The origins and fate of life on Earth are intimately
connected to the way the Earth responds to the Sun's variations. We live
in the extended atmosphere of an active star, as we can realize from the
simplified illustration of the Sun–Earth connection of Figure 1 (Marhavilas,
2004). While sunlight enables and sustains life, the Sun's variability
produces streams of high-energy particles and radiation that can harm life
or alter its evolution. Under the magnetosphere (a protective shield of a
magnetic field) and atmosphere, the Earth is an island in the Universe
where life has developed and flourished (http://sec.gsfc.nasa.gov/sec_science.htm).
As the sphere of the human environment and exploration continues to expand
towards space, understanding the effects of our active Sun and “space
weather” on astronaut safety, satellite operations, power and
communications, and climate change, becomes day after day more important.
We need to understand and be able to predict the effects of solar activity
on Earth and society (more information on NASA¢s LWS initiative:
questions, associated with the Sun-Earth connected system, which must be
answered, are as follows: How and why does the Sun vary? How does the
Earth respond? What are the impacts on humanity?
This paper reviews present understanding of the dynamics of the
solar-terrestrial environment and its impacts on the human activity.
THE STORMY SUN
The closer we get to the Sun¢s secrets, the more we
admire the luminous body. The ancient Greek philosophers Anaxagoras
(500-428 B.C.) and Democritus (460-370 B.C.) thought that the Sun was a
crag, getting hot by its fast rotation. Heraclitus (544-470 B.C.)
estimated that the Sun was not bigger than a footstep. According to
Aristotle the Sun and the other luminous bodies consist of constant and
everlasting matter-ether. Nowadays, we know that the Sun is a class G star
(J.F. Graham, 1995) composed of ionized hydrogen and helium gas located
~150 million km from the Earth. Current calculations give its age at about
of 5 billion years, i.e. about half its life span. It is a second
generation star which means that it formed from the remains of some other
stars which may have exploded between five and ten billion years ago. The
Sun has a mass of 1.98 x 1030 kilograms about 333,000 times
larger than the Earth's mass of 5.98 x 1024 kilograms. The
Sun's radius is about 695,000 km compared to the Earth's radius of 6378
Because the Sun is an enormous gaseous sphere it rotates faster around the
equator than it does at the poles, 27 days versus about 35 days. Its
surface temperature is about 5,770º Kelvin and its interior temperature is
about 16 million degrees Kelvin. Its total radiated energy is equivalent
to 100 billion tons of TNT exploding per second or the same as a
significant portion of the Earth's entire nuclear arsenal exploding every
second. This power is obviously important to us because it supports all
life on the Earth. It causes seasonal changes, ocean current flows, and
atmospheric circulation. It also is responsible for photosynthesis for
plant life from which is derived all food and fossil fuels.
The Sun contains several major sections: the core along
with the radiative and the convective
layers. The Sun is a huge thermonuclear reactor. The process by which the
Sun gives off energy is the fusion i.e. the conversion of hydrogen into
helium by nuclear reactions, which release energy. Even though the nuclear
burning occurs in the core, the heat and light generated from this process
take about 10 million years to reach the Sun's surface. Once the photons
depart the core, they must travel through the radiative
layer to the convective zone
where the temperatures go from 8 million to 7,000º Kelvin. After reaching
the Sun's surface also known as the photosphere, the
photons travel through the chromosphere and it eventually
reach the corona. The last three regions with their
different physical properties constitute the solar atmosphere. Most of the
solar radiation comes from the photosphere (its name comes from the Greek
word “öùò” (“phos”) that means “light”), which emits a continuous spectrum
with superimposed dark absorption lines. The photosphere is the visible
surface of the Sun. This is what we see in a clear day from the ground.
Its temperature is ~5,800 ïK. The chromosphere lies above the
photosphere up to a height of ~1,500 km with a temperature of ~10,000 to
~500,000 ïK. When the Sun is observed through filters of
different wavelengths, pictures can be obtained of the Sun¢s structure at
a variety of levels. The lower chromosphere is shown up by using an Ha
filter. In the beginning of an eclipse we can see light that has emitted
from the photosphere and is then scattered towards us at the chromospheric
levels as well as the intrinsic chromospheric emission. This colourful
effect (it appears bright red), led Young in 1870 (Priest, 1984) to give
the chromosphere its name (from the Greek word “÷ñþìá” (“chroma”) that
means colour). The corona (from the Latin word for “crown”) is the upper
layer of the solar atmosphere. In this layer the temperature rises to more
than ~1,000,000 ïK.
