Eos, Vol. 7 3 , No.

7, February 18, 1992

Eos, T R A N S A C T I O N S , A M E R I C A N G E O P H Y S I C A L U N I O N

VOLUME 73, NUMBER 7

FEBRUARY 18, 1992

PAGES 81-88

The Impact of Solar Flares and proton events—especially those with > 1 0
MeV fluxes exceeding 100 pfu—are associ­

Magnetic Storms on Humans ated with flares, but only a small percentage
of all flares are associated with proton
events.
PAGES 81. 84-85 Energetic particles pose a special hazard
at low-Earth orbit and above, where they can
Jo Ann Joselyn penetrate barriers such as spacesuits and
aluminum and destroy cells and solid state
electronics. The penetration of high-energy
The Sun shines, and Earth and its inhab­ have been ionized. X-ray wavelengths partic­
particles into living cells, measured as radia­
itants benefit. But the Sun radiates more ularly contribute to ionization at altitudes
tion dose, leads to chromosome damage
than light, and these radiations are variable between 90 and 160 km. Radio waves can be and, potentially, cancer. Large doses can be
over time scales of seconds to days. The reflected from the ionosphere like light re­ fatal immediately. Factors in calculating the
consequences for people range from glori­ flects from a mirror. However, the efficiency hazard to human beings include the compo­
ous celestial displays—auroras—to subtle of reflection depends on the radio wave fre­ sition and thickness of any shielding mate­
but potentially damaging effects on the tech­ quency and the properties of the ionosphere rial and the elemental composition of the
nological systems that are increasingly im­ itself, which can change over a time scale of impacting particles, which are mainly pro­
portant for daily living. For example, electric minutes. Extremely high frequencies, such
power transmission systems and communi­ tons but with some higher-mass particles, as
as those used to communicate with satel­ well as their energies and numbers. Solar
cation links have proven vulnerable to solar lites, pass right through the ordinary iono­
phenomena. And outside of Earth's protec­ protons of energies between 10 and 100 MeV
sphere^ Radio waves from terrestrial sources are particularly hazardous. In October 1989,
tive atmosphere and magnetic shield, there with frequencies below about 30 MHz are
is a small but genuine risk of a solar ener­ the Sun produced enough energetic particles
directly affected by the ionosphere. During that had there been an astronaut on the
getic particle burst that would be lethal to
large flares, which can occur at the rate of Moon, wearing only a spacesuit and caught
satellite sensors and command and control
several per day during peak activity condi­ out in the brunt of the storm, death would
systems and astronauts.
tions, all high-frequency (HF) radio waves have been probable. Astronauts who might
It has been known since the time of Gali­ (3-30 MHz) reaching the daytime ionosphere have gained safety in a shelter beneath
leo that the Sun is neither featureless nor
are absorbed for the duration of the flare. moonsoil would have absorbed only slight
steady. Besides ordinary sunlight, there are
The flare can last minutes to hours. Such amounts of radiation—on the order of the
three classes of solar emanations that can
radio "blackouts" are significant because HF lifetime dose of an average, nonspacefaring
be directly associated with effects at Earth—
photon radiation from solar flares, solar en­ radio propagation permits communication citizen. The October 1989 event was so ex­
ergetic particles, and inhomogeneities in the over long distances, such as short-wave traordinary that it produced elevated dosime­
solar wind that drive magnetic storms. Be­ broadcasting by the Voice of America and ter readings—but less than the annual dose
low, the emanations and their effects are the BBC as well as by amateur radio enthusi­ limits set for the general public—onboard
described. asts. At the same time, the phase of very low supersonic transports flying at high altitudes
wave frequencies (VLF, 3-30 kHz) reflecting over the polar caps. At sea level, there was
Solar Flares from the bottomside of the ionosphere ad­ no noticeable radiation increase because
Sunspots are dark areas on the solar sur­ vances as the propagation paths shorten. Earth's atmosphere has the absorption equiv­
face that are transient concentrated magnetic alent of 10 m of water. However, over their
fields. Groups of sunspots, especially those Solar Energetic Particles lifetimes, satellites at high altitudes or in
with complex magnetic field configurations, Most of the time, satellite sensors at geo­ low-altitude polar orbits accumulate doses
are preferential sites for "flares." A flare is a synchronous altitudes (approximately 36,000 many times that of the lethal human limit
km above Earth's surface) that count protons with results ranging from damaged surface
A
spontaneous release of energy (up to 10 32
ergs) in time spans of seconds to hours. and helium nuclei with high energies (above materials to logic circuit upsets in computer
Flares are seen at ground-based observato­ approximately 5 MeV/nucleon) measure only memories and control mechanisms. Ener­
ries as bright areas on the Sun in optical "background," a threshold level of instru­ getic particle events like those of October
wavelengths and as bursts of noise at radio ment noise. However on occasion, signifi­ 1989 are relatively rare, but they are, as yet,
wavelengths. During solar flares, the flux of cant counts of energetic particles from the unpredictable. The most recent one of a
photons emitted at X-ray wavelengths (below Sun are detected, where "significant" is de­ comparable size occurred in 1972. The only
a nanometer) increases up to several hun­ fined as at least 10 particle flux units (pfu; strategies for mitigation are shielding, careful
dred times its usual level. Radiation at wave­ particles/cm^2-s-sr) with energies above 10 selection of designs and materials, and the
lengths shorter than 100 nm does not reach MeV. Proton energies can exceed 100 MeV. use of redundancy and self-checking in logic
the surface but is absorbed in Earth's upper The record of the number of significant sat­ systems.
atmosphere (above approximately 40 km), ellite proton events that have occurred each Energetic solar particles also influence
producing a layer where some of the atoms year since 1976 varies from one to 23, and terrestrial radio waves propagating through
the fluxes range from the event threshold, polar regions in a separate process than the
which is 10, to approximately 40,000 pfu. one caused by solar flare X-ray radiation,
Jo Ann Joselyn, NOAA Space Environment Labora­
These events can last from hours to more which affects the entire sunlit side of the
tory, 325 Broadway, Boulder, CO 80303 than a week but typically last 2-3 days. Most Earth. Although energetic particles are

