Changes in the fading speed of low-frequency radio waves

l~esearoh notes

qui est k la h a u t e u r du m a x i m u m d'ionisation. I1 suffira de reporter cette h a u t e u r sur la verticalef~ du graphique et d'extrapoler la courbe j u s q u ' k ce point en la terminant tangentiellement £ la vertieale. Bien entendu les fr~quences fl et f2 d e v r o n t ~tre choisies assez proches de fro. Nous avons m a i n t e n a n t £ justifier cette construction; de (1) nous pouvons ~crire: ( h ~ - - hl~ ~

(2)

1 - K~ = 1`h~-:--4o-4! 1 - - K , 9":

(3)

1`h~-h0]

et enfin: h m -- h a = (1 - - K12~ 1/2_ hm-h~

1 1 - - K 2 ~]

(4)

--a.

I)'aprbs la Fig. 2, nous aurons, le point 0 6tant suppos6 ~ la h a u t e u r hm: h m - - h x ~- A P

et

(5)

h~ - - h 2 = B Q

et par similitude des triangles, ainsi que d'apr~s (4): OA

AP

hm -- hi

OB

BQ

hm -- he

- - a,

(6)

Or, par suite de l'6chelle logarithmique des frgquences: OA OB

log f= - - log f l ----a logf~ -- log f2

(7)

et nous pouvons ~crite:

(s)

1`f~!

d'ofi: f2a l/(a-1)

K2afma l/(a--1)

/K2a\ll(a-1)

Centre d' t~tudes Ggophysiques Garchy, Ni~vre, France

DOVBT.ET J. KET.SO J . H . KELSO J . H . SCHMERLING E . R .

(9) A.

R~F~RE~CES 1951 1952 1954 1958

HAUBERT

Note pr~liminaire du L.N.R. No. 149. J. Gdophys. Res. 57, 357. J. Atmosph. Terr. Phys. 5, 11. J. Atmosph. Terr. Phys. 12, 8.

Changes in the fading speed of low-frequency radio waves (Received 25 April 1960)

THIS paper is concerned with the speed at which low-frequency radio waves fade. W h e n a fading curve appears to be made up of two components with v e r y different correlation times and when different radio frequencies are to be compared, it is n o t desirable to estimate 339

Research notes

fading speed by counting the number of maxinla on the fading curve. It, however, it is desired to estimate the way in which fading speed varies with time on one frequency, the counting of maxima gives a simple and satisfactory measure of the speed of the fastest component. Records made on frequencies of 70.8, 113.3 and 200 kc/s were investigated in this way. Examination of fading records on a single frequency shows that the fading speed, as measured by counting maxima, changes over quite a wide range and in general the fading can be described as either slow, medium or fast. I t will here be suggested that the fast

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1957.

fading is associated with the incidence of meteors on the ionosphere. Although it was noticed on many occasions that unusually high magnetic activity was associated with high fading speed, no clear correlation between the two variables was found. We shall consider fading records of the wave reflected at steep incidence from the ionosphere covering 24 hr periods in winter. In summer the daytime strength of the reflected wave was too weak to be recorded. The fading speed was measured in terms of number of maxima per hour and the monthly average magnitude was determined for each hour of the twenty-four and denoted by S,,,. The fading speed S m was compared with the monthly average value (Mm) of the number of meteors (sporadic and shower) which left ionized trails in the atmosphere sufficiently dense to reflect radio waves with a given intensity on a given wavelength. These numbers were kindly provided by Professor A. C. B. LOVELLat Jodretl Bank. In Fig. 1, S~ for each hour of the day is given for the frequencies 70, 113 and 200 kc/s for the month of December 1957. Each point on these curves is the average of between twenty-five and thirty individual values. The shapes of the curves for the different frequencies are very similar, and it is reasonable to suppose that fading on these frequencies had a common origin. Since the speed of the slow component of fading is found to be roughly constant at about eight maxima per hour, these curves are known to represent the changes in the fast component. Fig. 1 shows that the fading speed is greatest round about 0(D0 GMT and is least at about 1800 GMT. The curve for M m against time of day for December 1957 is given in Fig. 2 together with the curve of S,~ for 70 kc/s for the same month transferred from Fig: 1. 340

Research

notes

These curves are remarkably similar and the correlation coefficient was found to be 0-85. Similar curves were evaluated for November 1957 and J a n u a r y 1958 and these confirm the high correlation found for the curves in Fig. 2. I t is known that the number of meteor trails formed decreases quite sharply with height. I n the light of the correlation mentioned above, the height of reflection of a particular frequency will then be an important factor determining the fading speed on this

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Fig. 2. l~[ean fading speed on 70 ke/s (taken from Fig. 1) compared with mean number of meteors observed by radio methods, for December 1957. O- - -O Fading speed (70 kc/s); O~ C) Numbers of meteors. frequency. Fading speed will change with frequency as a result of the change of wavelength and a]so because of the change in height of reflection. I t is thus not expected to find a linear relationship between fading speed and frequency over a range of frequencies. I t is observed that on frequencies less than about 70 kc/s, the fast component of fading is completely absent for most of the time. One can then, taking the above-mentioned experimental facts into account, perhaps make the statement that at heights where fiequencies lower than 70 kc/s are reflected either no (or very few) meteor trails are formed. I t m a y also be that the irregularities present at these heights, are of dimensions smaller than a wavelength of the radio frequencies concerned.

Acknowledgements--The author is indebted to Mr. J. A. RATCLIFFEfor help and suggestions in connection with this paper. The work was made possible by a grant from the South African Council for Scientific and Industrial Research.

Cavendish Laboratory Cambridge

W . A . C~,LIERS

An auroral emission in the 2.0-2-2 ~ region* (Received 27 April 1960) I~-R~D emission near 1.5 # has been observed by HARRISOI~ and V ALT,A~CE J o ~ s (1959) in very bright active aurora. Two features at 1.46 # and 1.52 ~ were definitely * Supported by the Geophysics Research Directorate of the Air Force Cambridge Research Center, Air Research and Development Command, under Contract No. AF 19(694)-1831. 341