Those of us that like to hunt European DX on 160m from the west coast know that the best time for this is during the 'solar low' years, those quiet periods between the end of one solar cycle and the beginning of the next.
From the west coast, openings to Europe on 160m are not something that happens with much regularity and, unlike the more frequent paths to Europe enjoyed from the east coast, are almost exclusively limited to this quieter part of the solar cycle. The weakening of the Sun's magnetic field at these times allows for less prop-killing D and E-layer signal absorption, particularly through the northern auroral zone path required from the west coast.
In his October, 2016, posting to the Topband reflector, propagation guru Carl Luetzelschwab, (K9LA), suggested that the coming years of solar lows may actually be too low and that because of the likely unprecedented low levels not seen in our lifetimes, the planet could receive higher cosmic-ray bombardment than normally associated with these periods.
"Since galactic cosmic rays are mostly *very energetic* protons, they can get down to low atmospheric altitudes, causing collisional ionization in the D region (and lower E region). A cursory estimate using cosmic ray ionization rates confirms more ionization in the lower atmosphere. 160m is not very tolerant of more absorption, so we may see an adverse effect of the weakened solar magnetic field."
K9LA's Topband comments seems to have its roots in his May, 2015 article, "What's Going On With-160 Meters?", where he compares the solar minimum period between Cycles 22 and 23 to the minimum years between Cycles 23 and 24. Carl noted that the best 160m propagation period that he had seen in his lifetime was during the years between Cycle 22 and 23 and pondered why, during the even deeper prolonged low between Cycles 23 and 24, was it not producing the same levels of great propagation observed 11 years earlier. One possibility he puts forward was that ...
" ... it involves galactic cosmic rays (GCRs). At solar maximum, the Sun is more active, causing more geomagnetic field activity that is believed to be detrimental to 160-Meter propagation. Coupled with the Sun being more active is the fact that the Sun's magnetic field is stronger, which shields the Earth from galactic cosmic rays. Going the other way, when we're at solar minimum, the Sun's magnetic field is weakest, letting in more cosmic rays."
His graph shows the yearly trend of only the low Ap index days (geomagnetically quiet) versus smoothed sunspot numbers for several recent cycles. The blue line plots the trend of low Ap index values with the black line showing the smoothed values; the red line indicates the smoothed sunspot number (solar activity levels).
Carl's earlier observations indicating that the best 160m propagation he had ever observed was during the low period between Cycle 22 and 23 and not during the much quieter low period between Cycle 23 and 24 are very much different than my own ... perhaps because of our different locations.
From the west coast, the most challenging topband path is over the pole to Europe. This only occurs during 'best propagation' periods as this path will only open during prolonged periods of very low geomagnetic activity. Unlike Carl's path to Europe, west coast signals need to traverse the signal-killing auroral zone.
During the first low period, I did experience several openings to Europe but nothing compared to what they were during the second low period, between Cycles 23 and 24, the one Carl did not experience propagation as good as the previous low. For several winters in a row, during the 23-24 low, I often found night after night of amazing propagation to Europe, the quality of which I had never heard before. Interestingly, on almost all of these nights, there were no other signals on the band but Europeans and nearby Washington or Oregon state W7s ... no signals at all from the rest of North America. At times it mimicked the sound of 20m CW to Europe, with signals often reaching S9 on my FT-1000 S-meter. I even worked one SM station on CW while running just 10W output!
With this long intense low, cosmic ray bombardment should have been at an all time high ... maybe it was, but it didn't seem to be bothering the west coast path to Europe, via the seemingly dormant auroral zone.
I was prompted to address this topic after reading a recent report on the GeoSpace website, siting a new study led by Nathan Schwadron, professor of physics at the University of New Hampshire’s Space Science Center. In the study, recently published in the journal Space Weather, a publication of the American Geophysical Union, the researchers found that large fluxes in Galactic Cosmic Rays (GCR) are rising faster and are on a path to exceed any other recorded time in the space age.
The author's study predicted a 20% increase in radiation bombardment but their newest research shows current conditions exceed their predictions by about 10 percent, showing the radiation environment is worsening even more than expected.
With cosmic ray levels now predicted to increase by a whopping 30%, Carl and the rest of us may soon get some clarity on his original postulation that "maybe a solar minimum can be too deep for 160 meters."
With the next few cycles expected to be even poorer than the present one, the large increase in radiation levels from space may have profound impacts on more than just propagation ... satellites and, with a new appetite to return human activity to the moon, astronauts could be exposed to much higher radiation levels than ever before.
The next few years of (ultra?) solar-quiet should be very interesting!