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shielded from lower latitudes of Earth's am­ Geomagnetic storms are extraordinary at geostationary orbits—portions of the satel­
bient magnetic field, they gain access to the variations, albeit only a small percentage, in lite surface can charge up. Differential
ionosphere over the polar caps, where the the surface magnetic field. For example, charge exceeding 10,000 V has been mea­
magnetic field is nearly perpendicular to consider the compass—a rudimentary instru­ sured, and arcing can occur. The effects of
Earth's surface. Polar cap absorption (PCA) ment that reveals the direction of the Earth's arcing on the satellite are not predictable
events are troublesome to radio navigation magnetic field. Workers have learned to and can be damaging. Over time, the physi­
techniques making use of the nearly con­ make use of sophisticated compasses called cal properties of the surface materials can
stant height of reflection of very low fre­ magnetometers to assist with navigation and be altered.
quency waves to find the propagation time, geophysical exploration. To those workers, Another result of the energy deposited in
and hence the distance, to the beacon. Dur­ even moderate fluctuations in Earth's rela­ the upper atmosphere during geomagnetic
ing PCA events the height of reflection low­ tively steady field are a concern. storms are ionospheric storms. Like flares,
ers. Positioning errors on the order of kilom­ Further, rapidly fluctuating fields induce ionospheric storms affect radio communica­
eters are possible on transpolar paths if a currents in long "wires" (for example, power tion at all latitudes, but these storms last for
PCA event is unrecognized. lines, pipelines, cables, and even train hours to days and disturb frequencies from 3
tracks) that have led to equipment failures in kHz to 30 GHz. Patches of transient ioniza­
Magnetic Storms the past. A recent case is the loss of the tion occur as a strong function of latitude
power grid in Quebec, Canada, on March 13, and time of day. Some frequencies are ab­
Although flare radiation and solar ener­
1989; 6 million people were without com­ sorbed and others are reflected, leading to
getic particles have important and noticeable
mercial electric power for nine hours. The anomalous propagation paths and rapidly
effects, the most pervasive human effects can
geomagnetic storm that bears responsibility fluctuating signals. Long-range radars experi­
be attributed to magnetic storms, which are
for that outage produced total deviations in ence unusual signal retardation and refrac­
the response of the Earth's magnetic field to
compass heading of several degrees even at tion, causing distance and pointing errors.
specific inhomogeneities in the solar wind.
middle latitudes. Was the March 1989 storm Even satellite communication systems oper­
Stated simply, the solar wind is the expan­
singular in its intensity? It was the largest in ating through the disturbed ionosphere may
sion and escape of the outer solar atmo­
recent memory, but in the years since 1868, experience phase and amplitude scintilla­
sphere into interplanetary space. Inhomoge­
the first year of the longest series of geomag­ tions.
neities arise because the outer solar
Finally, there is a growing body of evi­
atmosphere—the corona—is structured by netic index records, comparable or larger
dence that changes in the geomagnetic field
the strong solar magnetic fields. Where the storms were observed on September 25,
affect biological systems. In particular, hom­
magnetic field is relatively weak or aligned 1909, September 18, 1941, and November 13,
ing pigeons and other migratory creatures
with the gravity field, the atmosphere can 1960.
appear to use the magnetic field as at least a
readily escape, leading to high-speed solar An associated consequence of Earth's backup navigational aid. Other studies indi­
wind streams. In other places, for example response to blasts of solar wind is the ener­ cate that physically stressed human biologi­
above sunspot groups, closed magnetic gization of a population of electrons and cal systems may respond to the minute but
fields impede or confine solar wind flow. ions resident in the magnetosphere. These measurable fluctuations of the geomagnetic
However, the confined atmosphere can be trapped particles, guided by the roughly di­ field. Interest and concern in this subject has
released in bubbles or tongues of plasma polar geomagnetic field, usually enter the led the Union of Radio Science International