The Solar Wind
Near its surface, the Sun is like a pot of boiling water, with bubbles of
hot, electrified gas – actually electrons and protons in the forth state
of matter known as “plasma” – circulating up from the interior, rising to
the surface, and bursting out into space. The steady stream of ionized
plasma, which continuously escapes from the solar corona and pervades the
whole interplanetary space (Hundhausen, 1972, 1995), is known as the
solar wind. This gas, composed of
electrons and protons with approximately 10% helium ions, also induces
geomagnetic activity by variations of its pressure and magnetic field (N.
Vilmer, 1998). At the orbit of Earth, this wind has a density of about 10
particles per cc, a temperature of about 1 keV (equivalent to about
10,000,000 oK) and an average speed of about 500 km/sec. The
solar wind gas also has magnetic fields from the sun imbedded within it.
At Earth¢s orbit, these fields have a strength of ~10 nT (about 3,000
times less than the Earth¢s magnetic field at its surface).
Blowing at ~400 to ~2,500 km per second, the solar wind (see Figure 2)
carries a million tons of matter into space every second (NASA & ESA,
2002). Although the solar wind carries mass away from the Sun at rate of
1.6x1012 gr/sec and energy at a rate of 1.8x1027
erg/sec, it is negligible in the overall mass and energy balance of the
sun. It¢s not the mass or speed, however, that makes the solar wind
potent. In fact, the solar wind would not even ruffle the hair on the head
because there are too few particles in the breeze (our air is millions of
times denser than the solar wind). Instead, it is the energy stored in the
plasma and the magnetic fields associated with that plasma that allow the
wind to shape and impact Earth¢s protective magnetic shield in space (the
magnetosphere). Though less than 1% of the solar wind penetrates the
magnetosphere, that¢s enough to generate millions of amps of electric
current in our atmosphere and to cause occasional storms in the space
around Earth (see: NASA¢s brochure, 1998).
The Sun-Earth system is driven by the 11-year
solar cycle. This means that every 11
year the Sun reaches a peak period of activity called “solar maximum”,
followed a few years later by a period of quiet called the “solar
minimum”. During solar maximum there are many sunspots, eruptive
prominences, solar flares, and coronal mass ejections (CMEs), all of which
can affect communications and other technology here on Earth. The last
maximum occurred in 2000 during Cycle 23.
One way of tracking solar activity is by observing the number of sunspots.
During solar maximum there are hundreds of sunspots and during solar
minimum only a dozen can be found. In the photosphere the sunspots are
characterized by the most intense concentrations of magnetic flux that
have been formed during the emergence of flux in one day or so. They are
relatively cool areas that appear as dark patches like freckles on the
solar surface formed when magnetic field lines just below the Sun¢s
surface are twisted and poke through the solar surface (see Figure 3).
They appear dark because they are not as hot or bright as the area
surrounding them (4,000 º Kelvin vs. 6,000 º Kelvin). Sunspots can last
from a few hours to several months, and a large sunspot can grow several
times the size of Earth.
Though the Chinese recorded some observations as early
as 28 B.C., scientists have been observing and recording sunspots since
the 17th Century. (Galileo, who first performed scientific
observations in the early 1613, concluded that the Sun did indeed have
spots.) The scientists care about sunspots because they are visible signs
of the turmoil inside the Sun that lead to space weather effects on Earth
(see: NASA¢s brochure, 2000).
However sunspots, which are surrounded by areas with enhanced brightness
called active regions, are not the only
element of solar activity. The Sun emits more energy when it is active.
Increased solar activity also means stronger and more frequent eruptive
prominences, and solar flares (a dramatic release of energy equivalent to
a million hundred-megaton nuclear explosions).
Prominences are the most impressive objects on the Sun because they are
located in the corona but possess temperatures a hundred times lower and
densities a hundred or a thousand times greater than the coronal values.
They are structures in the corona, consisting of cool plasma supported by
magnetic fields (see Figure 4). They are bright when seen at the Sun¢s
edge. However, when seen against the bright solar disk they are dark and
are called filaments. If they have broken away from the sun, they are
called eruptive prominences.
