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ARRL Propagation Bulletin ARLP012 (2004)

ARLP012 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 12  ARLP012
From Tad Cook, K7RA
Seattle, WA  March 19, 2004
To all radio amateurs 

ARLP012 Propagation de K7RA

Average daily sunspot numbers rose nearly 13 points last week (March
11-17) to 66.1.  Average daily solar flux rose by a negligible
amount from 106.3 to 106.8.  A solar wind stream caused geomagnetic
disturbance from the last reporting week into the early part of this
week, but conditions quieted.  Mildly unsettled conditions may
return over the weekend, with Friday through Monday (March 19-22)
planetary A index predicted at 8, 12, 15 and 10.  Solar flux is
expected to moderately peak this weekend around 120 on both March 19
and 20, then 115 and 110 on March 21 and 22.

Sunspot 570, mentioned in last week's bulletin, split into two parts
this week as it moved toward the sun's western limb.  About now it
is disappearing from view.  A holographic image showing the far side
of the sun revealed a large sunspot group a few days ago, so perhaps
we'll see more activity soon.

Last week's bulletin mentioned polar cap absorption, but it turns
out that strictly speaking, this doesn't occur often, only a couple
of times a year on average.  Polar cap absorption only applies to
the polar cap and is caused by protons from big solar flares.  The
phenomenon in Alaska that is most common is called auroral
absorption, in and near the auroral oval.

This week Doug Gehring, WA2NPD wrote to ask about a related topic,
bad conditions on 80 and 160 meters.  He lives in Southern New
Jersey and has been having trouble working DX on the lower
frequencies over the past few years.  No doubt, Doug will notice
better conditions when there are fewer geomagnetic storms.
Geomagnetic activity will be less frequent as the solar cycle
declines over the next few years.

The spring equinox begins tonight, around 10:49 PM PST (0649z).
Spring conditions are here, a great time for HF DX if the
geomagnetic conditions are stable.  Regarding the time of the vernal
equinox, an interesting article in National Geographic News (see says that equinoxes migrate through a
period that is about six hours later from year to year.  Due to the
leap year cycle, (this year was one) the system resets every four
years.  However, because the average year is really 365.24219 days
long, over time the vernal equinox would slip.  To solve this, Pope
Gregory XIII in 1582 instituted the new calendar wherein century
years (such as 1800 and 1900) are not a leap year, except in century
years divisible by 400, such as 1600 and 2000.  This adjusted the
average calendar year from 365.25 days to 365.2425, which is only
about 26.8 seconds longer than the real 365.24219 days.  This yields
a gain (or error) of only one day over a period of about 3,200

Remember the big solar flare last fall?  It occurred from 1929z and
1950z on November 4, 2003, and was so large it overloaded
instruments and was estimated to be an X28.  This was much larger
than previous record flares on April 2 2001 and August 16 1989,
which were both rated as X20.  Now it appears that last year's flare
was more than twice as large as the previous record.  It has now
been adjusted upward to X45.  See the BBC report at

Sunspot numbers for March 11 through 17 were 67, 71, 61, 61, 49, 53
and 101 with a mean of 66.1.  10.7 cm flux was 113.2, 107.5, 103.8,
102.5, 101.4, 109.6 and 109.8, with a mean of 106.8.  Estimated
planetary A indices were 26, 23, 15, 16, 13, 8 and 6, with a mean of


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