An
email received from Paul Harden, NA5N
9/18/2006
6:46 AM via
QRP-L
> Bob Henning
writes:
> Hi Gang!!
> I was
wondering if anyone could
explain the what Solar flux, Sun Spot
> number, A
index and K index actually mean, and how do I use them. I use
> HFProp in
my shack, and would really like to know how these numbers can
> help to
determine when the best
times are to operate.
During the
active sun, I often try to
post various explanations of solar activity and HF propagation. Now
being in
the solar doldrums, it seems there's not much to talk about. However,
we often
forget how newcomers to QRP come into the hobby. So for those getting
interested in the solar phenomenon during the minimum, the following is
a
general description of things you will hopefully find to answer your
questions.
As others have mentioned, I would also recommend reading my FDIM
presentation
(.pdf) on the ARCI website,
www.qrparci.org.
The sun is
basically a thermonuclear
reactor at its core, producing ionizing gamma and x-ray radiation. By
the time
this radiation reaches the cooler surface of the sun, the wavelength is
much
longer ... in the radio spectrum. This produces a wideband radio output
from
the sun, which is measured on earth to represent the overall
electromagnetic
energy being generated by the sun.
It is measured
at 2880MHz, or 10cm, as a
frequency to use as the standard reference, and a frequency for which
our
ionosphere is fairly opaque and not generally effected by the effects
of solar
flares. The solar radiation measured at 2880MHz is the SOLAR FLUX,
measured
periodically and averaged over the 24-hour day. It seldom changes much
hour-to-hour (or even day-to-day).
During the quiet
sun, solar flux in the
60-100 range is typical. During the active sun, 150-200 is typical. The higher the solar flux, the more ionizing radiation
that is
striking our ionosphere, producing free electrons that stratify into
the D, E
and F layers. The more free electrons in the E and F layers, the
more
reflective they are to HF frequencies and the higher the MUF (Maximum
Usable
Frequency).
Right now, with
solar flux in the 60-100
range, the E and F layers are poorly ionized, yielding a lower MUF and
not
acting as a very good mirror for bouncing HF signals back to earth.
Very
generally, when the solar flux is around 100, 15M will be open; above
150 10M
will be open. Below 100, 20M will usually die shortly after sunset.
IMPORTANT: The
MUF seldom drops below
10MHz. Therefore, the solar flux has very little effect on 30, 40 and
80M
propagation. These bands are fairly immune from the solar flux and the
11-year
solar cycle. Magnetic disturbances on the sun produce sunspots (cooler
areas).
Occasionally,
the magnetic field lines
of the disturbance(s) grow to such an intensity
that
it produces a small hole in the solar surface, allowing hot solar mass
to
escape. This is a SOLAR FLARE. While this hole is present (usually in
the order
of minutes to tens of minutes), energetic electrons and ionizing
radiation
(that is, x-rays and sometimes gamma rays) are allowed to escape. This,
of
course, quickly increases the overall radiation output of the sun.
The ionizing
radiation, when it strikes
the earth 8 minutes later, will ionize the E and F layers, making them
more
reflective to HF and raise the MUF, usually for the rest of the day
until local
sundown. The radiation from especially strong flares can penetrate into
our
ionosphere to the D-layer. When the D-layer is highly ionized, it
becomes very
absorptive to HF signals, and in extreme cases, can produce a temporary
HF
blackout.
Most flares will
not appreciably
increase the daily solar flux; therefore, the solar flux alone is not a
good
indicator following a flare to increased E and F layer reflectivity
(and hence,
good skip DX).
As the number of
sunspots increases,
there is a higher chance of solar flares, and the daily solar flux
tends to
increase. However, there is *no* direct mathematical relationship
between
sunspot count and the solar flux. They follow the same trend when
plotted, but
no one can say 10 sun spots equals xxx
solar flux
units.
The solar flux
will vary from a minimum
to maximum value over 28-days, related to the solar rotation. It also
varies
from minimum to maximum over the 11-year solar cycle. Thus, it is a
slowly
varying indicator that is used to show the general trend of the sun for
the
current 28-day cycle, and for the current solar cycle. It is not used
for an
hourly or daily predictor.
Propagation
programs use solar flux
values primarily for calculating the MUF and what bands will be open,
or
closed, at different times of the day. When a solar flare occurs, it
often
produces a shockwave carrying electrons and other solar mass away from
the sun.
