

The above notes are given as an aid to identifying a sighting as a man-made object, and there is no need to record these in the meteorological logbook. However, towards the end of a satellite's life it will enter the denser regions of the Earth's atmosphere and decay or burn up like a natural chunk of rock. These decays are of great interest and accurate reports of these events are extremely important and should be logged in the greatest detail possible. A fireball produced by the decay of a satellite and that from a natural object can easily be distinguished by the speed of transit across the sky. A satellite may take as long as several minutes to cross the sky compared with the 3 to 4 seconds of a natural object. Apart from the speed, the general appearances are more or less the same. The recording of observations of natural fireballs applies equally to decays of satellites.

The reporting of phenomena associated with the launching of satellites and research rockets is also of interest because the rocket exhausts can sometimes be seen from points well away from the launch site. Research rockets are launched from various sites around the world and can carry equipment for research into the environment, the upper atmosphere and for astronomical observations. They are usually projected vertically or nearly so and, after attaining their maximum height (which may be 160 km or more), fall back to the Earth's surface. The direct light from the burning propellant during the upward leg of the flight will be readily identified but some of the experiments carried by the rockets may themselves give rise to effects which are visible at night. For example, equipment may be carried which releases chemicals into the high atmosphere.

Some chemical vapours are ejected in the form of a long bright trail on the upleg or downleg of the rocket path. Night-time trails usually appear white but at twilight, about 30 minutes after sunset or before sunrise, the trails are generally yellow, red or greenish-blue. Although their size depends on the altitude at which they are released and, of course, on the position of the observer, it is not unusual for a trail to be as much as 40º long and from 1º to 5º wide. These vapour trails which may remain visible for more than 20 minutes, are generally distorted by winds in the upper atmosphere.

Sometimes 'grenades' are ejected from the rocket during the upward flight, generating acoustic waves whose reception at the ground enables the wind and temperature structure of the upper atmosphere to be calculated. At night a grenade burst would appear similar to an anti-aircraft shell burst, being much brighter than a planet but of very short duration. After 2 or 3 seconds another burst would appear displaced in the direction of flight. The number of bursts may total about 20. No sounds are likely to be heard as the acoustic waves are generated at sub-audible frequencies and are very weak in intensity on reaching the ground.

In some rocket firings the grenade ejections are extended above 100 km altitude and the gas products of the explosion then react chemically with the atmosphere, producing a faintly luminous spherical cloud about the size of the full moon although much fainter. Such clouds may last for 5 to 10 minutes before diffusing and disappearing, and during this time the wind speed and direction can be found from their drift. On other occasions a faintly luminous trail may be produced for the same purpose by the release of a certain chemical from the rocket. The trail, like the grenade clouds, is soft white in colour. Such experiments are held under twilight conditions with sunlight falling on the releases against a still dark sky. The sunlight is reradiated in certain parts of the spectrum from the cloud or trail providing a spectacular blue- green hue, or the well known yellow coloration of sodium light, if sodium vapour has been released.

The Latin word 'aurora' means dawn, and equatorwards of the polar auroral zones a glow like a false dawn may sometimes be seen along the poleward horizon by a casual observer during the course of a dark night. 'Aurora borealis' is the name given to the phenomenon in the Northern Hemisphere and 'aurora australis', first recorded by Captain Cook in 1773, to that in the Southern Hemisphere. During a big auroral storm the glow may develop into one or more homogeneous arcs from which searchlight- like beams called rays may be emitted. The arcs may twist into rayed bands, the rays extending upwards to the zenith. At the peak of the display auroral forms may actively change location and shape while the light emitted from them may pulsate or appear like flames or flickering. As the storm subsides the rays may be superseded by diffuse patches of light without sharp borders. The whole storm may die away but there is the possibility that it will repeat itself later in the night. (Figure 25).

The lower border of the aurora is usually situated at an altitude of between 90 and 120 km above the surface of the Earth. The vertical extent of the aurora varies from between a few kilometres to about 1000 km. Height is reduced with increase in brightness and there is a diurnal tidal effect. The base of the auroral arc is at about 105 km and a red tinge thereon might be as low as 90 km, or even 65 km. Red aurorae are associated with altitudes of several hundred kilometres.

The aurora is caused by the bombardment of the upper atmosphere by electrified particles either coming directly from the Sun or due to the release of particles trapped in space adjacent to the Earth by the Earth's magnetic field. The particles are guided into the atmosphere by the magnetic field lines and accelerated by the associated electric fields. The impact of particles and their reaction with the atoms and molecules of oxygen and nitrogen convert particle energy into light energy, each reaction with its own specific colour, rather like the activity present that makes a neon tube glow.

