ARRL

ARRL Propagation Bulletin ARLP019 (1998)

SB PROP @ ARL $ARLP019
ARLP019 Propagation de K7VVV

ZCZC AP19
QST de W1AW
Propagation Forecast Bulletin 19  ARLP019
From Tad Cook, K7VVV
Seattle, WA  May 8, 1998
To all radio amateurs

SB PROP ARL ARLP019
ARLP019 Propagation de K7VVV

It has been an amazing week for solar flares and geomagnetic
disturbances.  Suddenly the earth is being bombarded by protons, and
the intense solar wind just doesn't let up.  Average sunspot numbers
this week were over twice the week before, solar flux was almost
30%, and the average planetary A index was more than double the
previous week.  The average solar flux for the previous 90 days went
from 104 to 106, and solar flux levels were above this average for
six out of seven days this week, indicating an upward trend.

May 2-5 had severe geomagnetic storms, with global A indices of 56,
57, 96 and 36 and K indices as high as 9.  The K index is updated
every three hours, and is an indication of geomagnetic stability.  A
single unit change in the K index represents a big jump in activity.
The A index is updated daily, and is based on the K index for the
previous 24 hours.  A small change in the K index is represented as
a big change in the A index.

This past week has had extremely high levels of activity, but
actually the worst day (May 4) is not near the top of the historic
list of disturbed days.

The worst day on record had a planetary A index of 312.  That was
September 18, 1941.  The second worst planetary A index was 293 on
November 12, 1960, and the third highest was 285 on March 13, 1989.
The last time the planetary A index was 96 was on February 20, 1992,
and a few months later on May 10, 1992 the Ap index shot all the way
up to 193.  You can see a chart of records of geomagnetic storms
since 1932 via ftp from the National Geophysical Data Center on the
web at http://www.ngdc.noaa.gov, then select Solar Terrestrial
Physics, then Geomagnetic Variations, then Magnetic Data and
Indices, then Ap Index, then select FTP, then apstar.lst.

The direct ftp path via your web browser can be found at
ftp://ftp.ngdc.gov/STP/GEOMAGNETIC_DATA/APSTAR/apstar.lst.  An
interesting chart that plots a correlation between sunspots and
magnetic storms is on the same National Geophysical Data Center site
at http://www.ngdc.gov/stp/GEOMAG/image/ap_ssn.gif.

We are seeing a big increase in solar activity, but with the
increased sunspots comes a downside, with flares disrupting HF
communications, often to the point of total blackout.  The
absorption of radio waves, rather than the hoped for reflection, has
been so extreme that many HF users at first thought their radios
were broken.  On May 4 conditions were so bad that the author could
not hear WWV in the evening on 5 or 10 MHz, only a 1,000 mile path.

The predicted solar flux for May 8-10 is 120, 115, and 110, and the
planetary A index for these days is forecast at 50, 25 and 15.  If
there are more solar flares or coronal holes, then the A index will
be higher following the activity.  Monitor WWV at 18 minutes after
the hour, and note the K index, which is updated every three hours.

When the K index is 3 or lower, conditions should have recovered
somewhat.  The forecast for the next few weeks, which may be in
doubt given the sudden appearance of new active regions, shows solar
flux declining below 100 by May 15, then rising above 100 about five
days later, above 120 by May 25, and above 130 by the end of the
month.  Predicted disturbed days are May 21-23.

Sunspot Numbers for April 30 through May 6 were 74, 89, 110, 123,
117, 105 and 111 with a mean of 104.1.  10.7 cm flux was 102.5,
113.4, 117, 117.4, 121.1, 133.4 and 130.1, with a mean of 119.3, and
estimated planetary A indices were 12, 8, 56, 57, 96, 36, and 8,
with a mean of 39.
NNNN
/EX