This is called a coronal mass ejection or CME.
If the solar
flare is located towards
the center of the sun (as opposed to the limbs or edges), the
trajectory of the
shockwave will intercept with the earth, usually about 50-55 hours
later. When
this happens, the shockwave will compress the Earth's geomagnetic
field,
triggering a GEOMAGNETIC STORM, generating huge electric currents
flowing along
the Earth's magnetic field lines, causing increased noise levels.
IMPORTANT: This
effect is more
pronounced on the lower frequencies, such that 30M, 40M, 80M are more
affected
by the "noise storm" than is 20, 15 and 10M. The amount of
"wiggling" or disturbance to our magnetic field is the K-Index. It is
measured every 3 hours to show what the present state of our
geomagnetic field
is. K=1 to 3 is fairly quiet to unsettled. Higher numbers (K>4) is a
geomagnetic storm. K>7 is a severe to extreme storm.
The K-Indices
throughout the day are
averaged over the UTC day to form the A-Index. It basically tells you
what our
geomagnetic field did YESTERDAY. It is usually expressed as the "Ap," or planetary A-index, being averaged over
24
hours and from all the reporting stations.
THEREFORE, The
SOLAR FLUX tells you the
general radiation output of the sun. But, don't expect it to make a
sudden jump
to open up 15 or 10M. That takes years ... or an M or X-class solar
flare.
The A-Index
tells you what our
geomagnetic field did YESTERDAY. It tells you almost nothing about what
the bands sound like TODAY. For that, find
out what the current
K-Index is. The lower the number, the better.
Above
about 6 or 7, conditions on the LOWER bands will be very rough.
WHEN TO OPERATE
For QRPers, a
good time to operate is right after a solar flare. Once the solar flare
is
over, our E and F layers are highly ionized for good reflectivity and
higher
MUF, and will stay that way for the duration of your local sunlight.
This can
cause several hours of unexpected openings on the higher bands and
fairly quiet
signals on 20M.
Conditions will
be normal the following
day or two ... until the shockwave from the flare arrives, triggering a
geomagnetic storm. This can last from a few hours to over a day. Once
the
geomagnetic storm is over (when the K-Index falls back to 1-3), our
geomagnetic
field tends to get very quiet for a day or two (unless triggered by
another
CME, though unlikely where we are right now in the solar cycle).
Therefore,
another good time for QRPers to operate is
following a geomagnetic storm when
conditions can often be fairly quiet on 30 and 40M, and sometimes even
80M. Especially at night. This is really
all you need to know to
understand the effects of HF propagation due to solar flux and the K-
and
A-Indices.
The only other
real variable is
throughout the year, our sun "shines" on the earth at different
latitudes (higher in the summer, lower in the winter for northern
hemisphere).
This changes the paths signals bounce off the E and F layers ...
whether the
"skip" is more east-west from your location, or other directions. This changes throughout the day and throughout
the year.
This is what the propagation programs primarily exploit to determine
good times
to work South America or the Middle East from your QTH.
Let's be honest.
For a QRPer, if your propagation program
says conditions are good
to the Pacific and you end up in a QSO with Israel ... who's going to
be
disappointed over that! Heck, from New Mexico, I get excited when I
work Alaska
or Nova Scotia :-)
72, Paul NA5N
> Bob Henning
writes:
> Hi Ron, I
have been doing some
research on this very subject, Is this what
> they call
a "Ten-flare" ??
Bob, and others,
In my post
yesterday, I pointed out that
2880MHz, or 10-cm., was the frequency used to measure the solar flux
since that
frequency is seldom affected by solar flares. However, once in a while
a
particularly large flare or one in the right geometry will cause the
daily
solar flux measurements to suddenly increase.
When a solar
flare contaminates the
10-cm. solar flux measurements, it is called a "ten flare" (for
"10-cm. emission flare"). NOAA tries to "smooth out" the
reading for that day, but reports it as a "ten flare" so you know the
value was likely contaminated by the solar flare.
As reported
numerous places, the solar
flux measurements are made at the Penticton Observatory in Canada.
Actually,
the proper and legal name of the facility is the Dominion Radio
Astrophysical
Observatory, or DRAO. They have been the official radio observers of
the sun
and recording the daily solar flux since the 1950s. They have a proud
and
outstanding record of virtually no downtime or missed measurements over
the
past 50 years.
72, Paul NA5N
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