Topband: Bad 160m Conditions

[Frank W3LPL]

There's some confusion about the effects of increasing solar
activity on 160 meter DX propagation.

160 meter DX propagation is often badly affected by nighttime propagation
degradations, especially as Solar Cycle 25 becomes much more active
from now through solar maximum in about 2024-2025 and as it slowly
declines to current ionization levels through about 2027-2028.

Solar flares have no know impact on 160 meter DX propagation.
Solar flares produce electromagnetic radiation that travels from
sun to Earth at the speed of light - in about 8 minutes. Solar flare
electromagnetic radiation (mostly X-rays) affects only the sunlit
side of the earth and ionosphere.  There are no known physical processes
that extend solar flare effects into the night time ionosphere.

While solar flares have no relevance to 160 meter DX propagation, solar
flares often occur coincident with (but are not caused by) coronal mass
ejections that can cause severe post-midnight absorption in the D region
on propagation paths that cross the auroral oval (e.g., North America to
northern Europe and Asia).  CMEs cause the auroral oval to dip to much
lower latitudes causing post-midnight increased D region absorption on
propagation paths crossing lower latitudes.

Unrelated to CMEs, coronal hole high speed stream effects also cause
increased D region absorption in the post-midnight auroral oval and
occur very frequently compared to geo-effective CMEs (thankfully most
CMEs never strike the Earth or its magnetosphere, they usually miss
our tiny planet).

But what about 160 meter absorption usually present much earlier in the
night, from sunset through midnight and later?

The E region usually retains enough ionization to degrade 160 meter
night time propagation especially during the more active years of the
solar cycle. The ionized night time E region causes increased absorption
at the bottom of the E region (just above the D region) and blankets
propagation that would otherwise pass through the E region to the
F region. Blanketing causes many shorter hops that suffer increased
loss from multiple lossy passes through the ionized E region.

73
Frank
W3LPL