Solar Flares are huge explosions in the Sun¢s atmosphere. They appear to
our instruments as bright flashes in visible light, often followed by a
burst of high-energy protons and radiation. Moreover their characteristics
can include bursts of radiowaves, EUV and X-rays. A large solar flare can
release a thousand million megatones of energy [more precisely 1028
to 1034 ergs (N. Vilmer, 1998)] in a single explosion. The
released energy is transformed into: 1) thermal energy (localized heating
leading to an increased brightness of e.g. the Ha and X-ray emission), 2)
particle kinetic energy leading to the acceleration of electrons to
energies of 10 keV to 1 GeV and ions to energies from a few MeV/nuc to
GeV/nuc, 3) mechanical energy leading to several kinds of plasma ejecta.
Solar flares sometimes occur together with other signatures of solar
activity e.g. prominence eruptions, CMEs and interplanetary shock waves.
However the exact relationship between these phenomena is not yet
completely understood. In fact the solar flares are one of the main
challenges of space weather prediction. Figure 5 shows the unusually large
flare, which took place on 14/7/2000 (F. Jansen & R. Hippler, 2003). This
event was so remarkable that it has become known as the “Bastille Day”
Coronal Mass Ejections (CMEs)
One of the most important solar events from the Earth¢s perspective is the
coronal mass ejection,
the solar equivalent of a hurricane (NASA¢s brochure, 2000). A CME is the
eruption of a huge bubble of plasma from the Sun¢s outer atmosphere. It
can occur with or without solar flares, and can threaten Earth¢s
atmosphere. Once it escapes the Sun¢s gravity, a CME speeds at velocities
approaching 400 km/sec (~1,000,000 miles/hr) up to 2,000 km/sec
(~5,000,000 miles/hr). A typical CME can carry more than 10 billion tons
of plasma into the solar system. Just hours after blowing into space, a
CME cloud can grow to dimensions exceeding those of the Sun itself, often
as wide as 20 million km across. As it ploughs into the solar wind, a CME
can create a shock wave that accelerates
particles to dangerously high energies and speeds. Behind that shock wave,
the CME cloud flies through the solar system bombarding planets,
asteroids, and other objects with radiation and plasma. If a CME erupts on
the side of the Sun facing Earth, and if our orbit intersects the path of
that cloud, the results can be spectacular and sometimes hazardous. Figure
6 shows a large CME erupting from the Sun on 2nd April 2001.
The CME is seen as a bright cloud expanding towards the right hand side of
the image taken using the LASCO coronograph onboard SOHO. In this case an
EUV image of the Sun has been superimposed onto the LASCO image to show
the size and location of active regions on that day.
Geomagnetic storms and substorms
The region near-Earth space, where the dynamics is governed by the
internal geomagnetic field, is called magnetosphere. The
solar wind flow past the Earth distorts the dipole field to compress it on
the dayside and elongate it to a long geomagnetic tail on the nightside.
The geomagnetic tail plays a key role in magnetospheric dynamics; for
example, it acts as an energy reservoir for the dynamic processes (T.I.
Pulkkinen, 1998). The magnetosphere (Figure 7) comprises distinct regions,
which all have their characteristic plasma properties: The tail lobes at
high latitudes are regions of low plasma density and energy, whereas the
plasma sheet is characterized by denser and hotter ~keV plasma. The most
hazardous region for technological systems is the inner magnetosphere,
where trapped populations of high-energy (from hundreds of keV to
multi-MeV) electrons and ions reside in the ring current and in the Van
Allen radiation belts.
The dynamic response of the magnetosphere to varying
solar wind and interplanetary magnetic field conditions is the
magnetospheric substorm (Rostoker et al. 1980). Energy input from the
solar wind is largely controlled by the interplanetary magnetic field
orientation: During periods of southward interplanetary field, the energy
input is enhanced and the energy extracted from the solar wind is stored
in the magnetosphere in the form of magnetic field energy in the
magnetotail. This is the substorm growth phase. After typically 30-60 min,
the magnetotail undergoes a change of state from stable to unstable, and
the stored energy is dissipated via a highly dynamic process. This
substorm expansion phase involves an injection of energetic (tens to
hundreds of keV) electrons and ions to the vicinity of the geostationary
orbit, strong electric currents in the auroral regions, and rapid
fluctuations and configurational changes of the magnetospheric magnetic
field. All these phenomena are potential space weather effects. The
substorm process ends when the energy dissipation ceases and the
magnetosphere recovers its initial state after about two to four hours
from the beginning of the event (for recent reviews: McPherron, 1991;
Baker et al. 1996; Pulkkinen, 1998).
