1958, the physicist James Van Allen found himself thoroughly puzzled by
data from a newly launched satellite, Explorer l. It carried a Geiger
counter, an instrument to detect radiation in space, but its data could
only be received for a few minutes at a time when it was over a ground
station. The intermittent data raised more questions than it answered. It
showed plausible values of radiation intensity while the craft was low in
its orbit. But at high altitude, there were no counts at all! Moreover,
other data showed sharp transitions, with the count suddenly stopping and
then just as suddenly resuming.
concluded that there might be a problem with the satellite's equipment. It
relied on a battery for electric power, and the battery would shut down if
onboard temperatures were too high or too low. Still, he was prepared to
learn more. When Explorer 3 reached orbit, two months after its
predecessor, it carried an onboard temperature sensor. It also carried a
tape recorder that could hold radiation readings from an entire orbit. Van
Allen got the first such data two days after launch, and found his
data showed the count rising rapidly as the craft climbed upward. Then it
dropped to zero, as had happened with Explorer l. The count stayed at zero
until Explorer 3 returned to lower altitude, and then resumed. Yet through
it all, measured onboard temperatures remained at moderate levels,
assuring good battery operation. Hence the results could not have resulted
from extreme onboard temperatures.
Mystified, Van Allen talked it over with two of his colleagues. The three
men quickly realized they were seeing the consequence of a quirk in the
Geiger counter itself. It could not respond if the radiation was too
intense; it would shut down and refuse to give a reading. They concluded
that Explorer 1 was flying through a zone of very strong radiation that
surrounded the planet, trapped by the Earth's magnetic field.
Substantiated by data gathered by later Explorer spacecraft, this zone
quickly became known as the Van Allen belt.
NASA's Transition Region and
Coronal Explorer (TRACE) spacecraft recorded a bright
but extremely short-lived explosion in the atmosphere of the Sun.
The explosion, called a flare, was observed on May 31, 1998,
in extreme ultraviolet light using the telescope on board TRACE.
spacecraft soon made further discoveries. Explorer 4, which flew in July
1958, carried radiation shielding. This screened out some of the
radiation, to keep its Geiger counter from being swamped. Van Allen
studied its data, and concluded that the peak radiation intensity would
kill an astronaut following exposure of only a few days. Then in December,
the space probe Pioneer 3 soared to an altitude of 63,000 miles (191,389
kilometres) and showed that there was a second radiation belt, some 10,000
miles (16,093 kilometres) above the Earth's surface. The inner belt was at
2,000 miles (3,219 kilometres) from the Earth. Fortunately, it was high
enough to allow astronauts to fly safely in orbit while remaining well
below the dangerous altitudes.
The solar wind flowing from the
Sun, from IMAGE.
did the radiation come from? The obvious answer was the sun, but it was
not immediately clear how. The physicist Eugene Parker responded to Van
Allen's discoveries with calculations of his own. He studied the corona, a
thin atmosphere that extends outward from the sun. Astronomers had already
shown that it was very hot, having temperatures of over a million degrees
Celsius. Parker showed that these high temperatures would make the corona
expand outward, at a speed that would increase with distance from the sun.
He called this expanding corona the "solar wind," and proposed that it
delivered electrically charged particles that produced the Van Allen
the solar wind be observed? Three Soviet space probes of 1959, called Luna
1, 2, and 3, carried charged-particle detectors and indeed made suitable
observations. However, these spacecraft returned data for only a few days.
In 1962 an American mission to Venus, Mariner 2, crossed interplanetary
space en route to that planet and made far more extensive measurements.
The speed of the wind ranged from 350 to 800 kilometres per second. It
consisted largely of protons and electrons, with an average density of
about ten of these particles per cubic centimetre. This was tenuous
indeed; the density of the Earth's atmosphere was a million trillion times
greater. Yet the existence of the Van Allen belts showed that this
rarefied wind could produce dramatic effects, as it interacted with the
Earth's magnetic field.
Solar Wind Speed from South
Pole to North Pole:
The upper panel contains an X-ray image of the Sun obtained by the
Soft X-ray Telescope on the Japanese Yohkoh spacecraft; the lower panel
shows the solar wind speed and density observed by the Ulysses spacecraft from the
South Pole to the North Pole.
The latitudinal region indicated by the yellow bar in the lower panel
is the region previously explored by in-ecliptic spacecraft.
geographers mapping a newly discovered continent, scientists such as
Parker and Van Allen laid the groundwork for further discoveries by
mapping the shape of this field in space. They did this using
magnetometers, instruments that could measure the strength of the field.
From 1959 to 1963, the space probes and satellites that contributed to
this work included Explorers 6, l0, 12, and 14; Pioneer 5, which probed
interplanetary space; and the Interplanetary Monitoring Platform (IMP).
magnetometers of these craft showed that the Earth's magnetic field was
confined within a "magnetosphere." When spacecraft crossed its boundary,
the measured strength of the magnetism fell sharply and dropped to the far
weaker values that were observed in interplanetary space. The
magnetosphere proved to have the shape of a very long teardrop, with a "magnetotail"
that was several million miles in length. The solar wind formed this tail,
which pointed outward from the sun.
