HF PROPAGATION FALLING VICTIM TO STRONG SOLAR WINDS

http://www.arrl.org/news/stories/2004/11/10/7/?nc=1

NEWINGTON, CT, Nov 10, 2004 --- As HF radio conditions drift in the
doldrums, the Space Environment Center (SEC) reports that geomagnetic
storm activity spiked into the ``extreme`` (G5) category over the
past 24 hours. A result of disturbances in Earth`s geomagnetic field
caused by gusts of ``solar wind`` blowing past the planet,
geomagnetic storms have adversely affected HF radio propagation over
the past few days and even resulted in limited radio blackouts.
Although things had calmed to ``moderate`` (G2) by 2000 UTC,
geomagnetic storm activity at the G5 level can affect more than just
radio propagation. Severe geomagnetic storms also can abruptly
increase drag on spacecraft and even cause electrical power blackouts
on Earth.

``I don`t know when it will end,`` said Tad Cook, K7RA, who compiles
the weekly ARRL propagation bulletin. He said during heavy solar wind
conditions, amateurs typically experience north-south propagation.
``It isn`t that north-south connections are enhanced during heavy
space weather,`` he explained. ``It is the only propagation that still
works during conditions like this.``

The recent space weather conditions also have resulted in auroral
displays visible as far south as the Middle Atlantic states over the
past weekend. The SEC estimates that G5-scale geomagnetic conditions
will occur on four days of each 11-year solar cycle.

The forecast shows the Planetary A index increasing over the next 24
hours, from 40 today to 50 or greater on November 11, although it`s
predicted to drop to 10 on November 12 and move even lower over the
weekend. During the height of the current solar cycle, values topped
150.

The Planetary A index relates to geomagnetic stability. Magnetometers
around the world are used to generate a number called the Planetary K
index. Every 24 hours the K index is summarized in the Planetary A
index. Small changes in the K index are significant. For example, a
full day with the K index at 3 will produce an A index of 15, a K of
4 would mean an A of 27, a K of 5 would generate an A of 48, and a K
of 6 would produce an A of 80.

``Generally, the higher the latitude of the measuring station, the
higher the K and A indices reported,`` Cook says. ``This is because
the effects of geomagnetic instability tend to concentrate toward the
polar regions of the globe.``

In terms of radio blackouts --- disturbances of the ionosphere caused
by X-ray emissions from the sun --- the SEC reports conditions as
moderate (R2) over the past 24 hours but nil as of 2000 UTC. The SEC
determines the radio blackout value using X-ray measurements from the
primary GOES satellite. The chance of an ``extreme`` R5-level radio
blackout is less than one per 11-year solar cycle.

Solar radiation storm activity has been ``moderate`` (S2) over the
past 24 hours and continues at that level. This degree of activity
can have small effects of HF propagation through the polar regions,
the SEC says.

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