JOURNAL OF RESEARCH of the National Bureau of Sta nda rds-D. Radio Propaga tion

Vol. 64D, No. 4, July- August 1960

Natural Electromagnetic Energy Below the ELF Range 1

Wallace H. Campbell

(February 23, ]960)

The t ransit ion of natural signals from sferics slow ta ils to geomagnetic mi cropulsations was observed bet ween 2. 0 a nd 0.2 cy cles per seco nd . M icropulsations wi t h periods of 5 to 30 seconds have cha racteristics which closely relate to solar terresk ial disturba nce phenomena. The low latitude diurn al a mpli t ude va riat ion has max imums at 0945 a nd 1000 l.m.t . Sim ilar g roups of oscillat ions appear in Alas ka a nd Califo rnia. Simu ltaneous pulsation of A 3914 a urora and m agnetic fi eld micropulsations has bee n obser ved in Alas ka.

1. Introduction

The purpose of this paper is to review som e studies of th e characteristics of natural electromagnetic fields w hich are ell co untered a m C<Lsurem ents ftre made to frequen cies lower t ha n those domin ated by the sferi cs slow tails. In p:U"ticuLu', magnetic fi eld oscilla tio Il S with periods of 5 to 30 sec will be

. discussed. The greftter par t of this paper is ft digest of experimental res ults which h ave been previously published. New developments in th e r elation of m agnetic fi eld mi cropulsatio ns to t he pulsating a urora will be reported .

Only the ftut)lOr's work will be discussed in this paper . However , i t should be men tioned that recent comprehensive studi es of the shor t period pulsating m agneti c field )) ftve been mftcl e by Holmberg [1 ),2 Angenheister [2], Troyi9kaya [3], K ato and 'Watanabe

) [4], and Shand , Wrigh t, a nd Duffus [5].

2. Methods

For t he fi eld str ength observa tions in frequen cies below 20 cps the an tenn as were large loops, 2 m in diam eter with 21,586 turns [6]. The loop natural

~ r eson an ce was a t approximately 150 cps. In opera­tion the r ela tively fl at system response was down by 3 db at 0.4 cps and 20 cps for oscilloscope film r ecordings and at 0.04 cps and 0.4 cps for slow run E sterline Angus pen recorders. Wind velocity and WWV timing were also r ecorded on the film [6, 7] . The maximum noise level was equivalen t to 0.02

, gamma and decreased with in creasing frequency. Middle latitude observations were made at a southern

I California site (33°2l.5' N, 11 6°17 ' W) and high la titud e experim ents were carried out in Alaska (64°42' N, 148°29.5 ' vV).

~

( I Co ntribu t. ion from Geophys ical Institu te, Un iversity of Alaska; papel' pre-sent.ed at Conrerence on the Propagation of E LF' !lad io \Vavcs, Boulder, 0010., J an. 25. 1960.

, Figures in brackets indieate the literatu re references at t ile end of this paper .

3. Observations

In California the tran ition of the domin ate received natural signals from slow tail sferics to geomagnetic m ieropulsations was fo und to occur between 2.0 and 0.2 cps [7]. On a few rare occasions dispersive tmi n oscilbtions wi th periods of approxi­miLtely l. 5 cps a nd ampli tudes reachi ng 0.015 gamma were observed [6]. Sixty pOl'cent of the 4808 hours ampled between M arch and Sep tember 1958,

sbowed sinusoidal type pul ations appearing in groups or packets of fo ur or six 0 cillations with periods of 20 to 25 sec during the midd ay h ours fLncl falling to 5 to 8 sec by midnigh t [6]. TIle periods did not seem to be ampl itude dependent. I n so uthern Californ ia th e signals i ncreased in amplitude at dawn, rose to a maximum of about one-quarter gamma fL bou t 0945 to 1000 l.m.t., then decreased to a slight minimum after 1200 In-, showed a econdary m aximum at approxim ately 1400 hI', and faded away graduallyin the IfLte afternoon (fi g. 1). From comparison with data of resefLl"chers [2, 3] in other parts of the world i t is apparen t that the diurnal pattern of micropulsations is a local mean t ime variation [7].

