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Why do HF band conditions as the A index increases.


farm24

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Hello, long-time lurker farm back and now an amateur radio operator wanting to get into HF, and I noticed that as the A index increases some if not most HF band conditions worsen, why is that? is it just coincidental?

Edited by farm24
me dummy not capitalizing
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There's a good explanation here:

https://www.swpc.noaa.gov/products/station-k-and-indices

The way I look at it, the K-index is short term and represents the maximum spikes in the magnetic field variations during the preceeding 3 hours. The number is traditional and comes from a table. I view it as sort of the logarithm of what's been going on over the past 3 hours.

The A-index is a much longer-term 24-hour average of what's really going on. The value is directly related to the magnetic variations in nT. So it swings over a much wider range but is more representative of the effects on propagation. Except for radiation bursts from flares, Earth's magnetic field and the ionosphere do not respond instantly. It takes sustained inputs to effect change and the A-index reflects that. When the A-index gets really high it means strong sustained magnetic variations have been going on for half a day or more and there will be serious effects on propagation.

When the A-index goes really high, you get some weird effects like we had on the bands today and tonight. For example, from West Virginia tonight, the 40 meter band, which is always open at night for short hops under 700 miles, even with very low power, it was not open. It was open 1,000 miles and farther but was stone dead to surrounding states I can normally reach easily. North Carolina, Georgia, nothing. Not a whisper. But Texas, Louisiana, even California, were okay.

That's my own personal interpretation that serves me.

I'd be interested in comments and disagreements with my assessment above.

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In general, high A index, high Kp index, and geomagnetic storms (all of which are correlated to the same phenomenon) increase noise on the HF bands and impair propagation.

During the recent geomagnetic storm the 10m band (28Mhz), which has been great during the day, was just noise.  The maximum usable frequency (MUF) measured at Boulder, CO had dropped significantly from the mid-30MHZ to around 15MHz during the storm.

The solar proton event https://www.spaceweatherlive.com/en/help/what-is-a-solar-radiation-storm.html may have also adversely affected propagation.

Another good web page describing how space weather affects radio  is https://www.swpc.noaa.gov/impacts/hf-radio-communications.  The pdf document linked there is particularly good.

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I can't confirm elevated noise levels (I'm logging it 24/7 from 10 kHz to 30 MHz). Apart from solar radio emissions of course, but they are usually short-lived.
The noise power spectral density was actually significantly lower due to lack of propagation.

The MUF is decreased due to heating in the F-Layers, which affects the refraction index.
On the lower frequencies, an increase of free electrons in the D-layer increases attenuation.

There are more factors, but I don't know them by heart, I'd have to look it up. IEE Ionospheric Radio from Kenneth Davies, the book goes in depth regarding ionospheric wave propagation. I can scan the chapter about geomagnetic disturbances, if there is interest.

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3 hours ago, helios said:

I can't confirm elevated noise levels (I'm logging it 24/7 from 10 kHz to 30 MHz). Apart from solar radio emissions of course, but they are usually short-lived.
The noise power spectral density was actually significantly lower due to lack of propagation.

The MUF is decreased due to heating in the F-Layers, which affects the refraction index.
On the lower frequencies, an increase of free electrons in the D-layer increases attenuation.

There are more factors, but I don't know them by heart, I'd have to look it up. IEE Ionospheric Radio from Kenneth Davies, the book goes in depth regarding ionospheric wave propagation. I can scan the chapter about geomagnetic disturbances, if there is interest.

Yes, that's my observation also. At times like last night, after a very high A-index, when propagation is suppressed, the noise is reduced because you're not hearing the aggregate noise from dozens or hundreds of distant thunderstorms. My noise last night was dominated by local power line noise.

That's not to say the ionosphere doesn't generate more noise when its upset, it does. But ionospheric noise is usually swamped by other noise sources. Solar noise is also a factor that's insignificant most of the time. But if you're in the center of a strong radio blackout (from a flare) then solar radio noise can become quite loud. You can recognize it from the rapid rise in noise level. It can rise several S-units over a period of 10 to 15 seconds, like someone turning up the volume knob. It remains constant for a few minutes, then slowly fades back to normal. When I know of a strong flare around local noon and am expecting a blackout in my region, I'll tune to a vacant frequency and listen to the noise. Kind of cool to pick up a radio signal from 93 million miles.

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I agree KW2P!  It is really cool DX (distance) However if I ever get a cw ( morse code) reply with the callsign LGM ( little green men) I’m definitely going to ask the operator if his first name happens to be Rod! 

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During the halloween-storms 2003 i was in a shack in the middle of nowhere doing ham radio, when suddenly the band went quiet. After that there was S9+ noise on all bands from 20m to 10m for over half an hour. I guess it was a type-IV burst or type-III storm. I didn't had mobile internet, so i could only guess what happened. I think it was the X17 flare.
The bands didn't recover for the rest of the day, so I went back home to check on the internet what happened.

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Good clarifications about perceived increase in noise may instead be mostly a decrease in signal.  In such a case, with little or no signal, noise dominates, so what we hear is only noise - and the absolute level of that noise in terms of S-level defined by voltage at the receiver may not be higher.