Geomagnetic storms are large
disturbances in the near-Earth environment caused by coherent solar wind
and interplanetary field structures that originate from solar disturbances
such as CMEs (Gonzalez et al. 1994). Storms are associated with major
disturbances in the geomagnetic field and strong enhancement of the fluxes
of energetic (tens to hundreds of keV) ions and high-energy (up to several
MeV) electrons in the outer Van-Allen radiation belt (Baker et al. 1998).
IMPACTS ON HUMAN LIFE AND
As A. Frank has written (A. Frank, 1999), humanity is maturing into a
space-faring race. The response of the space environment, particularly
around the Earth, to the stormy Sun, is known as Space
Weather, which is a hot topic today, because of the
increasing awareness that many modern technological systems are
potentially vulnerable to the effects from solar storms. On the other
side, the danger of some aspects of space weather has slowly been
recognized, and is studied by the scientists in order to help protect
space and ground systems (technical and biological) from space environment
hazards. Space weather disturbances are generally caused by transient
events in the solar atmosphere. There are two different types of events,
which trigger disturbances in the Earth¢s environment (Brekke, 2001): a)
solar flares and b) CMEs. However, not all solar flares result in
geomagnetic storms, and even more significantly, not all geomagnetic
storms can be associated with solar flares. CMEs are some of the most
dramatic space weather effects. The emission from the two types of
disturbances can be divided into two classes: a) particle radiation and b)
electromagnetic radiation, which will have different effects on the
Particle Radiation: A continuous flow of charged
particles (protons and electrons), the solar wind, is streaming out from
the Sun. Moreover, several types of solar events can cause particles with
high velocities to be superimposed on this background solar wind. CMEs
carry billion tons of matter at high speeds, considerably greater than the
normal solar wind velocities. The cloud of charged particles (which also
bring with them parts of the solar magnetic field) interacts with the
Earth¢s magnetic field when it reaches the Earth¢s orbit. This results in
a disturbance of the Earth¢s magnetic field, and the auroral particle
precipitation into the atmosphere increases. The aurora (as discussed
below) is a dynamic and delicate visual manifestation of solar-induced
Electromagnetic Radiation: The energetic radiation bursts
from flares, travel at the speed of light well ahead of any particles or
coronal material associated with the flares, arrive at Earth just 8
minutes after leaving the flare site. Moreover, unlike the electrons and
ions of the solar wind and the solar energetic particle populations, the
passage of electromagnetic waves is not affected by the presence of
Earth¢s magnetic field. The direct response of the upper atmosphere to a
burst of solar flare ultraviolet and x-ray emissions is a temporary
increase in ionisation (as well as temperature) in the sunlit hemisphere,
lasting from minutes to hours and called a sudden ionospheric disturbance.
This can cause disruption of short-wave radio communication at HF
frequencies (3-30 MHz), which is still extensively used by the military
and for overseas broadcasting.
The earliest evidence for the impact of solar-terrestrial phenomena on
technical systems appeared in the first half of the 19th
century (Lanzerotti et al., 1999). Beginning with the invention of
telegraphy in 1841, during solar storms,
"earth currents" induced by the changing terrestrial magnetic field, were
so powerful that telegraphers didn't need a battery to send their messages
down the line. Any relationship of the sun to the appearances of the
“spontaneous” currents that were measured on the telegraph wires was not
clarified until the occurrence of the large white light solar flare of
late August 1859 (Carrington, 1863; the first such flare ever recorded by
astronomers). Within a day following this flare, large geomagnetic
disturbances and wide-spread aurora were observed over the Earth,
including at low geomagnetic latitudes in Hawaii and Rome. The advancement
in communications provided by radio stimulated a significant need to
better understand the medium that was critical in bending the radio waves
around the curvature of Earth, the ionosphere. The same ionosphere
currents that could produce “spontaneous” Earth currents could also affect
the reception and the fidelity of the transmitted, long-distance wireless
signals. Marconi in 1928 noted, with respect to wireless communications,
that “… times of bad fading practically always coincide with the
appearance of large sun-spots and intense aurora-boreali usually
accompanied by magnetic storms …” He also wrote that these were “… same
periods when cables and land lines experience difficulties or are thrown
out of action”. Such concerns have persisted throughout the twentieth
century (e.g. Gassmann, 1963), with considerable present-day research on
the ionosphere being motivated by engineering considerations similar to
those encountered in the early days of trans-ocean wireless
The March 24, 1940 storm caused a temporary disruption of electrical
service in New England, New York, Pennsylvania, Minnesota, Quebec and
Ontario. A storm on February 9-10, 1958 caused a power transformer failure
at the British Columbia Hydro and Power Authority. On August 2, 1972, the
Bureau of Reclamation power station in Watertown, South Dakota was
subjected to large swings in power line voltages up to 25,000 volts.
