книги из ГПНТБ / Pushkov N.V. Quiet Sun

measured the electron density at 5,500 km above the earth’s surface.

Our natural satellite the moon is also used in studies of the ionosphere. The first experiments were conducted in 1957 by the radio observatory at Jodrell Bank in England. Radio waves reflect­ ed from the moon told us a lot about the proper­ ties of the ionosphere that they traversed on the way out and back. Similar investigations were conducted in the United States in 1959 and 1960. Radio reflections from the moon were obtained on radar frequencies of about 150 and 400 MHz.

These measurements were found to agree with the material obtained in radio observations of the first satellite. The electron concentration in the outer ionosphere proved to be about three times that in the lower ionosphere. During the IQSY lunar radar studies were carried out systemati­ cally from moonrise to moonset.

Sounding from above, from a satellite, opened up great opportunities for investigating the upper part of the ionosphere. This was done for the first time on a joint Canadian-American satellite, Alouette, launched into orbit at the end of Sep­ tember 1962. The ionospheric station of the sat­ ellite with a frequency varying from 0.5 to 12 MHz was manufactured in Canada, while the launching rocket was made in the United States.

Alouette is in a nearly circular orbit 1,000 km from the earth, the orbit inclined 80° to the equa­ tor. Data gathered by this space vehicle are re­ ceived by 14 ground stations, and have so far supplied great quantities of information about the distribution and behaviour of the electron density above the principal maximum.

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struments carried on satellites and rockets. Now chemical robots like magnetic and radio-fre­ quency mass spectrometers do the job. They oper­ ate from 100 km upwards. A mass spectrometer can sort out charged particles according to atomic mass units and determine their quantitative ra­ tios. It can also determine the composition of neutral particles at altitudes of about 200 km, and charged particles at about 1,000 km and even higher.

Numerous analyses of the gaseous composition of the atmosphere demonstrate that up to ap­ proximately 90-100 km the atmosphere may be considered homogeneous, since, with the excep­ tion of such slight components as water vapour and ozone, its composition is practically the same. The explanation is that up to these heights the atmosphere gets thoroughly mixed.

Upwards of roughly 100 km the composition of the atmosphere begins to change perceptibly. Un­ der the impact of the sun’s ultraviolet radiation, a dissociation process sets in: first oxygen molecules break up into separate atoms and then, at greater altitudes, nitrogen molecules. As a result, we be­ gin to encounter atomic oxygen and nitrogen in increasing quantities along with molecular oxy­ gen and nitrogen.

As has already been mentioned, there is also a parallel process of ionization of atoms and mol­ ecules of the air. Neutral and charged atoms and molecules of the air combine chemically to form new chemical compounds. In the E layer of the ionosphere, mass spectrometers noted both posi­ tive ions of molecular and atomic oxygen and large quantities of positive ions of nitrogen oxide.

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Above 100 km the density of the atmosphere falls off appreciably, and so collisions between particles are fewer, and the free-path lengths of the particles increase. Diffusion and gravitational forces become increasingly important in the dis­ tribution of gases in altitude: the lighter gases strive upwards, the heavier ones move down­ wards.

American measurements in 1963 confirmed the findings obtained by the Soviet Sputnik 3 that at less than 250 km (at the surface as well) the principal role in the composition of the atmo­ sphere is played by molecular nitrogen; above 600 km, the light gas helium is dominant, and in the intermediate region, it is atomic oxygen.

An extensive programme of mass-spectrometric investigations was carried out during the IQSY by the Soviet satellites Electron 1 and Electron 2 and the American space vehicle OGO 1 (Orbit­ ing Geophysical Observatory). The Soviet Elec­ tron satellites recorded positive ions between 400 and 3,000 km.

Curiously, no molecular ions of nitrogen, nitro­ gen oxide or oxygen were detected, although Sputnik 3 registered them. This indicates that at 400 to 500 km the number of molecules of these gases diminished when passing from maximum to minimum solar activity. As usual, it is the sun with its whimsicalities.

Positive ions of hydrogen were registered at all altitudes, and at 500 to 700 km up the number of positive hydrogen ions was equal to the num­ ber of positive ions of atomic oxygen. Helium ions at any of these altitudes made up only a small portion of the total ionic population

20ft

W h e n c e r e s e a r c h e r s c o n c l u d e d t h a t d u r i n g m i n i ­

t h a t t h e t h i c k n e s s o f t h e r e g i o n o f p r e d o m i n a n t

a b l e a t l o w t e m p e r a t u r e s ( d u r i n g s o l a r m i n i m u m ) .

T h e t y p e o f o r b i t s o f t h e s a t e l l i t e s a n d t h e

p r o b e s a b o a r d t h e S o v i e t C o s m o s v e h i c l e s a n d t h e A m e r i c a n E x p l o r e r s s h o w e d t h a t b e s i d e s c h a n g e s f r o m d a y t o n i g h t a n d t h r o u g h t h e s e a s o n s o f t h e y e a r , t h e t e m p e r a t u r e o f t h e u p p e r a t m o s p h e r e i s v e r y s e n s i t i v e t o c h a n g e s i n t h e a c t i v i t y o f t h e s u n . L i k e w i s e , i t v a r i e s w i t h t h e l a t i t u d e , i n c r e a s ­

i n g f r o m e q u a t o r t o t h e p o l e s .

