Hi Bill,
Yes, large solar flares can cause radio blackouts. However, C-class
flares are not large enough to cause blackouts. It takes a large
M-class or an X-class flare to cause such blackouts.
Yesterday, propagation was negatively affected by a coronal stream
(energetic particles) from a large coronal hole on the sun. The K
index is the right thing to look at, but you need a source that shows
it throughout the day, not just at one time.
This URL shows the K index in three hour increments (as well as being
the source of the solar flux chart you referenced):
http://www.sec.noaa.gov/today.html
You can click on any of the charts to get a larger chart with an
explanation of the data it depicts.
The "Estimated Kp" chart shows that the K index was at 6 from
0900-1500 UTC and at 4 from 1800-2100 UTC. These K index readings
reflect the effect of the above mentioned coronal stream that is the
most likely cause of the poor propagation you observed.
Note that the K index shown here is a Kp, or planetary index, derived
by averaging magnetometer measurements taken at a number of locations
around the globe. The actual K index at your location and along the
path you want to use may be higher or lower than the average and thus
the propagation conditions you experience may be better or worse than
the Kp suggests.
Here is another URL that gives an excellent daily analysis of solar
and propagation conditions:
http://www.dxlc.com/solar/index.html
You can learn a lot about the sun and how it affects propagation by
exploring the links on the above two pages and the links that you
will find on those pages, etc, etc.
Best regards,
Jimmy
>Been a ham quite a while, but learned something new this weekend. I was
>operating the Volta RTTY contest which started at 1200 UTC, and for the first
>two hours, propagation was good. At around 1400, it got very poor and stayed
>that way for about two hours, then returned to near normal for a while. At
>about 1800 it got poor again and once again, stayed that way for a while,
then
>returned to normal.
>
>All this while I was checking the solar flux, A and K-indexes and
>forecasts for
>the next 24 hours but they showed little change. What's going on here???
>
>To make a long story short, take a look at:
>
>http://dx.qsl.net/propagation/
>
>and scroll down to the chart showing GOES X-ray flux (5 minute
>data). Read the
>time on the X-axis carefully and you'll see two C-class solar flares,
right at
>the times when the bands went out.
>
>Now to some folks I suppose this is old hat, but I had not realized how
much a
>solar X-ray flare could wipe out propagation without affecting the SFI, A
or K
>indexes, something I will be watching more carefully in the future.
>As it says
>in the text which introduces the chart, "If the bands seem to go dead all
of a
>sudden, it is always a good idea to check this chart to see if a large
>flare has
>occurred recently."
>
>Ain't that the truth!! :-)
>
>73, Bill W7TI
--
Jimmy Weierich, KG2AU <kg2au@stny.rr.com>
Vestal, NY USA FN12xa
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