Similar voltage swings were reported by Wisconsin Power and Light, Madison
Gas and Electric, and Wisconsin Public Service Corporation. A 230,000-volt
transformer at the British Columbia Hydro and Power Authority exploded,
and Manitoba Hydro in Canada recorded power drops from 164 to 44 megawatts
in a matter of a few minutes, in the power it was supplying to Minnesota
(St. Odenwald, 1998).
The aurora is beautiful, spectacular, splendid, and appears quite
frequently – almost nightly – in the polar sky (Daglis & Akasofu, 2004).
Appearing in the form of majestic, colourful, irregular lights in the
night sky, the aurora has a variety of shapes, colours, and structures
(alike shimmering, colourful curtains), and continuously changing in the
time (see Figure 8). Although many theories existed, it wasn¢t until a
hundred years ago that scientists discovered that they were caused by
interactions with the Sun. It is a large-scale electrical discharge
phenomenon in the high-altitude atmosphere, resulting from quantum leaps
in oxygen and nitrogen atoms. What exactly happens? In the highest reaches
of the atmosphere, above about a hundred kilometres, oxygen and nitrogen
atoms and molecules are energized and/or ionised by energetic electrons.
In this transition region between the earth¢s atmosphere and near-earth
space free electrons abound. Accelerated by electric fields in the
magnetosphere, energetic electrons streaming geomagnetic field lines hit
and excite atoms and molecules. The auroral light results from the
de-excitation of these particles. The colour, shape, and intensity depend
on the electromagnetic forces that shoot electrons downward into the upper
Impacts of Solar-Terrestrial Processes on
During a space weather storm electric currents flowing in the
magnetosphere and ionosphere change rapidly. The variations produce
temporal changes in the geomagnetic field. These changes are known as
(geo)magnetic disturbances or storms. According to Faraday¢s law of
induction, magnetic disturbances are accompanied by an electric field,
which drives currents within the conducting earth. These currents affect
the magnetic disturbance and the (geo)electric field occurring at the
earth¢s surface, too. The electric field also creates currents in man-made
conductor systems, such as electric power transmission networks, oil and
gas pipelines, telecommunication cables and railway equipment, in which
they are called geomagnetically induced currents (GIC). Inconveniences to
the system may result from GIC (Pirjola et al. 1998). Large CIC occur most
frequently in the auroral regions, in particular in North America. The
increasing number of technological systems vulnerable by GIC and the
approaching sunspot maximum with a higher geomagnetic activity make GIC
research very actual and important now. In Table 1, many of technological
systems are listed, which must include processes and parameters from the
solar-terrestrial environment in their design and/or operations. These
systems are grouped into categories that have similar physical origins.
Some of the effects of the solar-terrestrial environment on technical
systems deployed on the Earth¢s surface and in space, and/or whose signals
propagate through the space environment are depicted in the paper of L.
Lanzerotti et al. (1999).
Table 1 Impacts of
Solar-Terrestrial Processes on Technology
● Ionosphere Variations
Wireless signal reflection, propagation, attenuation
Communication satellite signal interference, scintillation
Interference with geophysical prospecting
Source of electrical currents in the Earth
Power distribution systems
Long communications cables, land and ocean
Solar cell damage
Semiconductor device damage and failure
Misoperation of semiconductor devices
Spacecraft charging, surface and interior materials
Airline passenger safety
● Magnetic field variations
Attitude control of spacecraft
● Solar radio bursts
Excess noise levels in wireless communication systems
Low altitude satellite drag
Attenuation and scatter of wireless signals
Electric Power Systems: As compared to
the 50 (or 60) Hz frequency used in electric power transmission,
geomagnetic variations are slow with typical frequencies in the mHz range.
