|The auroral oval goes right over my head and most of the time we're right in the middle of it.|
What I had a much harder time understanding, though, was why it happened frequently that the auroral activity numbers would suggest things should probably be ok but, in fact, HF propagation would be lousy. Then I discovered the D-RAP. "D Region Absorption Predictions" are provided by NOAA and they opened up a whole new world of understanding about what's going on up there in the ionosphere especially at high latitudes. In the context of amateur radio, the lower ionosphere is cited as the reason the low bands are only open at night. The sun causes it to absorb low frequencies during the daytime. What is actually happening is that the same aurora-causing high energy protons given off by the sun tend collect in a specific region of the upper atmosphere known as the D Region. Since the earth's magnetic field tends to concentrate these particles towards the poles we have this layer overhead almost all the time. What the D-RAP predictions do is to quantify exactly what the effect of the particles (and x-rays as well) will have on any specific frequency at any particular point on the Earth. It takes the solar wind strength into account and also measures the effects of other solar particle events like flares and coronal mass ejections.
|The little chart on the right side of the image breaks down the amount of attenuation in the red areas. Click on the image for a larger version.|