The Polar Aurora and Storm Aurora. Around each of the Earth's magnetic poles lies an oval of auroral activity. The dayside of each oval is at about 15º of latitude from the magnetic pole and the nightside is at about 20º. The auroral zone on the Earth's surface is the locus of the ground swept by the nightside of the oval as the Earth turns beneath it. The dayside of the aurora can only be observed by ships and landsmen under the polar winter sunless sky. Certain interactions between the interplanetary and the Earth's magnetic fields can produce a westward surge in the nightside auroral oval activity followed by a retreat polewards. This is known as an auroral substorm. Other magnetic and particle reactions between interplanetary space and the Earth produce a full-blown magnetic storm when the auroral oval expands equatorwards into mid latitudes, and in extreme cases produces visible aurora in the Tropics such as on March 13 1989, when a number of ships observed activity in the Caribbean Sea, and in the Indian Ocean from the Cape of Good Hope to Madagascar.

Observation of the aurora need not involve any particularly specialized equipment. The position of auroral forms is best described in terms of true azimuth and to angular altitudes above the horizon, rather than relative to the positions of visible stars. To this end a sextant is helpful with angles measured to the nearest degree, especially when measuring the altitude of the lowest edge of any arc present. For observers without instruments to hand an outstretched handspan from thumb to little finger approximates to 20º and the knuckles, like the handspan held at arm's length, approximate to 2º.

It is particularly important to record the UTC date and time, and the ship's position when an aurora is seen. The simplest record of an observation is to make a written description of what is seen, including forms, their location, colours and changes with time. The notes may be enhanced by making a series of sketches to scale, adding notes on colour, azimuth and altitude, brightness, obstructing cloud or other phenomena. Artistic skill is not called for but many mariners succeed in producing praiseworthy efforts. Some observers draw a circle to represent the horizon with the centre as the astronomical zenith and make a drawing equivalent to photography made with a fisheye lens. Note that visual observers tend to overestimate the brightness of auroral forms.

A more detailed record of an active storm is effected by making use of the International Aurora Code and the standard form used for this purpose. Copies of the Code and Report Form are given in Appendix II. An observer would make an entry on the form at about once every 5 minutes or at every change in the appearance and activity of the aurora. There is space on the form for sketches to amplify the report, for an active display may be such that the rate of change may be faster than the observer can fill in the form and he has to make judgements as to the time and nature of the major changes in the apparition. Details of the sky condition and interference to visibility due to cloud, haze, moonlight, precipitation and the like should be noted. Any unusual radar images, distortion or loss of radio signals on HF or long distance VHF reception are worth recording, with the bearing of the incoming signal, when possible.

The observer should note particularly if rays on the horizon appear to fan out like spokes of a wheel from a hub apparently below the horizon or if they stand vertical or appear to converge towards a hub point above the horizon. Note particularly the times when aurora is overhead, either as bands, corona1 converging rays or spirals. Be aware that during a strong auroral storm, in higher latitudes such as the Norwegian Sea, the auroral oval may have swept equatorwards to the extent that the aurora is on the equatorward and not the poleward side of the ship, an all round check of the sky being necessary.

Unless anchored or moored alongside, in calm waters a ship does not present a very stable base for time exposure photography, but this has been successfully carried out, both from ships at sea and aircraft overflying Canada. Any camera with a lens ratio of f2.8 or faster can be used. Film may be colour or black-and- white but some makes have a better colour balance and grain structure than others. Film speeds of IS0 200 to 400 are normal but high speeds of 1000 have been used. Speeds of ISO 100 having suitable properties for aurora work are available.

Depending upon the brightness of the aurora, exposure times of between 5 and 30 seconds may be required. Longer exposures may suffer from fogging by nearby urban lighting or moonlight and star images will start to trail across the photograph, but 60-second exposures have been used on faint quiet aurora with slow film. If unsure of the exposure required it is best to take a number of photographs in quick succession using progressively longer exposures, say from 10 to 30 seconds. Most beginners tend to overexpose, especially with faint aurorae, and background light tends to fog the picture. Colour photography is useful in detecting the various hues and their locations that might not be visible to the naked eye, such as the red lower fringe of a strong rayed arc.

Observers on board ships of the UK Voluntary Observing Fleet may enter the details of auroral sightings in the meteorological logbook making use of the recording methods described above. On receipt of the logs at the Met. Office the information is extracted and copies used by aurora researchers. Copies of the log entries are also sent to the University of Aberdeen for storage with the existing Balfour Stewart Auroral Laboratory and other national auroral records.

