Technical Topics > Propagation
foF2 Map Discrepancy
KaySeeks:
Hey Chris,
I see quite a discrepancy between the Australian SWS real-time foF2 map:
http://www.sws.bom.gov.au/HF_Systems/6/5
and the equivalent map on the HFU propagation page:
https://www.hfunderground.com/propagation/#ionosphere
For reinforcement, I got some contemporary data from North American ionosonde data and it much more closely aligns with the SWS than the HFU maps.
Since there is really only one source for real time ionosonde data, I am going to assume that map on the HFU page is predicted by the software mentioned on the map and not based upon real time measurements. Correct?
I realize that recent ionosonde data is featured here, later on the page:
https://www.hfunderground.com/propagation/#skipzone
but I would think that this could be made a bit more clear.
ChrisSmolinski:
Hi KaySeeks,
I'm honestly not sure of the origins of the map data, but I'm happy to add any text you think would be useful to clarify things.
Exo:
The maps show different things.
One is the Critical F2.
The other is the foF2.
KaySeeks:
--- Quote from: Exo on November 27, 2018, 0521 UTC ---The maps show different things.
One is the Critical F2.
The other is the foF2.
--- End quote ---
OK great. However, if you go onto their web page for a description of what Critical F2 is (http://www.spacew.com/www/fof2.html) , you get this:
--- Quote ---The following image is a recent high-resolution global map of F2-layer critical frequencies. This corresponds to the maximum radio frequency that can be reflected by the F2-region of the ionosphere at vertical incidence (that is, when the signal is transmitted straight up into the ionosphere). ...
This map can be used to determine the frequencies that will always be returned to the Earth. Transmitted frequencies higher than the indicated contours (which are given in MHz) may penetrate the ionosphere, resulting in lost power to space. Frequencies lower than the indicated contours will never penetrate the ionosphere. Lower foF2 values indicate a weaker ionosphere and correspond to regions with lower Maximum Usable Frequencies (MUFs). Higher foF2 values indicate a stronger ionosphere and correspond to regions with higher MUFs.
--- End quote ---
"the maximum radio frequency that can be reflected by the F2-region of the ionosphere at vertical incidence (that is, when the signal is transmitted straight up into the ionosphere)" is the definition of foF2. Note that Spacew.com use "foF2" and "critical frequency" interchangeably in that last quoted paragraph.
When you look up the definition of foF2 elsewhere, it is defined as "the critical frequency", which I take to be "critical F2": https://www.wmo-sat.info/oscar/variables/view/64
So we can pretty well dispense with the thought that the two things are different.
And we are back to my original question as why these maps would be be so different. I don't know. Given that spacew.com is updating their map on their website every 5 minutes, it doesn't seem like it is is any sort of prediction of the future but rather more real time. All I can say is that spacew.com's map is a heck of a lot higher frequency than the SWS map and the ULowell databases.
Exo:
--- Quote from: KaySeeks on November 28, 2018, 0252 UTC ---
And we are back to my original question as why these maps would be be so different. I don't know. Given that spacew.com is updating their map on their website every 5 minutes, it doesn't seem like it is is any sort of prediction of the future but rather more real time. All I can say is that spacew.com's map is a heck of a lot higher frequency than the SWS map and the ULowell databases.
--- End quote ---
All of those maps are quasi-realtime, and not predictive.
The biggest difference is how each index or key is weighted in averaging, spatially, and over what timeframe, which is a matter of choice for the map programming.
The maps are based on a melding of ionosonde data from various geographically separated ionosonde sources.
The map algorithms average the differential between the ionosonde curves and that forms contours.
Also, there's the "Spread F index, or fxI, upon which the MUFs are calculated, and may be utilized for map source data.
fxI is above foF2.
Normally, on the Lowell Digisonde ionograms, you see it as a green color, scattering of points.
* 1978. Spread F index, fxI: The URSI/STP Committee, noting that a measure of the top frequency of
spread F is urgently required for CCIR purposes and also has scientific interest, and that a proposal
to introduce such an index has been widely supported by those responsible for stations, recommends
that a new ionospheric parameter denoted fxI (with computer symbol 51) be adopted for international
analysis, tabulation and normal circulation through WDCs and other publication methods, defined and
applied according to the instructions following. It is recommended that all stations at high latitudes
or subject to equatorial spread F tabulate and circulate this parameter, and that stations at
other latitudes be invited to volunteer to analyze'the parameter as a trial. Tests are particularly
important at stations where the spread of frequencies of spread F often exceeds fB/2 at certain hours.
It is very important to measure fxI at stations where spread F causes the foF2 count to be small at
certain hours. Spread F rules are given in Section 2-8.
* The parameter fxI is defined as the highest frequency on which reflections from the F region are
recorded independent of whether they are reflected overhead or at oblique incidence. Thus, fxI is
the top frequency of spread F traces including polar or equatorial spurs, but not including ground
back scatter traces. Since this parameter can be gain sensitive it should always be measured using
the normal gain ionogram. Special care is needed when foI (foI = fxI - fB/2) is near or below fB
since absorption can then hide fxI. Detailed rules are given in section 3.3.
For more information on how the foFI may affect the drawing of the ionogram or maps, see https://apps.dtic.mil/dtic/tr/fulltext/u2/a128934.pdf
The best NVIS propagation frequency usually happens between the foF2 and the fxI.
You may see it as:
(foF2+fxI) /2
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