and magnetic fields called coronal mass upper atmosphere near the polar regions. to create a new commission entitled "Elec­
ejections (CMEs). CMEs add to the complex­ They strike the molecules and atoms of the tromagnetics in Biology and Medicine."
ity of the ambient solar wind. They are asso­ thin, high atmosphere, exciting some of It has been realized and appreciated only
ciated with some flares, but more often they them to glow. These are auroras, dynamic in the last few decades that solar flares and
occur independently from flares. These and delicate displays of colored light seen in magnetic storms affect people. The list of
structures in the solar wind have been ob­ the night sky. The incoming particles deposit consequences is growing in proportion to
served and identified by ground-based opti­ their energy in the neutral atmosphere, heat­ our dependence on technological systems.
cal and radio measurements and in-situ ing it. The heated "air" rises, and the density The subtleties of the interactions between
spacecraft measurements. at the orbit of satellites up to about 1000 km the Sun and Earth and between solar parti­
As observed near Earth, solar wind increases significantly. As a result of the cles and delicate instruments have become
speeds are typically about 400 km/s, but added frictional drag, satellites lose energy factors affecting our well-being.
speeds exceeding 1000 km/s have been mea­ and their orbits change. All low-altitude sat­
sured. Proton and electron number densities ellites are slowly falling back to Earth owing
3
are typically near 5 cm but occasionally to atmospheric drag; this process is acceler­
3
exceed 100 cm" . The solar wind flows ated during geomagnetic storms. For exam­
around obstacles such as the planets, but ple, the NASA Long Duration Exposure Facil­ Suggested Reading
those planets with intrinsic magnetic fields ity satellite, which was recovered from an
altitude of 340 km in January 1990, lost 500 Davies, K. Ionospheric Radio. London: Peter Pere-
respond to the solar wind in a specific way. grinus, 1990.
In effect, Earth's magnetic field activity m of altitude in one day as a result of the Eather, R. H. Magestic Lights. Washington, D.C.:
senses the solar wind—its speed, density, March 1989 storm. Normal daily loss rates AGU, 1980.
and magnetic field. Because the solar wind were about 200 m/day at that time. Individ­ Garrett, H. B., and C. P. Pike, eds. Space Systems
ual satellites respond differently to increased and Their Interactions with Earth's Space Envi­
is variable over time scales as short as sec­
density, so agencies that monitor the posi­ ronment. New York: American Institute of Aero­
onds, the interface that separates interplane­ nautics and Astronautics, 1980.
tary space from the magnetosphere is re­ tions and identify the approximately 6000 Gauthreax, Jr., S. A. Animal Migration, Orientation,
markably dynamic. Normally, this interface— objects in low-Earth orbit, satellites and de­ and Navigation, Chap. 5. New York: Academic
the magnetopause—lies at a distance bris larger than about 10 cm in diameter, Press, 1980.
equivalent to about 10 Earth radii in the di­ require additional resources during magnetic Harding, R. Survival in Space. New York: Rout-
rection of the Sun. However, during episodes storms. Lower-altitude navigation satellites ledge, 1989.
can be affected to the point that they are Johnson, N. L., and D. S. McKnight. Artificial Space
of elevated solar wind density or velocity, Debris. Malabar, Florida: Orbit Book Co., 1987.
the magnetopause can be pushed inward to useless until their new orbits stabilize.
Lanzerotti, L. J., ed. Impacts of ionospheric/mag-
within geosynchronous altitudes (6.6 Earth During magnetic storms, some of the en­ netospheric process on terrestrial science and
radii). As the magnetosphere extracts energy ergized magnetospheric particles are trapped technology, in Solar System Plasma Physics,
from the solar wind, internal processes pro­ above the tangible atmosphere; they circu­ Vol. III. Edited by L. J. Lanzerotti, C. F. Kennel,
and E. N. Parker. New York: North Holland Pub­
duce geomagnetic storms, increase the prob­ late around Earth and form a ring of current
lishing Co., 1979.
ability of auroras at low latitudes, and that can be sensed on the ground by its as­ Parkinson, W. D. Introduction to Geomagnetism.
change the properties of the ionosphere and sociated magnetic field. When these ener­ New York: Scottish Academic Press Ltd., Elsevier
upper atmosphere. gized particles impact satellites—especially Science Publishing Co., 1983.

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