IGY IN RETROSPECT—ROBERTS Dil 
strength and flare occurrence have been found. Finely detailed map- 
ping of the solar magnetic fields showed such small intensities as 1 
gauss or less, with resolution in the order of 1 second of arc. 
The corona is of vast extent and influence. Observations of electro- 
magnetic waves of various frequencies from radio stars at times of 
near conjunction with the sun indicated a coronal structure alined 
with residual magnetic fields out to distances of 20 or more solar radii. 
It has no definite limits and may extend indefinitely outward beyond 
the outer planets. It is believed to reach 7,000,000° F. in some parts, 
and Sidney Chapman has speculated that much thermal energy is, 
in fact, transferred to the earth’s atmosphere through direct contact 
with the thin but hot gases of the corona, perhaps at 350,000° F. 
Radiation belts—Space, we have seen, is far from empty. Among 
its features are the Van Allen radiation belts, intrinsically remarka- 
ble as well as an outstanding IGY discovery. Based on almost fan- 
tastically sparse probing up to the time of this writing, these two 
belts were tentatively described as annular shrouds about the earth, 
shaped by the typical force lines of the terrestrial magnetic- 
dipole field, and having northern and southern terminal cusps or 
edges pointing inward toward the auroral zones. Totally unexpected 
radiation intensities blocked the initial rocket probes with impos- 
sible counting loads, until Van Allen, with brilliant insight, provided 
modified counters. The vast outer belt of relatively soft, low-energy 
particles is most strongly developed in the zone between 3 and 4 earth 
radii distant from the earth’s center, with maximum intensity perhaps 
in excess of 25,000 counts per second. It consists of charged particles— 
protons and electrons—captured by the geomagnetic field from clouds 
of plasma spewed forth from the sun. The inner belt, 2,200 to 5,500 
kilometers from the earth’s surface, consists of very high-energy par- 
ticles of an origin yet unknown but suspected to be the decay products 
of cosmic-ray collisions. The particle count is somewhat lower than 
that of the outer belt. It has been said that despite the vast extent 
and intensity of the Van Allen belts, which would require billions of 
X-ray machines to duplicate, the materials present would amount to 
perhaps one-fifteenth of 1 ounce of hydrogen! Not enough is known 
about the identity and energy of the particles to provide good estimates 
of the radiation intensities. 
The implications are tremendous. Notwithstanding our slender 
knowledge, it appears that the energy budget of the outer atmosphere 
and the theory of magnetic storms and aurorae will be dominated by 
these belts. Their presence means that many types of instruments 
must be shielded and that severe problems may confront human space 
travelers, although little is yet known of the biological effects. (It is 
because of this that one hears suggestions that future space voyagers 
