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CX2SA > SOLAR 19.09.09 03:04l 134 Lines 7345 Bytes #999 (0) @ WW
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Subj: More to Solar Cycle than...
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More to Solar Cycle than Sunspots; Sun Also Bombards Earth with High-Speed
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Streams of Wind
===============
Septiembre 17, 2009
BOULDER-Challenging conventional wisdom, new research finds that the number of
sunspots provides an incomplete measure of changes in the Sun's impact on
Earth over the course of the 11-year solar cycle. The study, led by scientists
at the High Altitude Observatory of the National Center for Atmospheric
Research (NCAR) and the University of Michigan, finds that Earth was bombarded
last year with high levels of solar energy at a time when the Sun was in an
unusually quiet phase and sunspots had virtually disappeared.
"The Sun continues to surprise us," says NCAR scientist Sarah Gibson, the lead
author. "The solar wind can hit Earth like a fire hose even when there are
virtually no sunspots."
The study, also written by scientists at NOAA and NASA, is being published
today in the Journal of Geophysical Research - Space Physics. It was funded by
NASA and by the National Science Foundation, NCAR's sponsor.
Scientists for centuries have used sunspots, which are areas of concentrated
magnetic fields that appear as dark patches on the solar surface, to determine
the approximately 11-year solar cycle. At solar maximum, the number of
sunspots peaks. During this time, intense solar flares occur daily and
geomagnetic storms frequently buffet Earth, knocking out satellites and
disrupting communications networks.
Gibson and her colleagues focused instead on another process by which the Sun
discharges energy. The team analyzed high-speed streams within the solar wind
that carry turbulent magnetic fields out into the solar system.
When those streams blow by Earth, they intensify the energy of the planet's
outer radiation belt. This can create serious hazards for weather, navigation,
and communications satellites that travel at high altitudes within the outer
radiation belts, while also threatening astronauts in the International Space
Station. Auroral storms light up the night sky repeatedly at high latitudes as
the streams move past, driving mega-ampere electrical currents about 75 miles
above Earth's surface. All that energy heats and expands the upper atmosphere.
This expansion pushes denser air higher, slowing down satellites and causing
them to drop to lower altitudes.
Scientists previously thought that the streams largely disappeared as the
solar cycle approached minimum. But when the study team compared measurements
within the current solar minimum interval, taken in 2008, with measurements of
the last solar minimum in 1996, they found that Earth in 2008 was continuing
to resonate with the effects of the streams. Although the current solar
minimum has fewer sunspots than any minimum in 75 years, the Sun's effect on
Earth's outer radiation belt, as measured by electron fluxes, was more than
three times greater last year than in 1996.
Gibson said that observations this year show that the winds have finally
slowed, almost two years after sunspots reached the levels of last cycle's
minimum.
The authors note that more research is needed to understand the impacts of
these high-speed streams on the planet. The study raises questions about how
the streams might have affected Earth in the past when the Sun went through
extended periods of low sunspot activity, such as a period known as the
Maunder minimum that lasted from about 1645 to 1715.
"The fact that Earth can continue to ring with solar energy has implications
for satellites and sensitive technological systems," Gibson says. "This will
keep scientists busy bringing all the pieces together."
Buffeting Earth with streams of energy
--------------------------------------
For the new study, the scientists analyzed information gathered from an array
of space- and ground-based instruments during two international scientific
projects: the Whole Sun Month in the late summer of 1996 and the Whole
Heliosphere Interval in the early spring of 2008. The solar cycle was at a
minimal stage during both the study periods, with few sunspots in 1996 and
even fewer in 2008.
The team found that strong, long, and recurring high-speed streams of charged
particles buffeted Earth in 2008. In contrast, Earth encountered weaker and
more sporadic streams in 1996. As a result, the planet was more affected by
the Sun in 2008 than in 1996, as measured by such variables as the strength of
electron fluxes in the outer radiation belt, the velocity of the solar wind in
the vicinity of Earth, and the periodic behavior of auroras (the Northern and
Southern Lights) as they responded to repeated high-speed streams.
The prevalence of high-speed streams during this solar minimum appears to be
related to the current structure of the Sun. As sunspots became less common
over the last few years, large coronal holes lingered in the surface of the
Sun near its equator. The high-speed streams that blow out of those holes
engulfed Earth during 55 percent of the study period in 2008, compared to 31
percent of the study period in 1996. A single stream of charged particles can
last for as long as 7 to 10 days. At their peak, the accumulated impact of the
streams during one year can inject as much energy into Earth's environment as
massive eruptions from the Sun's surface can during a year at the peak of a
solar cycle, says co-author Janet Kozyra of the University of Michigan.
The streams strike Earth periodically, spraying out in full force like water
from a fire hose as the Sun revolves. When the magnetic fields in the solar
winds point in a direction opposite to the magnetic lines in Earth's
magnetosphere, they have their strongest effect. The strength and speed of the
magnetic fields in the high-speed streams can also affect Earth's response.
The authors speculate that the high number of low-latitude coronal holes
during this solar minimum may be related to a weakness in the Sun's overall
magnetic field. The Sun in 2008 had smaller polar coronal holes than in 1996,
but high-speed streams that escape from the Sun's poles do not travel in the
direction of Earth.
"The Sun-Earth interaction is complex, and we haven't yet discovered all the
consequences for the Earth's environment of the unusual solar winds this
cycle," Kozyra says. "The intensity of magnetic activity at Earth in this
extremely quiet solar minimum surprised us all. The new observations from last
year are changing our understanding of how solar quiet intervals affect the
Earth and how and why this might change from cycle to cycle."
Correction - September 18, 2009 | The headline for this release was changed
from "Solar Cycle Driven by More than Sunspots" to clarify that sunspots are
not the cause of the solar cycle but rather one measure of its strength.
PRESS RELEASE
Date Released: Friday, September 18, 2009
Source: http://www.ucar.edu/news/releases/2009/solarminimum.jsp
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