There appears to be two issues with sunrise and propagation on the low
bands.
First, the signal enhancement around sunrise that W7LR mentioned is
believed to be due to tilts in the E and F regions at sunrise, bringing
signals out of the duct in the electron density valley above the E
region peak in the nighttime ionosphere. See Nick VE7DXR's feature in
the July/August 2001 QEX, which includes plots of medium frequency
enhancements and a side-view look at the ionosphere around sunrise.
Second, the D region ultimately limits daytime propagation on the low
bands (to roughly 1500km on 160m CW for good stations), which is mostly
due to solar radiation at 121.5nm ionizing nitric oxide at D region
altitudes. When this starts is not only dependent on when the D region
"sees" the sun, but it also is affected by the ozone layer delaying this
radiaton in reaching the D reigon. Bob NM7M has written several articles
about this issue.
Carl K9LA
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