If propagation suddenly drops out inexplicably (e.g. not due to gray line), especially in the lower frequency bands, I will check to see if there has just been a solar flare (or if a cable is loose).  So far I have not had the radio on while such a radio blackout has occurred (when things should go "quiet" due to absorption), nor have I heard radio bursts (when things should get "loud").  Just have to be listening at the right time (or have a SDR and computer making continuous recordings ).

Edited by Drax Spacex
remove AGC add SDR
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The effects vary greatly with your locations relative to the auroral belts and the path to the station you want to hear/work.
But when the A-index rises after the onset of geomagnetic disturbances the attenuation will increase in any path near the polar areas.

For me up at 63N in Norway, the sectors to the east, west, and especially to the north will be attenuated by many dB.  In many cases, we can hear the other stations, but it is almost impossible to be heard due to the "auroral bubble" we are in or close to.

Here is a map of auroral attenuation from LGB at 58N.  North-east and North-West are most attenuated, and only the southern sectors are close to normal.




image.thumb.png.25c1ef74a6ab82b0268c7eeb4cb94877.png

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I've been analyzing the effect of the recent geomagnetic storm (after 26/2) on the WSPRnet spot count. It seems that mostly the upper bands were affected, but it has recovered by yesterday.

wsprnet-all.png.decbc39727bae6abe2d4ccc2fe18dcc4.pngwsprnet-upper.png.263492fac3b05671e3b965277f19a393.png

Edited by helios
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Radio blackout in progress now in North America from X2 flare.  Empirical observation from my station:  No signals from WWV at 5MHz or 10MHz.  No signals on 40m (7.0-7.3Mhz).  20m OK

S7 @ 7.2MHz

S5 @ 10Mhz

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2 minutes ago, Drax Spacex said:

Radio blackout in progress now in North America from X2 flare.  Empirical observation from my station:  No signals from WWV at 5MHz or 10MHz.  No signals on 40m (7.0-7.3Mhz).  20m OK

S7 @ 7.2MHz

S5 @ 10Mhz

Darn I was gonna do some shortwave listening 

On 3/2/2023 at 8:08 AM, helios said:

During the halloween-storms 2003 i was in a shack in the middle of nowhere doing ham radio, when suddenly the band went quiet. After that there was S9+ noise on all bands from 20m to 10m for over half an hour. I guess it was a type-IV burst or type-III storm. I didn't had mobile internet, so i could only guess what happened. I think it was the X17 flare.
The bands didn't recover for the rest of the day, so I went back home to check on the internet what happened.

Did the storm induce enough voltage in your radio to cause any damage, or is that not possible?

On 3/1/2023 at 5:27 PM, KW2P said:

Yes, that's my observation also. At times like last night, after a very high A-index, when propagation is suppressed, the noise is reduced because you're not hearing the aggregate noise from dozens or hundreds of distant thunderstorms. My noise last night was dominated by local power line noise.

That's not to say the ionosphere doesn't generate more noise when its upset, it does. But ionospheric noise is usually swamped by other noise sources. Solar noise is also a factor that's insignificant most of the time. But if you're in the center of a strong radio blackout (from a flare) then solar radio noise can become quite loud. You can recognize it from the rapid rise in noise level. It can rise several S-units over a period of 10 to 15 seconds, like someone turning up the volume knob. It remains constant for a few minutes, then slowly fades back to normal. When I know of a strong flare around local noon and am expecting a blackout in my region, I'll tune to a vacant frequency and listen to the noise. Kind of cool to pick up a radio signal from 93 million miles.

I wonder if the sun would accept a QSO card 

On 3/1/2023 at 9:13 AM, Sam Warfel said:

Welcome back, @farm24!

Thank you!

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11 minutes ago, farm24 said:

Darn I was gonna do some shortwave listening 

You can hear some at higher frequencies.  The lowest frequency shortwave broadcast station I can hear right now is at 12.050MHz.  Daytime reception is of course better at higher frequencies, and higher frequencies are not attenuated as much by radio blackout.

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2 minutes ago, Drax Spacex said:

You can hear some at higher frequencies.  The lowest frequency shortwave broadcast station I can hear right now is at 12.050MHz.  Daytime reception is of course better at higher frequencies, and higher frequencies are not attenuated as much by radio blackout.

Yea it seems like cb is open right now, but I’m mobile right now and my antenna isn’t as effective as I thought it would be so not much listening anyway lol

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1 hour ago, Drax Spacex said:

Radio blackout in progress now in North America from X2 flare.  Empirical observation from my station:  No signals from WWV at 5MHz or 10MHz.  No signals on 40m (7.0-7.3Mhz).  20m OK

S7 @ 7.2MHz

S5 @ 10Mhz

https://www.swpc.noaa.gov/communities/radio-communications shows D region absorption is subsiding.  I hear WWV at 10MHz and QSOs on 40m.  The noise floor S-levels are unchanged compared to during the radio blackout.

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2 hours ago, farm24 said:

Did the storm induce enough voltage in your radio to cause any damage, or is that not possible?

No, that won't happen. Geomagnetic storms can induce currents in large structures like pipelines or high voltage transmission lines.
My Antennas are too small for that¬†ūüėČ


Notice the shortwave blackout at the arecibo spectrogram today during the flare:

Arecibo-Observatory_20230303_174500_63_fit_gz.thumb.jpg.ce9163144bad5e07f180e0913c77108c.jpg

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