ARRL

ARRL Propagation Bulletin ARLP041 (2000)

SB PROP @ ARL $ARLP041
ARLP041 Propagation de K7VVV

ZCZC AP41
QST de W1AW  
Propagation Forecast Bulletin 41  ARLP041
From Tad Cook, K7VVV
Seattle, WA  October 13, 2000
To all radio amateurs 

SB PROP ARL ARLP041
ARLP041 Propagation de K7VVV

Solar flux and sunspot numbers were down again this week.  Average
sunspot numbers were down by over 66 points and average solar flux
by almost 43 when compared to the previous week.

Last week's bulletin ARLP040 pointed out that the average daily
solar flux values for the past three calendar quarters were all
about the same, around 182 (180.5 for the first quarter, 182.9 for
the second quarter, and 181.9 for the quarter year just completed).
The average daily flux for September was 182.1, right at this same
value.  The past week's average solar flux was 152.6, well below
this average.

We are surely at the peak of this solar cycle.  Since there is so
much daily variation in solar indices, we won't know until much
later when the peak actually occurred.  This year's values, although
higher than last year's, look flat when examining the first nine
months of this year.

Solar watchers and HF radio aficionados wonder if during the final
quarter the sun will give us some more activity, yielding a later
peak.  Since the average radio amateur does not actually see many
solar cycles during a lifetime, this is a subject of keen interest.

As a young child, the author of this bulletin missed the most
exciting sunspot cycle of all, cycle 19 in 1958.  As a young ham at
age 12 in early 1965, I joined the amateur service at the solar
minimum between cycles 19 and 20.  The peak of cycle 20 around age
17 was a bit of a disappointment, especially after listening to the
stories of the older brethren who gloried in the peak of cycle 19.

Cycles 21 and 22 around age 27 and 38 were more exciting than cycle
20, but one could only hope for another cycle 19 or better.  Now at
age 48, the probable peak of this cycle is not standing out as
anything remarkable, and the question arises of how many more cycles
will I get to see, and what will they be like?  At an average 11
years per cycle, the probability of seeing much more than two
additional solar cycle peaks seems remote, at least at this stage in
life.  One can only hope.  At least by the era of cycle 23 we have
so many more tools available than we did in earlier cycles, and with
internet connections the availability of these observation and
forecasting aids has come right down to the level of the average
ham.

We may wish for more sunspots, but along with more activity comes
more solar flares and coronal mass ejections which increase polar
absorption of radio signals.  These events are often interesting to
aurora watchers and VHF enthusiasts, but can be a problem for HF
communications.  October 5 was a day of big geomagnetic disturbance,
when the planetary A index reached 96 and there was a sustained
period when the K index was seven, indicating a severe geomagnetic
storm.  The high latitude indices were worse, with Alaska's college
A index at 105 and K index as high as eight.

Geomagnetic indices were very quiet from October 6-9.  In fact, on
October 8 Alaska's college A and K indices were zero for the entire
day, which is highly unusual.  Solar flux reached a recent short
term minimum of 139.6 on October 10.

As this bulletin is written, activity is again increasing.
Planetary A index is expected to rise to 35 on Friday, October 13,
then hit 15 the next day and 12 for Sunday and Monday.  Solar flux
values for the same four days are predicted to be 170, 180, 195 and
205.  The current rise in geomagnetic activity is due to a full halo
coronal mass ejection at the end of the UTC day on October 9.  This
resulted in an interplanetary shock wave that hit earth's
magnetosphere at 2330 UTC on October 12.

Solar flux is expected to peak at 220 around October 18 and 19, and
not dip below 200 until October 28.  The next short term solar flux
minimum is expected around November 5-7.  Average solar flux
predicted for the next 45 days is 191, which is a bit higher than
the average for the first three quarters of this year.  Of course,
this could be an artifact of the period we are looking at, which
includes two upcoming peaks based on the 27.5 day solar rotation.

Sunspot numbers for October 5 through 11 were 145, 127, 94, 128,
106, 95 and 131 with a mean of 118. 10.7 cm flux was 173.8, 158.1,
155.6, 148.9, 140.8, 139.6 and 151.4, with a mean of 152.6, and
estimated planetary A indices were 96, 6, 7, 5, 5, 12 and 14 with a
mean of 20.7.
NNNN
/EX