How to read graph?

[lovewill]

Dear fellows, thank you for your time, may I know how or where can I learn to read the graph below? 

For example : what does it mean by the color green, black, blue in layman's term?

Does the black area means the radio blackout occur that part of the earth or the opposite?

[Marcel de Bont]

This is not my specialty so anyone can chime in to explain it better but the colored parts are where the blackout is occurring. The colors indicate the highest frequency (see scale below) which are affected.

[lovewill]

3 hours ago, Marcel de Bont said:

This is not my specialty so anyone can chime in to explain it better but the colored parts are where the blackout is occurring. The colors indicate the highest frequency (see scale below) which are affected.

Hi thank you for your reply, does it mean that for example the graph in the first post:

Australia's affected,

1. the frequency of around 10-13 MHz? 
2. alter the original frequency I.E. 50MHz - 10 MHz => 40MHz? 
3. the frequency in Australia's that affected can no longer be reliable? 

[Philalethes]

12 hours ago, lovewill said:

For example : what does it mean by the color green, black, blue in layman's term?

As Marcel mentioned, it refers to the gradient you can see under the image to the left; the color in a given area on the map corresponds to the highest frequency which sees 1 dB of absorption in the D layer of the ionosphere when you send a radio signal of that frequency straight up. You can use this calculator to see how much of a signal loss that corresponds to; in this case we see that -1 dB refers to a loss of ~20% of the signal strength. So the color between purple and blue is where 5 MHz signals lose that 1 dB to absorption, between blue and cyan is where 10 MHz signals lose that, and so on. Signals of lower frequencies in those areas will be attenuated even more than 1 dB.

12 hours ago, lovewill said:

Does the black area means the radio blackout occur that part of the earth or the opposite?

Opposite, as should hopefully be clear by the above. The black areas are where radio signal propagation is unaffected by the immediate changes induced by Solar electromagnetic radiation. Note also that there will be some such attenuation even in the absence of flares or particle events, simply due to regular sunlight itself, but that this milder attenuation doesn't necessarily represent a blackout.

1 hour ago, lovewill said:

Hi thank you for your reply, does it mean that for example the graph in the first post:

Australia's affected,

1. the frequency of around 10-13 MHz? 
2. alter the original frequency I.E. 50MHz - 10 MHz => 40MHz? 
3. the frequency in Australia's that affected can no longer be reliable? 

1. For Australia, what the map is showing is that around ~10 MHz (give or take depending on where exactly in Australia) would be the highest frequency which experiences 1 dB of absorption there.
    
2. No, that would definitely not be how it works. The frequency of the signals in question always remain the same. What it rather means is that if you send a 10 MHz signal straight up, ~20% (1 dB) of the signal strength is lost due to absorption in the D layer.
    
3. They certainly become less reliable, but how less reliable depends on the current level of absorption. I don't operate radio myself, so I'm not sure exactly how bad 1 dB of absorption is in practice, but I would guess that it'd still be possible to transmit at those frequencies to at least some extent; lower frequencies would likely fare worse. I know there are a fair amount of radio operators who frequent this site, maybe some of them will weigh in on what it means in practice.

[hamateur 1953]

One decibel isn’t much,  we amateur radio operators typically use something called an “ S”  unit.  Six db equals a change of one S unit.  We usually consider one S unit to be the minimum understandable signal not including the racket that usually accompanies solar flaring.  That is a basic description, hopefully this helps you understand it.    The cool colours at both poles represent polar cap absorption.  Personally I don’t worry too much about this. Although our F layer is somewhat affected by particles at lower latitudes, it isn’t much generally speaking.  

Edited February 17 by hamateur 1953
Polar cap clarification

[lovewill]

Thank you @Philalethes, that was very clear, now I understand the effects, it feels like you were inside my mind seeing the unclear questions & answering them, thank you @hamateur 1953 , it feels like you saw the worry lines in my soul, finally thank you @Marcel de Bont, for bringing people together in this quest 💯💯💯

Edited February 18 by lovewill
better phrasing