>--~20 U1 z w o

~ .10 --' 'u.

00 04 08 12 16 20 24 1200 W. M.T.

F IG lJHE 1. A veral/e diurnal behavior of microJ)ulsations in California from Jl![al'ch through S eptember' 1958.

544:;2 ' GO--O 409

Average daily amplitudes behave in a manner similar to other phenomena in the solar-terrestrial disturbance family. Good correlation may be ob­tained with magnetic K and A indices and disturbed F-layer conditions [7]. The 27 day periods of solar activity stands out on the records.

Several occasions a month there appeared "storms" of sudden increases in pulsation amplitude reaching an average of 1.4 gamma and followed for several hours by higher activity than would have been ex­pected for the day [7]. The storm time variation showed a small secondary maximum at 65 min. The occurrence distribution of the 19 storms studied had maximums at 0700 and 1645 u .t. On many occa­sions the micropulsation storms were coincident with sudden commencement magnetic storms.

At 10:51:30 ± 30 u.t., August 1, 1958, a small group of micropulsations were initiated, lasting for approximately 4 min and attaining an amplitude of approximately 0.2 gamma. About 50 min later a second group of micropulsations commenced, ris­ing gradually to a maximum of 0.3 gamma in 23 min then rapidly disappearing. Since local night­time activity was rare in the low latitudes and pre­viously accompanied by high magnetic K indices, these oscillations were interpreted as a direct result of the high altitude (70 to 80 km [8]) nuclear explo­sions at Johnston Island. No micropulsations were observed on the August 12 Johnston Island explosion which was reported to occur at a lower elevation [8].

Twenty-three coincident active groups of micro­pulsations were observed in 6 days concurrent oper­ation of California and Alaska stations [9]. Al­though the oscillation periods were similar, peak for peak correspondence was not evident. The diurnal pattern of micropulsations in the auroral zone is dominated by a post midnight maximum typical of the aurora itself [6]. Large nighttime activity in Alaska gave oscillations 10 to 20 times larger than

California. Daytime activities were similar at the two stations. In Alaska there seems to be a close ' relationship between the pulsating aurora and mag­netic field micropulsations [10]. Using a lens, inter­ference filter, photomultiplier, and appropriate ampli­fiers, observations were made of pulsations of the ;\3914 and A5577 auroral lines. Figure 2 shows typ­ical records of corresponding magnetic field mic1"o­pulsations and pulsating aurora. Figure 3 illustrates the average diurnal behavior of magnetic field micro­pulsations and pulsating aurora for occasions of joint operation in the month of December 1959. Figure 4 is an illustration of oscillations of the two phenom­ena in the predawn hours with periods of approxi­mately 25 sec. The optical and magnetic measure­ments were made at sites separated by 30 miles.

4. Concluding Remarks

Magnetic field micropulsfttion, though certainly dependen t upon solar terrestrial phenomena for their excitation, show many local diurnal features. There are strong indications that the ionosphere is intimately involved in the generation mechanism. It appears that the pulsating aurora is closely related if not identical to the micropulsation phenomena.

The low-latitude work was carried out at the Institute of Geophysics of the Univers ity of California and the continuing program in Alaska at the Geo­physical Institute of the University- of Alaslm. The research has had the support of the Electronics Branch of the Office of Naval Research. The assist­ance of B. Nebel in California and of A. Belon, C. Deehr, M. Rees, and G. "Romick in Alaska is grate­fully acknowledged.

c w ~

L.i....l..--.L .L. I I II 1:7 F7.~. :'lc-l-T.~ '. '.c -, 7 7=7 "r~ Y-. 1 --:- ! / · .. :L.I .. 2· ....... :... .. ----'-'--'l_.. I'~'( j • i···· 1 j / .I I l ,: .j " . , I Li1 t '=k -/ -F:" " t - e' / ,' . i~-

/ ! I .' If ! Y =r . . ~ =t==E, "E ± c1:'~CT =f=::t ' i , ··L ; , : 'T o' !-fc i , .. ! ; I I I - /~ " I ."" :Or ~r~" " ' t.- t ,t'-=t ~ ~ , --jcc : -'!