Therefore GIC, when flowing through a transformer, affects as a dc
current. In normal conditions the ac exciting current needed to provide
the magnetic flux for the voltage transformation in a power transformer is
only a few amperes, and the transformer operates within the range where
the dependence of the exciting current on the voltage is linear. However,
the presence of GIC implies an offset of the operation curve resulting in
saturation of the transformer during one half of the ac cycle and in an
extremely large non-linear exciting current (even some hundreds of
amperes). The exciting current is asymmetric with respect to the ac
half-cycles and is thus distorted by even and odd harmonics, which in turn
may cause relaying problems in the system. The increased exciting current
also produces large reactive power losses in the transformer contributing
to a serious voltage drop. The harmonics and the reactive power demands
also affect the transformer itself. The noise level is increased, and due
to the saturation of the core, the magnetic flux goes through other parts
of the transformer possibly resulting in overheading. The hot spots may
permanently damage the insulators and cause gassing of transformer oil
resulting in serious internal failures (more information in the paper of
R. Pirjola et al., 1998).
Perhaps the most dramatic and famous GIC failure occurred in the
Hydro-Quebec power system on March 13, 1989 (Kappenman and Albertson,
1990) so that for nine hours, large portions of Quebec were plunged into
Oil and Natural Gas Pipelines: GIC currents flowing in
pipelines are known to enhance the rate of corrosion over time, and this
can have catastrophic effects (St. Odenwald, 1998).
Effects on Spacecraft and Aircraft Electronics:
Spacecraft systems are vulnerable to space weather through its influence
on energetic charged particle and plasma populations, while aircraft
electronics and aircrew are vulnerable to cosmic rays and solar particle
events. These particles produce a variety of effects including total dose,
lattice displacement damage, single event effects (SEE), noise in sensors
and spacecraft charging (Dyer and Rodgers, 1998). Dose is used to quantify
the effects of charge liberation by ionisation and is defined as the
energy deposited as ionisation and excitation per unit mass of material.
[SI units: J/kg or grays (=100 rads, where 1rad=100 erg/g)]. The majority
of effects depend on rate of delivery and so dose-rate
information is required. Accumulated dose leads to threshold voltage
shifts in CMOS due to trapped holes in the oxide and the formation of
interface states. In addition increased leakage currents and gain
degradation in bipolar devices can occur. A proportion of the energy-loss
of energetic radiation gives rise to lattice
displacement damage and it is found that effects
scale with NIEL (non-ionising energy loss per unit). Examples of damage
effects are reduction in bipolar transistor gain, reduced efficiencies in
solar cells, light emitting diodes and photodetectors, charge transfer
inefficiency in charge coupled devices and resolution degradation in
solid-state detectors. The primary cosmic rays are very energetic and are
highly ionising, which means that they strip electrons from atoms, which
lie in their path and hence generate charge. The density of charge
deposition is proportional to the square of the atomic number of the
cosmic ray so that the heavier species can deposit enough charge in a
small volume of silicon to change the state of a memory cell, a “one”
becoming a “zero” and vice versa. Thus memories can become corrupted and
this could lead to erroneous commands. Such errors are referred to as
single event upsets (SEU). Sometimes a single particle can upset more than
one bit to give what are called multiple bit upsets (MBU). Certain devices
could be triggered into a state of high current drain, leading to burn-out
and hardware failure; such effects are termed single event latch-up or
single event burn-out. These deleterious interactions of individual
particles are referred to as single event
effects (SEE). A classic example of hardware failure
occurred in the PRARE instrument carried on the ERS-1 (European Ranging
Sensing Spacecraft). Surface electrostatic
charging can occur when spacecraft are bathed in energetic
plasmas (several keV electron temperature) without the presence of
neutralising cold plasma. Numerous anomalies have occurred from both
surface and deep dielectric charging. Some of these have proved fatal
(e.g. ANIK E1). Spurious counts, which constitute the background
noise, are produced in many detector systems and these
depend on the size distribution of individual depositions and can occur
from both prompt ionisation and delayed depositions due to induced
radioactivity. In the last ten years it has been realised that single
event effects will also be experienced by sensitive electronics (avionics)
in aircraft systems, which are subjected to increasing levels of cosmic
radiation and their secondaries as altitude increases.