Beginning in 1962, space scientists placed increasing emphasis on studying
the sun itself from orbit, making observations at energetic wavelengths
that are absorbed in the Earth's atmosphere and cannot be seen from the
ground. The Orbiting Solar Observatory spacecraft, which first flew in
that year, was particularly significant. The work reached a high point in
l973 with the crewed Apollo Telescope Mount, a part of the Skylab space
station. Astronauts used its instruments to photograph the sun at x-ray
and ultraviolet wavelengths, and obtained new views of solar activity.
activity rose and fell on an eleven-year cycle, producing greater and
fewer sunspots and other features that amounted to storms on the sun's
surface. When the sun was active, the corona became brighter and emitted
more solar ultraviolet. This radiation was known to heat the Earth's upper
atmosphere, which grew warmer and expanded. Pushing itself outward, it
became denser at a given altitude. Satellites, orbiting just outside the
atmosphere, then experienced increased drag. Their orbits decayed, leading
to an untimely end as they fell back into the dense lower atmosphere and
burned up like meteors. This happened to Skylab itself in 1979, as it
plunged to its death before a rescue mission could boost it to a higher
orbit for safety.
This photo shows images from the
Far Ultraviolet Imager that were acquired by the
Berkeley Ground Station at the Space Sciences Laboratory, UC Berkeley,
or other tracking stations in Alaska and Japan, for the Imager for
Magnetopause-to-Aurora Global Exploration (IMAGE mission).
of the magnetosphere continued after 1980. There was particular interest
in the aurora, a brilliant display of lights in the sky that is often seen
at night in polar regions. As early as 1896, the Norwegian physicist Olaf
Birkeland had suggested that the aurora could result from electrically
charged particles shot outward by the sun and attracted by the Earth's
magnetic field. These particles proved to be electrons, which produced the
aurora by striking atoms of gas in the upper atmosphere and causing them
to fluoresce or glow.
the British physicist James Dungey launched a line of research that added
detail to these ideas. The new approach concluded that electrons for the
aurora are stored in the magnetotail, close to the Earth. These electrons
come from the solar wind, which continually adds energy to the magnetotail.
But his tail can only hold so much, and several times each day, it gets
rid of the excess. It does this by forming a "plasmoid," which amounts to
a large portion of the magnetotail that breaks loose and flies outward
with the solar wind.
plasmoid is an enormous blob of rarefied protons and electrons, many times
larger than the Earth. Before it forms, the near-Earth magnetotail
stretches as it swells with energy. As the plasmoid begins to break loose,
the near-Earth field collapses and dumps its electrons into the upper
atmosphere, to produce the aurora.
to this theory was whether plasmoids indeed could be observed. This
happened in l983. The satellite ISEE-3 detected them in space, at
distances from Earth of nearly a million miles.
spacecraft have made further observations. Between 1979 and 1988, the
weather satellite Nimbus 7 and the Solar Maximum Mission, a research
craft, made accurate measurements of solar brightness. They found changes
of about 0.2 percent from week to week, produced by dark sunspots and
small bright regions. Longer-term changes followed the eleven-year solar
cycle and came to 0.l percent. Neither of these variations was large
enough to have significant influence on the Earth's climate.
recent missions have included Ulysses, SOHO (Solar and Heliospheric
Observatory), IMAGE (Imager for Magnetopause-to-Aurora Global
Exploration), and TRACE (Transition Region and Coronal Explorer). Ulysses
swung around Jupiter in l992 and used that planet's powerful gravity to
enter an orbit that took it high over the solar system. This enabled the
spacecraft to study the solar wind above the sun's north and south poles.
It found this wind flowing at high speed, 750 kilometres per second, and
with reduced density. It also measured the sun's magnetic field near its
From SOHO, this photo shows a
collage of prominences, which are huge clouds of relatively
cool dense plasma suspended in the Sun's hot, thin corona. At times, they
escaping the Sun's atmosphere. The hottest areas appear almost white,
while the darker red areas indicate cooler temperatures. Going clockwise
from the upper left,
the images are from: 15 May 2001; 28 March 2000; 18 January 2000, and 2
launched in l995, has contributed to predictions of geomagnetic storms.
These are rare but powerful disturbances of the Earth's magnetic field,
produced by the sun, which at times have had enough energy to shut down
electric-power grids and leave people without electricity. Soho helped
provide warning of a particularly severe such storm in 2000. Its
instruments have taken photos of the sun in the ultraviolet that have
shown excellent detail.
the Sun, the corona is far hotter than the underlying solar surface.
Clearly, energy is being transferred from the Sun to heat the corona, but
the source of the energy has not been understood. The TRACE spacecraft,
which flew in 1998, has helped to show that the source lies in localized
disturbances within the Sun's magnetic field. The IMAGE satellite,
launched in 2000, complements TRACE by giving views of activity within the
Earth's magnetosphere. Among scientists, answers to one set of questions
often lead investigators to ask new ones. Researchers have been learning
more and more, but the SOHO, TRACE, and IMAGE spacecraft show that there
is a continuing demand for new and better observations. They show that
scientists still have far to go before they can claim complete