SB PROP @ ARL $ARLP003
ARLP003 Propagation de K7RA
ZCZC AP03
QST de W1AW
Propagation Forecast Bulletin 3 ARLP003
From Tad Cook, K7RA
Seattle, WA January 18, 2008
To all radio amateurs
SB PROP ARL ARLP003
ARLP003 Propagation de K7RA
We've seen another seven days with no sunspots. After observing the
first sunspot of Cycle 24, we hope to see more and more of these,
signaling the beginning of the next sunspot cycle and the end of
Cycle 23.
Dick Gird, K6PZE of San Diego, California wrote asking how to
distinguish Cycle 24 sunspots from Cycle 23 spots. There are two
features that differentiate spots from each cycle. Old Cycle 23
spots will appear near the Sun's equator. The first spots of Cycle
24 are at a high solar latitude, and will have magnetic polarity
opposite of the old spots. Last week there was a brief appearance
on January 11 by an almost-sunspot which disappeared by the
following day. It appeared to be a Cycle 24 spot, which had
polarity reversed from Cycle 23 spots, but it was near the equator,
which is the wrong place for a new cycle sunspot.
Tom Schuessler, N5HYP of Irving, Texas wrote to ask about the
differences between the geomagnetic A and K index. He asks, I know
that both of them are indications of the instability of the
geomagnetic field. The K index is logarithmic and the A index is
linear, and they track together -- kind of. K indexes are given
every three hours while A index readings are for a full 24 hour
period. Do the two indexes have different uses or tell a person
different things about what to expect on the air?
Based on three hours of magnetic data, a particular magnetometer or
group of them is used to track the change in nanoTeslas, which are
the international units for measuring magnetic flux density. K
index is based on changes in the flux density over a 3 hour period,
and the difference between the highest and lowest values at the
magnetometer is converted to a semi-logarithmic scale that runs from
0 to 9, yielding a K index between 0 (very quiet) and 9 (extreme
magnetic storm).
An example is the latest Boulder K index, as reported by WWV at,
http://tinyurl.com/3bsu74. Note at the end of the line giving K
index is a nanoTesla (nT) reading. At the end of a UTC day
(midnight in Greenwich Mean Time) a new A index is reported, based
on the latest eight K index values. A nomograph showing the
relationship between A and K index is at, http://tinyurl.com/3a5rmg.
Note that if you have K indexes for one day averaging 2, the A index
for that day would be 7. But if the average were 3, this
corresponds to an A index of 15, and an average of 4 equals 27. You
can see a table of three different A and K index readings at,
http://tinyurl.com/24psl3 .
They are both derived from the same magnetic readings, but the A
index is for a whole day, and has an expanded scale. When those
numbers are low, we expect less absorption and in general better
propagation of radio waves. But sometimes high geomagnetic activity
can signal improved VHF conditions, allowing distant propagation of
6 meter signals, for instance. Because there is a new K index every
three hours, this gives us a more immediate indication if conditions
are changing fast. So if WWV reports a K index of 2, then three
hours later reports a K index of 5, this indicates a dramatic event,
such as an earth-directed solar flare, or a blast of strong solar
wind, and a resulting geomagnetic storm.
Josh Sawyer, a shortwave listener, wrote, The sunspot minimum such
as in 2007 and 1986 seems to yield the best conditions for working
far away DX on the 80 and 160 meter bands, and at the same time the
worst conditions when 15 and 10 meters are dead. Does this mean
lower frequency bands such as 80 and 160 meters don't use ionized
F-layers or sunspots at all during these lows, that its just real
far E layer or groundwave and we can work these stations because
it's the period of minimum noise generated by the Sun?
160 and 80 signals do propagate through the F layer. During
daylight most of the RF energy is absorbed by the ionosphere's D
layer, but at night the D layer disappears, and 160 and 80 meter
signals can be refracted off the ionosphere. But it doesn't take
much ionization at those low frequencies to propagate signals. The
more energy charges the ionosphere, the denser it becomes, and it
will refract higher and higher frequencies. That is why the MUF
(Maximum Usable Frequency) increases as sunspot numbers go up.
With zero sunspots on January 16th, early in the morning (West Coast
Time) the MUF is at a minimum, 10 MHz, between San Diego and
Australia, as an example. So that supports 160, 80 and 40 meter
propagation.
MUF is calculated for a specific path at a specific time and season.
If you download W6ELprop (free at http://www.qsl.net/w6elprop/) you
can try out difference scenarios. If you change the sunspot number
from 0 to 150, and change the date to March 16 instead of January
16, instead of the MUF