Radiobiology in Space Research
For high-altitude flights beyond the magnetic shielding of the earth,
cosmic radiation i.e. high-energy particles from protons to iron ions
predominate. Space radiation reaches its maximum values at a solar
particle event where the lethal doses can be delivered within an hour or
less (Testard et al. 1998). These events however are rather unlikely. Much
more important is the permanent i.e. protracted exposure to the low-dose
radiation of the heavy charged particles of cosmic galactic rays. Because
of their high local dose these particles are able to create local damage
in bio-molecules that can manifest itself in long-term alterations like
genetic mutation and cancer induction. It is the induction of these
biological changes that determines the general risk of long-term missions.
For low altitude flights such as MIR or space station orbits, trapped
electrons and protons from solar origin predominate. To study the
biological radiation response, especially genetic alterations and
cancerogenesis, X-ray experiments can be performed in order to mimic
sparsely ionising electrons. The radiation environment is present both on
earth and in space but differs in quality and intensity. Spaceflights are
on average 300 times more exposure-intensive than our daily life.
Depending on dose, acute or long-term effects can be induced by radiation
exposure. Acute effects like nausea, vomiting, skin irritation, depletion
of white blood cells occur at doses of about 1.5 Gy or more (Testard et
al. 1998). These high doses are produced by solar storms. Long-terms
effects are genetic alterations, cancer induction, damage to the central
nervous system and peripheral neurons and accelerated aging. Among these
effects cancerogenesis and neural damage seems to be the most important.
The uncertaincies for the risk determination with regard to long-term
effects are large and the risk estimation is mainly based on
epidemiological data from the atomic bomb survivors.
Magnetic storms trigger myocardial infractions with mechanisms relating to
heart rate variability (Halberg et al., 2001).
Travelling outside of the Earth¢s atmosphere -
Travelling outside of the Earth's atmosphere, places one in extremely
hostile surroundings. Space can be defined in many ways. Using the
threshold at the point where humans can no longer survive without life
support, space begins in the stratosphere (18 km to 50 km). Travelling
beyond the stratosphere, astronauts encounter several more layers of
Earth's atmosphere before reaching the exosphere (above 300 km) and the
vacuum of low-Earth orbit. Heat is transferred only by radiation in space,
and temperature can vary from extremes near absolute zero (-273 degrees
Celsius) to over 1200 degrees Celsius. When humans travel into space,
temperature is not the only consideration. Two forms of radiation -
electromagnetic and ionizing - are prevalent. Ionizing radiation, composed
of high energy particles and photons, can be further categorized into the
radiation found in the Van Allen Belts, solar cosmic rays (SCR), and
galactic cosmic rays (GCR). All three can be potentially harmful to
astronauts in space.
Van Allen Belts: The Van Allen radiation results from
electrons and ions trapped in the Earth's magnetic field. They form donut
shaped rings around the Earth and are distributed nonuniformly within the
magnetosphere. The two belts are located at altitudes of 300 to 1200 km
and above 10,000 km. Extended stays in either can be fatal.
Solar Cosmic Rays: Regular and irregular forms of solar
cosmic rays, or solar particle events (SPR), occur as solar wind and solar
flares, respectively. This phenomenon contributes to the Van Allen Belts.
Solar flares, resulting from "storms" in the Sun's magnetosphere, yield
extremely high radiation doses of radiation ranging from hours to days.
Galactic Cosmic Radiation: Having the highest energy of
the three forms, GCR consists of protons, -particles, and heavy nuclei
and is the most penetrating. It is emitted from distant stars and
galaxies, diffusing through space and arriving at Earth in all directions.
The flux of GCR is indirectly related to the solar cycle, with the minimum
occurring at solar maximum (when the solar particles can best scatter the
GCR from Earth.) Extra-vehicular activity in low-Earth orbit is shielded
from this form of radiation, but in-transit crewmembers to the Moon or
Mars would be susceptible to its effects.
