Thanks, Bill, for noting solar cycle #17. I'm certainly no expert, but I've recently been reading-up on solar activity, and a possible relationship to Earhart's radio communication. I was wondering if the solar cycle could have been in some way responsible for the abrupt end of Earhart's communication with Itasca/Howland when she changed operating frequency from 3105 Khz to 6210 Khz.
In years when solar activity is peak, the maximum usable frequency of the F2 layer of the ionosphere is at its greatest, permitting very long 'skip' propagation at frequencies up to 30 MHz, or more. That would certainly be high enough to propagate any alleged harmonics produced by Earhart's transmitter.
I believe all layers of the ionosphere are similarly affected by high solar activity, increasing the effect of each layer. Absorption in the D-layer, which is greatest in daytime, is increased during times of higher solar activity.
The E-layer can become active and reflect radio signals from lower levels in the ionosphere than the F-layer.
Could Earhart's skywave signal, when talking to Howland/Itaska on 3105 Khz, have been bouncing back off the E-layer, at a distance greater than line-of sight? And, when switching to 6210 Khz, that frequency might have exceeded the 'maximum usable frequency' of the E-layer; A 6210 Khz signal would have to travel up to the F-layer before reflected back, returning to earth possibly hundreds of miles more distant than a signal reflected off the E-layer. With Earhart's plane beyond line of sight to Itaska/Howland, there would be no radio path available on 6210 Mhz.
I certainly don't have an answer for that. But if it were likely, or even feasible, it would put another nail in the 'ran out of fuel and crashed in the ocean' theory of Earhart/Noonan's demise.