The Missing Link

For years and years I tried to understand HF propagation from the Arctic.  At my location just above the Arctic Circle it was fairly easy to figure out when it would be bad.  The sun throws off high energy protons from the solar wind or solar energy particle events like flares and coronal mass ejections. The charged particles interact with the earth's geomagnetic field and the north and south poles of this magnetic field attract the particles just like iron filings on a magnet.  When the particles get dense enough to collide with molecules in the upper atmosphere they emit light energy visible at night as the aurora (although its still there in the daytime, just not bright enough to be visible).  Extreme levels of aurora tend to reflect higher radio frequencies (like 6m and VHF) back in the general direction of their source.  This can be great if the auroral oval is just to the North (or South) of you, not so great if you're right in the middle of it!  Radio frequencies in the HF range are scattered by the aurora and lose most of their energy with the majority of the attenuation occurring on the lower HF bands.

The auroral oval goes right over my head and most of the time we're right in the middle of it.

If the K-index was high that would indicate auroral activity and I knew to expect poor conditions. The 160 and 80 meter bands would go completely dead and usually 40 meters as well.  Sometimes the higher bands held up a bit better but as the aurora becomes more intense the degradation increases in frequency.  It was a simple matter to take a peak at the K-index and I knew at a glance how bad things would be.  

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.

The NOAA D-RAP site  provides the data in a pretty easy to understand set of graphics that show the level of absorption (in dB) at specific frequencies.  Now its easy to see the piece of the puzzle that had been eluding me for so long.  Solar particle events throw off huge amounts of charged protons and if they are directed towards the earth they stream into the upper atmosphere for days afterwards.  A quick check of the data at any time will show which bands are affected and how much.  By combining this lowest usable frequency (LUF) data with the maximum usable frequency (MUF) calculated by the solar flux index one can instantly know which is the optimum HF band for paths from (or through!) the Arctic in real time.  Unfortunately for me and the other high-latitude operators, periods of low solar activity tend to lower the MUF.  When a solar particle events occur right after or during these quiet times the net result of a low MUF and a high LUF is no HF propagation from up here on any band!  The trick is being on the air at the just right time to catch those opportune moments when the flux is high enough to support propagation on the upper bands but with low attenuation from the D Region.  That happens just often enough keep me tuning the bands waiting for these interesting opportunities to communicate.