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ARRL Propagation Bulletin ARLP043 (1996)

ARLP043 Propagation de KT7H

Propagation Forecast Bulletin 43  ARLP043
From Tad Cook, KT7H
Seattle, WA  October 18, 1996
To all radio amateurs

ARLP043 Propagation de KT7H

Solar activity is still low, and we have just seen still another
week of zero visible sunspots.  October 16 was the thirty forth
consecutive day with no sunspots.  This surely means that we must
still be at the minimum between solar cycles 22 and 23.  The
current forecast, based upon the mean values of solar cycles 18 to
22 says that the next cycle will not have the same average sunspot
number value as January 1995 (24) until June 1997.  The same
prediction for solar flux shows that the value for January 1995 (81)
is not expected again until December 1997.  Remember that these are
only guesses based upon averages for previous solar cycles.

Current forecasts predict some unsettled to active conditions over
the next week due to recurring coronal holes, with a solar flux of
around 70 for the next few weeks.  Unsettled conditions will
probably return around November 9, with solar flux dipping below 70
again after November 2.

After lasts week's inclusion of the A index in the summary of weekly
numbers, I received some questions about what the A index is.

The A Index is based upon the K index over a 24 hour period.  A new
K index is announced every three hours on WWV, at 0000, 0300, 0600,
0900, 1200, 1500, 1800 and 2100 UTC.  K indices are an expression of
geomagnetic instability.  The scale changes very little, so that
each point is quite significant.

Ideally HF operators would love to see a constant K index of 0 and a
solar flux over 300, but it almost never works out that way.  During
periods of high solar activity, the K index tends to be higher,
because when the Sun is more active it spews out more protons,
plasma and flares, which can destabilize the Earth's magnetic field,
cause the ionosphere and polar regions to absorb radio waves and
generally make life miserable for HF hams.

Now that we are at the low point between solar cycles 22 and 23, the
K index is usually quite good, around 1 or 2.  During the high part
of a solar cycle, it tends to be 3 most of the time, 4 when
conditions are unstable, and 5 or even higher when there is a
geomagnetic storm or even a radio blackout.  Each point signifies a
big change, so that we almost never observe a K index of 6 or 7,
except when there has been a big solar flare.

The A index, based on all the K indices for the day, has a much
broader range.  The A index will move much more.  While I am writing
this, the Solar Flux is 69, A index is 4, and the most recent K
index is 2.  A typical condition during higher solar activity is a K
index of 3 and an A index of about 15, or perhaps a K index of 4 and
A of around 20 if conditions are disturbed.

During the past week The highest A index was 19 on October 13, and
the highest K index for the day was 5, which is quite disturbed.
The average K index for that day was only 3.7, as the K of 5 only
appeared during two three hour periods, and several other periods
had a K of only 2.

Sunspot Numbers for October 10 through 16 were 0, 0, 0, 0, 0, 0 and
0, with a mean of 0.  10.7 cm flux was 67.6, 66.4, 67.2, 67.6, 68.6,
69 and 68.6, with a mean of 67.9.  Planetary A indices for the same
period were 7, 7, 13, 19, 11, 5, and 8, with a mean of 10.


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