METEORS AND FIREBALLS
During the night watches the seaman has many opportunities for obtaining useful observations of meteors. The general term used to refer to all the small bodies orbiting the Sun between the planets is 'meteoroid', and from a point on the surface of the Earth their presence is recorded only when they approach too closely to the Earth and enter the upper regions of the atmosphere, when they become incandescent due to the heat generated by friction between the body and the molecules (and atoms) of the atmosphere. They are then referred to as 'meteors' and appear as luminous streaks in the sky, popularly known as 'shooting stars'. They have of course no connection with any star, being usually small fragments varying in size from a grain of sand up to a pea. It is estimated that many thousands of millions of these objects enter the atmosphere daily, but all except a minute fraction of them are much too faint to be seen by the naked eye. The vast majority of all meteors are entirely disintegrated and subsequently settle down slowly to the Earth's surface in the form of extremely fine dust. In general the brightness of a meteor depends on two factors, the size of the object and the speed at which it enters the atmosphere. The greater the speed the brighter the object.

Larger objects generate much more light and if the intensity of this light rivals or surpasses the brightness of the planets, and in particular the planet Venus, they are referred to as 'fireballs'. This is an unfortunate term as it is also sometimes used to describe ball lightning. If the light rivals that of the full moon or brighter they are often called 'major fireballs'. Some rival the Sun in brilliance but these are relatively rare. The majority of the objects producing fireballs are completely disintegrated high up in the upper regions of the atmosphere but some survive the atmospheric passage and reach the Earth's surface. These objects are called 'meteorites'. Meteorite falls are relatively rare but observational reports of such an event are of great importance.

Appearance and speed of meteors. The appearance and speed of meteors and fireballs is as varied as their brightness. Some travel quickly, others slowly. The apparent path across the sky is usually a straight line or arc, but it may take other forms. Some leave streaks of sparks or luminous vapour known as trains and also dust trails. In many cases the trail disappears immediately but in others it can remain visible for seconds or minutes, or in rare cases for periods up to two hours. In such cases, changes may be observed in it, these changes being caused by winds in the upper atmosphere, combined with the fall of the material due to gravity. Most bright meteors and fireballs are brightly coloured, with red and green being the most common colours, but often the distribution of the colours can change during the flight. Sometimes the fireball appears to break up with the detached portions then proceeding separately, or it may explode at the end of its visible track. The trains are often reddish, white or golden, reflecting the coloration of the object itself. A common reported event is the appearance of a red fireball travelling slowly across the sky and hitting the surface of the Earth. This is an optical illusion. The eye cannot by itself estimate accurately distance, and range is recorded by the brain in terms of the brightness of the object. The brighter the object, the nearer it appears to be. There is no foundation to this. As an example, the planet Venus, when low in the sky near the horizon, is often mistaken for an atmospheric object, simply because it can be very bright. In the case of the red fireball it is simply that it is passing over the horizon but still many tens of kilometres up in the atmosphere. As a general rule for objects of average size which do produce meteorites, they will have slowed up sufficiently during their atmospheric passage to have cooled down and therefore become invisible before they reach the surface.

The duration of a meteor's flight is rarely more than 3 seconds, and is often greatly overestimated. The theoretical range of speeds for objects entering the atmosphere is from 11-70 km per second but the majority lie within the approximate range of 15-50 km per second. The average height above the Earth's surface is 120-130 km at the time of appearance, to 70-80 km at the time of disappearance, but some bright fireballs can penetrate much lower than this before burning out. The height of the beginning and end of a meteor's visible path in the atmosphere and its speed are determined by observations made by two observers some distance apart, up to 100 n.mile or more. At much greater distances the same meteor could not be seen by both observers, since an individual meteor is only visible over a small part of the Earth's surface and would thus be below the horizon of one of the observers. In this joint observation each observer notes the points of appearance and disappearance of the meteor in the sky as accurately as possible, and the duration of its flight. The information derived from such observations is valuable, not only in extending our knowledge of meteors, but also in making inferences about the temperature of the attenuated atmosphere at very great heights above the Earth's surface.

Frequency of meteors. Meteor showers. The number of meteors seen in a given time is usually greater on nights of higher atmospheric transparency, since more of the fainter meteors are seen and these are much more numerous than the brighter ones. On nights of equal clarity, about twice as many are visible in July to December as in January to June. Furthermore, on any single night of the year, the hourly rate of meteor appearance is greater after local midnight than before it. These remarks refer to average conditions and the numbers actually seen can vary considerably. On certain nights the number of meteors seen is far more numerous than expected and a high proportion of the tracks when produced backwards seem to converge from the same point or small area in the sky. This point is called a radiant and such a group of meteors with the same radiant belong to the same meteor shower. Each of these showers is given a name, with the letters – id(s) added to the name of the constellation in which the radiant is situated. For example, the active meteor shower seen in August and having a radiant in the constellation Perseus is referred to as the Perseid shower, or just the Perseids. Many of these showers are active each year but not necessarily with the same intensity. Some are highly active at intervals of a few years. For example the Leonids in November give a display of many thousand meteors per hour at roughly 33-year intervals. The particles producing a meteor shower were moving in the same general orbit round the Sun and the meteors are seen at times when the Earth is at, or near to, the point of intersection of this orbit with the Earth's orbit round the Sun. In a few cases the orbit of the shower has been found to be the same as that of a known comet, of whose material the meteors originally formed a part. For example, the Orionids, seen in October, are associated with Comet Halley.

