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Space Weather: A Research Perspective
Space Weather: A Research Perspective
What the Future Holds
Many space weather events occur around the peak of the solar cycle, with stronger
cycles (higher sunspot maxima) expected to produce a greater number of major
disturbances. Yet sunspot numbers themselves have exhibited considerable
variation in intensity from one solar cycle to the next, in addition to small changes in
the cycle periods, throughout their recorded history. There also appear to have
been intervals in the fairly recent past when sunspots disappeared altogether for
several cycles. The most recent of these was the Maunder Minimum (1650-1715),
seen in the extended sunspot number record below.
Historical sunspot number record. Early sunspot numbers have been scaled
to recent numbers using information given on the method of counting
(courtesy of the National Center for Atmospheric Research High Altitude
Observatory).
It is notable that the Maunder Minimum was also a period of global cooling on Earth
known as the Little Ice Age. Recent spacecraft measurements have shown that the
solar activity cycle is accompanied by a small change (about 0.2%) in the amount
of heat radiated by the Sun, with lower values corresponding to solar minimum.
However, the existence of effects of the solar activity cycle on Earth's climate is still
a matter of debate. The reason is that the physical mechanisms by which the
related solar variations might cause such changes are not obvious. Subtle changes,
such as the alteration of polar ozone chemistry and aerosols by the deeply
penetrating ionization of the atmosphere accompanying some solar proton events,
may have cumulative influences that are yet to be understood.
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Space Weather: A Research Perspective
Painting by European artist Hendrik Avercamp (circa 1585-1663) of skaters on a frozen river during the Little
Ice Age.
In contrast to the effect of the solar activity cycle on Earth's climate, there is clear
evidence of the consequences of nonuniform solar cycles on space weather. Great
magnetic storms occurred on February 4, 1872, and August 5, 1972. These were
accompanied by exceptional auroral displays at middle and low latitudes. During
the 1872 storm some telegraph communications were sent using the induced
currents in the system, without power sources, while at other times they were totally
disrupted. Space-based measurements available during the 1972 storm revealed
potentially lethal doses of radiation had astronauts been outside the
magnetosphere enroute to the moon. An extraordinarily active cycle when coronal
material from numerous CMEs can overtake and reinforce each other's effects in
interplanetary space, and flares occur frequently, should maintain a state of
increased geomagnetic, atmospheric, and ionospheric disturbance. On the other
hand, a period of reduced solar activity should be accompanied by increased
access of galactic cosmic rays to Earth, and perhaps by intense electron radiation
belts if the solar wind streams remain strong.
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Space Weather: A Research Perspective
One prediction of the sunspot number for the coming cycle. Predictions such as this are
based on analyses of the previous cycles and differ considerably from one another (courtesy
of Interstellar Propulsion Society Radio and Space Services, Australia).
The figure above shows one of many projections for the approaching solar
maximum of the sunspot number, which is related to the amount of solar activity.
How will the solar cycle behave during future generations of cycles? Observations
of Sun-like stars suggest that the solar cycle may continue in roughly its current
state for a long time, but our understanding is insufficient to confidently predict the
extent of its future variations.
Solar activity, of course, is not the only part of the space weather equation. Earth's
magnetic field is also constantly changing because, like the Sun's field, it originates
in a complex dynamical system in the fluid-like interior. Convective motions in the
molten core that are responsible for the field's existence respond to variations in
internal chemical and radioactive heat sources and gravitational "stirring" by
solidifying material settling toward Earth's center. Magnetization left in crustal rocks
that solidified at different times in the past indicates that these motions have
periodically undergone sudden adjustments (sudden on geological time scales of a
billion years, at least). These adjustments affect Earth's field strength and even
reverse the field polarity (exchanging the north and south magnetic poles). The
dipole field strength appears to have varied on time scales measured in ten
thousands of years, while the polarity reversals occurred less frequently with
hundreds of thousands of years between them.
The last major change in the strength of Earth's dipole field 800,000 years ago
seems to have been a 90% reduction accompanied by a polarity reversal. The
records in the rock suggest that during this reversal the magnetic poles appeared to
wander across the equator and the usually weaker, non-dipolar parts of the field
became dominant. Such periods of reduced field intensity last on the order of 1000
to 10,000 years. We can only speculate about how our space environment and
space weather would change in response to such an event. It seems clear that the
radiation environments currently confined to the polar regions would expand
equatorward, that the solar wind would more closely approach Earth, and that the
aurora would become a more nearly global phenomenon.
Geomagnetic reversals are likely to continue to occur, albeit not within our lifetime.
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Space Weather: A Research Perspective
As illustrated below, measurements show that the locations of the magnetic poles
are drifting today. One estimation based on the current rate of decay of Earth's
dipole field strength is that the next reversal could occur within the next 2000 years.
While it is difficult to imagine what life on Earth will be like so many generations into
the future, humankind present in the year 4000 may have quite different
perspectives on their space environment than we have today.
The changing location of the north magnetic pole determined
by measurements over the last decade and a half (courtesy
of Canadian National Geomagnetism Program
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