Above 200 km, the mean monthly night tem­ perature during maximum solar activity reaches 1,500°K (Kelvin), during minimum, only 600°K. In the daytime the temperature of the neutral at­ mosphere is 30 per cent above that at night. Ap­ proximately 300 km from the earth the electron temperature in the daytime is roughly twice that of positive ions. Which means that the atmo­ sphere here is no longer a uniform medium: dif­ ferent particles move almost independently of one another.

w h a t w a v e l e n g t h t o u s e f o r c o m m u n i c a t i o n b e ­ t w e e n s p e c i f i c p l a c e s a n d w h a t c h a n g e s t o m a k e

20R

s u n ’s b e h a v i o u r , i t i s p o s s i b l e , b y m o r e o r l e s s

r e l i a b l y p r e d i c t i n g e v e n t s o n t h e s u n , t o m a k e u p a s c h e d u l e o f r a d i o c o m m u n i c a t i o n s f o r a w h o l e

s p h e r i c d a t a w i l l h e l p i n t h e d e s i g n i n g o f t r a n s ­

s e a r c h h a s s a v e d t h e w o r l d h u n d r e d s o f m i l l i o n s

e a s i l y m e a s u r e d i n m o n e y . B u t o n e t h i n g i s c e r ­ t a i n , a n d t h a t i s t h a t t h e e x t e n s i v e a n d s y s t e m a t i c

y i e l d e n o r m o u s d i v i d e n d s t o p u r e a n d a p p l i e d s c i ­

m a n y s i d e s o f h u m a n a c t i v i t y . R a d i o c o m m u n i c a ­

t i o n a n d b r o a d c a s t i n g , r a d i o n a v i g a t i o n o n l a n d ,

a t s e a , i n t h e a i r a n d i n c o s m i c s p a c e a r e i m p o s ­

s i b l e w i t h o u t a d e e p k n o w l e d g e o f t h e b e h a v i o u r o f t h i s t e m p e r a m e n t a l l a y e r o f t h e e a r t h ’ s a t m o ­

s p h e r e : t h i s i s c o l d p l a s m a i n t h e m o s t c o m m o n

w h i c h t h e i r s u r f a c e s w i l l i n t e r a c t .

G e o p h y s i c i s t s a r e n o w b u s y p r o c e s s i n g t h e e x ­

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tensive materials and findings of the 1QSY ob­ tained by 270 ionospheric stations scattered over all continents. There are 25 Soviet stations among them, including such important ones as the drift­ ing “North Pole” stations, the Antarctic “Vostok” base and many others.

Scientists are focussing attention on the An­ tarctic because this was the first time that such ob­ servations were carried out there during minimum solar activity. No less important are the present observations in the outer ionosphere that are being conducted on an unprecedented scale.

Atmospherics—child of thunderstorms

You are sitting back listening to your favourite radio programme when all of a sudden a ter­ rible crackling noise spoils the whole show. What could it be? There is no industrial noise or other interference close by. The actual source of trou­ ble is not on any adjacent street but in distant Argentina or on the island of Java.

Every day there are some 16,000,000 thunder­ storms on this planet. Even if each one lasts only an hour, there will be about a thousand eight hundred storms raging at any one time; every second, lightning flashes at least a hundred times and there are just as many crashing claps of thunder.

Most thunderstorms break out in the equatorial belt, in Africa, South America, Indonesia. There are vast expanses here with over two hundred thunderstormy days a year. And though these vio­ lent events take place far away, their effects are felt round the world.

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The reason is that thunderstorms are accom­ panied by lightning discharges between clouds and between clouds and the earth. Every flash of lightning is a discharge of electric current of tre­ mendous intensity and enormous instantaneous power output.

Every lightning stroke is attended by the emis­ sion of radio waves of a great variety of lengths from the very short to the extremely long, long­ er than any broadcast by man-made radio sta­ tions. These waves spread out in all directions and create the characteristic clicks we hear in our radio sets. They are called atmospherics (gener­ ally, simply static).

The short waves produced by lightning are propagated in exactly the same way as those broadcast by short-wave radio stations: in jumps between the source (lightning) and the ionosphere, and between the ionosphere and the ground. In this process they traverse the lower absorbing layer of the ionosphere a number of times and thus decay very rapidly.

Unlike short waves, the long radio waves gen­ erated by lightning flashes are propagated in a spherical waveguide formed by the ionosphere and the earth’s surface. Here they travel like in a pipe, expending very little energy en route and decaying very slowly with distance. That is why the interference on long wavelengths is worse and active over much larger distances than on the short wavelengths.

Studies of atmospherics help us to develop noise-proof devices, to determine the intensity that has to be created on a given frequency so that the transmission (its useful signal) can wipe

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out the noise and intelligibly get the information to the receiver without obnoxious atmospherics jostling it too much.

The intensity of atmospheric interference at large distances from the sites of thunderstorms depends on the state of the ionosphere, which in turn is determined by solar activity. Reduced ionospheric absorption at night leads to enhanced intensity of atmospheric interference on the short waves. Solar flares are accompanied by increased atmospheric disturbance on the long wavelengths. No wonder then that astronomers studying solar flares utilize instruments for recording atmo­ spheric disturbances on the earth in addition to devices aimed at the sun.

Investigators studying atmospheric disturbances have discovered a hitherto unknown mode of ra­ dio-wave propagation. It turns out that part of the electromagnetic energy emitted by lightning seeps through the ionosphere and, following the force lines of the earth’s magnetic field from one hemisphere into the other, is disseminated into outer space. Here the energy may be reflected from the ionosphere or from the earth and return via the same route to the original hemisphere. There may be quite a number of these inter-hemi­ sphere trips along the force lines of the earth’s magnetic field.

An observer can sometimes note several atmo­ spherics due to one and the same lightning flash: one (the first) moving the shortest distance along the earth’s surface, and others that pass through cosmic space with a greater or smaller lag. The first atmospheric is of the nature of a crackle, the following ones are like whistles with diminish­

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