Meteorites
Very bright fireballs with intensities rivalling that of the full moon, consist mainly of two types. The most common, comprising a high percentage of the fireballs seen, consist of possibly a loose agglomeration of small particles and in most cases they are completely burned up during the passage through the atmosphere. A small percentage, however, comprise a solid lump of rock, and although a high percentage of this rock will be ablated during its passage through the atmosphere, the remainder could reach the Earth's surface and are called meteorites.

If the meteorite is large enough it could form a crater, the form of which being dependent on mass and the velocity of impact. For small meteorites the last part of the track through the atmosphere cannot be seen because it has lost most of its cosmic velocity and hence cooled down, losing its glow in the process. It is therefore exceptionally rare for a meteorite to be seen to land. If the splash and related phenomena are seen at sea, full details should be recorded. Before the object reaches the Earth's surface, it will be travelling supersonically and the sonic boom will be heard over a wide area. The recording of a sonic boom is a good indication of the fall of a meteorite, especially if a brilliant fireball is observed. Logging of such events should be made in the greatest possible detail.

Because of the suddenness of the occurrence of such events, it is usually difficult to record a full account, the following notes being given as guidance to the kind of useful information desired by those analysing this type of phenomenon.

3. Details of the physical appearance, brightness, colour, changes in brightness and colour, fragmentation (if it occurs), existence of a train and/or a dust trail.

4. Any interesting or unusual phenomena seen during the passage across the sky. Of importance, the recording of a sonic boom, which may be heard as much as two minutes after the visible passage, is of exceptional interest.

It must be mentioned that although the observer may be at sea, the track of the fireball may pass over land and so the marine observations could provide details of a section of the flight poorly recorded from land positions.

These beautiful ice-crystal 'clouds' occur under the mesopause at a height of about 83 km where the barometric pressure is only one hundred thousandth of the sea level value, and the temperatures are the lowest in the atmosphere, some -130º to -160 ºC. Under these conditions ice crystals grow from the very small amounts of water vapour present, to form thin tenuous clouds which have a superficial resemblance to cirrus or cirrocumulus.

Noctilucent clouds (NLC) are normally seen in summer in fairly high latitudes, and when the sun is between about 6º and 16º below the horizon. They cannot be seen in daylight but appear when the sky is sufficiently dark, in the twilight arch, shining brightly with a pearly-white or electric-blue radiance, sometimes golden or greenish when near the horizon. However, when NLC is overhead it is very faint and difficult to distinguish from cirrus lit by the twilight glow or the moon. When the sun is sufficiently far below the horizon, the much lower clouds of the troposphere are no longer illuminated and appear as black silhouettes, but the NLC shines out in the darkening sky. In the northern hemisphere NLC are usually seen between about the end of May to about mid-August, with a peak around the first week of July. They are seldom seen below latitude 50° and in Arctic regions their visibility is limited by the brightness of the midnight sky near the solstice. In the Southern Hemisphere there are few observers at such latitudes, but NLC have been observed from the Falkland Islands and the Antarctic, and more observations are needed from this region.

Interest in noctilucent clouds is growing. Studies of them are helping our understanding of the movements and physical processes of the upper atmosphere, and there is some evidence that they are on the increase. Data on NLC are much needed by upper-air physicists, and it is desirable to have as many observers as possible, spread over a wide area, for a greater chance of NLC occurrences to be seen and logged. It is important to note not only 'positive' but also 'negative' nights, i.e. those nights in summer above about latitude 54º with a clear or nearly clear sky during which no NLC is seen over the observing period; it must be remembered that NLC which is only just visible to the naked eye is easily visible with binoculars. Unlike aurorae, NLC change in shape very slowly during a night, and observations should if possible be made at 15-minute intervals, i.e. on the hour and the quarter-hours and so on. Sketches can be very useful, photographs more so; but on a moving ship the requisite long exposures make this impracticable. For an observer on a stable platform a 200 IS0 colour film, and exposure of 5 to 8 seconds at f2.8 should give good results. A note should be made of the time any photograph is taken, and a full report should include:

The simultaneous occurrence of NLC and aurora is very unusual and is of great theoretical interest because the connection between the two phenomena is not fully understood. Observers should describe changes in both phenomena in detail,

e.g. appearance and disappearance times, sudden brightenings brightenings and other obvious features.