I' I i .-J.. -~. f.;;i'_c·,LJ.~: ·--'-1-=.f ·.J. f:f.·/ ·J-J , '< :=c ; - - . -

FIGURE 2. Exarnple of magnetic field rnicl'opulsations and pUlsation amora data in Alaska.

410

" ~ 0.8

" " z - 0.6 r ... " z w a: 0.4 .... <1)

o ...J ~ 0.2 ...

I I I I

MIC ROPUL$ATIONS - I /4- 39[4 A AURORA

I ..6

p-/

"' o ::> t--

6 :::; 0."' ::;--' ,,<1

U "<fl a:>-

4 0 a: a: " ::>a: <1t--

"iii za: 2 f: <l

<1 <fl --' ::> 0.

O'~'tf':"'/L.J'''L.L----'---L---'-...L-'--'-~-'-......L-:' 0 o L.l----'----'--'--'-6'--'- 9 12 15 18 21 2 4 G.MT.

~ 13 16 19 22 4 10 L. M.T. TIME

FIG lIH E 3. Average diurnal behavior of magnetic field miao­pulsation and pulsating aurora jor the month oj December 1959, in Alaska.

• • II A.J II' JV

I

l

T • IV J

1 --I I--- I min

J -"-n

I1n III

1

J

[j

II

A 3914 AURORA PULSATIONS

MAGNETIC FIELD MI C ROPUL S ATIONS

FJGUHE 4. C01Tes ponding pulsations of the aurora and the north magnetic field at stations separated by 30 miles near College, Alaska.

411

5. References

[IJ E . R. R. H olmberg, R apid p eriodic flu ctuations in the geomagnetic fi eld, I , Mon t hly Notices R oy. Astron. Soc., Geophys . Supp!. 6, 467 (1953).

[2J G. Ange nh eister, R cgistrierun g erdmagnetiseher p ul­sationen Gottin gen, 1952/53, Gerla nds Beitr. Geop hys. 64, 108 (19.54) .

[3J V. A. TroyiQkaya, Eart h curren ts, P r iroda 5, 81 (1955). [4J Y. K ato and T . Watana be, A. survey of observational

knowledge of the geomagnet ic p 'ilsation, Sci. R ep t. Tohoku Imp. U ni v., scr 5, Geop hys. 8 (3), 157 (1957).

[5J J. A. Shand, C. S. Wrig ht, and TI . J . Duf-ru s, A study of t he dist ribution of geomag nel ic microp ulsaLions, Pacifi c Naval Lab. Rept. 15, D .R .B., Canada, pp. 1 to 29 (1959).

[6J W.II. Campbell , A sLudy of micropu lsaLions in t hc earth's magn etic fi eld, I nsL. Geo phys., Uiliv. Cal if. Los An­geles, Sci. Rept. 1, No nl'. 233 (47), 1 to 138 (1959).

[7] W. ll . Campbell , Stud ies of magnetic fi eld micropulsations with periods of 5 to 30 seconds, .r. Geophys. Resea rch 64, 1819 (1959) .

[8J S. :'I fatsu shi ta, On artific ial geomagnetic a nd ionospheric sLorms associated "'ith high-alti tude explosions, J. Geophys. ReseMch 64, 11 49 (1959) .

[9J W. 11. Campbell and B. Nebel, Mi cropu l alion meas­urement s in Cal ifol'll ia and Alaska, Xature 184 , 628 (1959) .

[LOJ l,y. II . Campbell , Magnetic microp ulsalions and the pu lsating aurora, J. Geophys. Researc h 65, 784 (1960).

(P aper 64D 4- 74)