Technical Topics > Propagation
Question about the ionosonde charts
MDK2:
I've been using the ionosonde charts a bit more frequently, now that we're getting into sporadic E season, but I've referred to them a bit over the past couple of years since they're on the propagation page, and thankfully I live very close to Boulder and can take their readings as essentially identical for my local conditions.
I have wondered about the spikes you see that don't follow the general line of the MUF. Earlier today there was a single reading on the foF2 chart that looks like it might have actually been higher than the chart can illustrate, since it tops out at 12 MHz. (Also, 1100 UTC is 5:00am here, about 50-55 minutes prior to sunrise. Just in case that matters.) But there are periodic spikes every day, maybe just not that high. My question is, are those readings some kind of outliers, or perhaps false readings? Or do they indicate very brief moments of sporadic E? (Or something else?)
ChrisSmolinski:
--- Quote ---I have wondered about the spikes you see that don't follow the general line of the MUF. Earlier today there was a single reading on the foF2 chart that looks like it might have actually been higher than the chart can illustrate, since it tops out at 12 MHz. (Also, 1100 UTC is 5:00am here, about 50-55 minutes prior to sunrise. Just in case that matters.) But there are periodic spikes every day, maybe just not that high. My question is, are those readings some kind of outliers, or perhaps false readings? Or do they indicate very brief moments of sporadic E? (Or something else?)
--- End quote ---
That's a good question. I have always interpreted the spikes to be false readings. I did a quick search, and found this potential answer (search for spikes): https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2007RS003742 "Nighttime above-the-MUF HF propagation on a midlatitude circuit"
--- Quote ---The spikes in power that rise above 70 dBW, such as at 06 UT in Figure 1, are probably the result of reflection by sporadic E (Es) layers at the circuit mid- point, although there is no one-to-one correspondence between their occurrence and the occurrence of Es on the ionograms. This lack of correspondence is not surprising, because the correlation distance for Es layers is 100 km, and the ionosonde is 240 km from the circuit midpoint. The blanketing frequency for the Es layer would need to be 3.0 MHz for the layer to reflect 7.335 MHz signals. The levels reached by these spikes tend to be higher for longer durations, suggesting that the Es clouds then have larger horizontal sizes.
--- End quote ---
The section in the paper in question is not about ionosondes, but measurement of signal strengths from CHU. The paper is titled NIGHTTIME ABOVE-THE-MUF HF PROPAGATION, and looks like a fascinating read, I'll be studying it later. I don't know that this confirms the ionosonde spikes are real (sporadic E) vs glitches, but it sounds possible.
MDK2:
Cheers Chris. Food for thought for sure.
ChrisSmolinski:
It was an interesting paper. To summarize, they propose the propagation mode when foF2 is too low is via ground side scatter. "In the ground side-scatter mode, the transmitted signals are scattered from the ground at a range exceeding the skip distance, and thence to the receiver. "
We do receive stations (on 43 meter for example) when it should not be possible via normal propagation, based on the real time foF2 values, so this could be one way we're hearing them. Another mechanism is sporadic E, of course.
ChrisSmolinski:
Speaking of propagation above the MUF... my logging of The Relay Station the other night: https://www.hfunderground.com/board/index.php/topic,42507.0.html
Based on foF2 values I shouldn't have heard anything. But I was actually getting a fairly decent signal at times. He said he was running 500 watts, so significantly more than the typical 20 plus/minus watts of a MOSFET type transmitter. Several S units right there, plus a decent antenna on each side of the circuit.
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