During the Apollo program, there were several near-misses between the
astronauts walking on the surface of the Moon and a deadly solar storm
event. The Apollo 12 astronauts walked on the Moon only a few short weeks
after a major solar proton flare would have bathed the astronauts in a 100
rem blast of radiation. Another major flare that occurred half way between
the Apollo 16 and Apollo 17 moonwalks would have had a much more deadly
outcome had it arrived while astronauts were outside their spacecraft
playing golf. Within a few minutes, the astronauts would have been killed
on the spot with an incredible 7000 rem blast of radiation.
The MIR space station has been inhabited for over a decade, and according
to Astronaut Shanon Lucid, the daily dosage of radiation is about equal to
8 chest X-rays per day. During one solar storm towards the end of 1989,
MIR cosmonauts accumulated in a few hours, a full- years dosage limit of
radiation. Meanwhile, the Space Station will be assembled in an orbit
which will take it through the South Atlantic Anomaly. Moreover, Space
Station assembly will involve several thousand hours of space walks by
astronauts. The main construction work will occur between the years 2000
and 2002 during the sunspot maximum period of Cycle 23. We can expect
construction activity to be tied to solar conditions in a way that will
frustrate the scheduling of many complex activities and the launches of
Space Station components.
Impacts on Earth¢s Climate
Scientists increasingly suspect that solar activity affects more than just
satellites and power grids. Although sunspots and active regions
themselves produce only minor variations in the energy output from the
Sun, the magnetic activity that accompanies these regions can produce
dramatic changes in the ultraviolet and soft X-ray emission levels. These
changes over the solar cycle have important consequences for the Earth¢s
upper atmosphere and are known to alter the dynamics, temperature and
chemistry (e.g., ozone) in these layers. This may have implications on the
Earth¢s climate (Brekke, 2001). Moreover, there have been suggestions that
climate is related to the appearance and disappearance of sunspots.
Researchers have found that the solar constant
(s = 1.37 kW / m²), which describes the solar radiation that falls on an
area above the atmosphere at a vertical angle, doesn¢t remain constant,
but varies slightly with sunspots and other solar activity. The total
solar invariance varies just as regularly as the sunspot activity over the
11-year-solar cycle. Satellite measurements showed that the solar output
variation is proportional to sunspot numbers. A research of Lane at al.
(1994) indicates that the combined effects of sunspot-induced changes in
solar irradiance and increases in atmospheric greenhouse gases offer the
best explanation yet for the observed rise in average global temperature
over the last century. Using a global climate model based on energy
conservation, Lane et al. constructed a profile of atmospheric climate
"forcing" due to combined changes in solar irradiance and emissions of
greenhouse gases between 1880 and 1993. They found that the temperature
variations predicted by their model accounted for up to 92% of the
temperature changes actually observed over the period -- an excellent
match for that period. Their results also suggest that the sensitivity of
climate to the effects of solar irradiance is about 27% higher than its
sensitivity to forcing by greenhouse gases. The long term increase in the
Sun's level of activity (both variations in the emitted energy and its
magnetic fields) may have played a significant role in the measured global
warming the last 150 years. It is important to quantify this effect before
one can determine any human influences on our climate. Thus, it is of
great importance to understand how the Sun works and how it varies over
time so that we can better understand how it will affect us in the future.
Perspectives in the future: Space Weather
Recently there has been a revolution in understanding the Sun due to two
major advances. The first is in theoretical modelling of the way the Sun¢s
magnetic field interacts with solar matter. The second advance is a series
of observational discoveries from three space born solar satellites:
ULYSSES, Yohkoh (a Japan-USA-UK Satellite), SOHO (the Solar and
Heliospheric Observatory, a joint ESA-NASA project), and TRACE (NASA).
These missions are providing high resolution observations of the Sun using
sophisticated imaging telescopes and spectrographs that separates out
observed light into the colours it is made of. This coordinated attack on
solar physics has provided breathtaking new views of the Sun and a wealth
Ôhe ULYSSES Mission (Figure 9) is a joint undertaking between the European
Space Agency (ESA) and the National Aeronautics and Space Administration
(NASA). Its goal is the exploration of the Sun's environment far out of
the ecliptic plane. ULYSSES is the only spacecraft to have visited this
unique region above and below the poles of the Sun.
SOHO has been leading the way into a new era in the field of
helioseismology – a study of the solar interior through the analysis of
vibrations on the surface.
The fact that the Sun is affecting us in so many ways makes it very
important to learn more about our own star. We need to monitor it
continuously to better understand the solar cycle and any long term
changes in the Sun¢s activity level.
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