On 01/23/2013 09:17 PM, k9la@frontier.com wrote:
I would think that there's a high correlation between the
polarity of the IMF and the K/A indices, so this polarity may not
tell us any more than the K/A indices.
Absolutely there is a very strong correlation. I suppose it comes
down to what one wants to know and how soon one wants to know it. A
index is a 24 hour value so it will usually not show much increase
until a disturbance has been going on for many hours, even a day. K
index (measured at Earth) is a 3 hour value but still looking at the
previous 3 hour period so there is a lag. Suppose a disturbance
starts at 0000 UTC. It won't be reported in K index derived from
Earth based magnetometers until 0300. It won't show up officially in
the A index until the following day, though SWPC does "estimate" the
daily A index starting at 1800 UTC if I recall correctly.
There is, however, a K index model which attempts to forecast K
index a few hours into the future using solar wind data as input:
http://www.swpc.noaa.gov/wingkp/
Raw solar wind data is more of an early warning system. It will show
what is coming some 30 to 60 minutes before any Earth based
magnetometer, depending on solar wind speed. It also shows what is
coming sooner than the Wing Kp model. For the average MF/HF DXer I
suppose the Wing Kp or even the previous 3 hour data of measured K
index at Earth is quite sufficient. For catching VHF auroral
propagation it most definitely is not enough, and I suppose years of
that have biased me toward monitoring the earliest information
available - even if it is in a raw state and requires considerable
experience to appreciate. I pretty much always have an eye on the
ACE plot:
http://www.swpc.noaa.gov/ace/MAG_SWEPAM_24h.html
NOTE, however, that following solar proton flares, the ACE data can
become severely corrupted and useless. This affects all downstream
models that depend on it, such as the Wing Kp and the much beloved
(by me :) OVATION models.
73
--
Paul Kelley, N1BUG
RFI Committee chair,
Piscataquis Amateur Radio Club
http://www.k1pq.org
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