SCIENTIFIC ERICAN

I.ONe E \l IT Il OL \I,E " \I ES

H/-71 n n 7:),

E stablished 1845

SCIENTIFIC .AMERICAN"

,\ Iarch, 1959

Volume 200

Numhcr 3

Radiation Belts around the Earth Instrum ents borne aloft by art!ficial satellites a.nd lunar prob es indicate that o"r plan.et is encircled by two zo nes of high- energy pa.rticles, against which space travelers will h.ave to be shielded by bmee A. Vall Alle n

O far, the most int crcstillg and least expected result o f man's exploration of the immedia te vici nity of the earth is the di$COvcry that olir p lanet is ri nged by a region- to be eXllct, two reg iOll S-Qf high-energy radiation cJllcnding many thousands of miles into space. The discovery b of course troubling to astronau ts: soluchow the human body will h ave to be shielded from this radi ation, even all a r,lpid transit through the region. But geophysic ists, astrophysicists, solar RstronOn1l'rs and msmic-ray physicists arc enthralled by the fresh implications of these find ings. The conllguration of the region and the radiation it contains bcs pc:lk ,I major physical phenome non involving cosmic rays li nd solar corp uscles in tho vicini ty of tho c:.rth. T his enonnous reservoir of clHlfged particles plays :I still _une:tplained role liS middleman in the interaction of e:lrth and Slill which is reflected in magnetic storms, in the airglow ~n d in the beautiful displays of the ou rora. The story of the i n v~tigat ion goes ba",k to 1952 and 1958, before ony of liS could thin k realistien]]y IIho ut the use of e3rth satellites to elt p lore the environment of the earth. Part ies from Ollr bu.oratory ot the Stole U nivertily of Iowa spe nt the summers of tJIOse years llboard Coast CUll rd and naval vessels, cruising along a 1,500-mile line from the waten of 8:lffin B:.y, neac the magnet ic pole in the far J\orthw~tern co rner of C reenland. southward to ti,e North Atl:Lntie off the coast of Newfou nd laud. Along the way we la unched n series of rocket-

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ca rrying ballooos-"rockoons." (The balloon lifts a small rocket to an alli t\.de of 12 10 15 miles, whence the rocket carries a modest payload of instruments to a height of 60 to 70 miles.) Our objective was to develop a profile of the cosmic-roil' in tensities at high altitudes and latitudes, and th us to learn the nature of the low-energy cosmic rays which at lower altitudes and latitudes are deflected 1.0)' the earth's magnetiC fiekl or absorbed ill the atmosphere. 1I.·l o.~t of the read ings radiocd down from the rockets were in aerurd wi th plausible eltpectations. T ..... o rockoo ns scnt aloft in 1953. however, provided us with a puzitc. Lau nched nea r Newfoundl and by }.Iclvin Cottlieb and Leslie Mered it h, they e ll(."o unl ered a 7.ontl of radia tion beginning a t an alt it ude of 30 miles tha t was far stronge r th:on we had eltp'ccted. At first we were uneas), abou t the proper operation of Ou r instrumcnts. But critical exarnin:ltion of the d at'l convinced us that we had un(Iuestionably encountered something new in the upper a tmosphere. Signi llca nt ly thCl>C meas uremenll;; were made in the northern auroral 7.one. In this zone, which focms a ring some 23 degrees south of the north geomagnctic pole. the incidence of visible auroras reachel' its maxim um. Sillce rockets fin.-d oorth lind sou th of the :rune had revea\t.-d nothing unllsua\, we speculated that the strong radi ation played some p arI in th e aurora. Showers of particles from t.he sun, it was thought, come pl unging in to Ihe atmosphere along magnctie lines of

force and sct off these displays [see "Aurora and Airglow," by C. T. Elver and Franklin E . Ro.1ch ; SCtE!'."'lU·' C A.!.u:llIc . . r--. September, 1955). But the theory underlying this explanation did not eltplain satisfactori ly why the aurora and the high-intensity rad iation we had dctcd:ed should occur in the auroral zone a'id nol in the vicinity of the geomagnetic pole itse lf. Nor coukl it accoun t for the high energies raluired to cany the solar pnrticlcs through the atmosphl'rc 10 ~uch rdatively low altitudes. The myslc ry dcepened when we found in b ter studies th at the radi ation persists almost contin uously in the zone above 30 milc~, irrespective of visi ble aurora l displays and other known hi ghaltitude dis turb:lIlllS in 41

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