Bob,
The official definition of sunrise is when the center of the sun is
0.8333 degrees below the horizon. Due to refraction in the atmosphere
and the size of the sun, this puts the upper limb of the sun on the
horizon under average atmospheric conditions with an unobstructed view.
Thus the official definition of sunrise is based on a visual criteria -
not an ionospheric criteria - thus confirming your suspicion that the
two are not necessarily tied together. The time of a sunrise peak more
than likely depends on when the various ionospheric regions start
becoming illuminated and then it depends on the day-to-day variation of
those same regions. Trying to pin this down could be tough.
Carl K9LA
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