Ionospheric layers and Ionisation basics

[hamateur 1953]

This is dealt with very well in an older article I came upon during my personal researching of the maximum frequency possible vs Solar Flux Index or SFI as reported daily by NRC.  It turned out to be a horribly difficult thing to predict and I refer the reader to the other topic 50mhz F2 propagation for the linked article that deals with the issue in depth.  But several other things arose which I hadn’t considered.  Our ionosphere as we describe it from the top consists of four main layers during sunlight.  What is it that actually ionises these layers??  In order of increasing frequency if you like:  The top layer(s) F and F2 are primarily ionised by UV radiation. Next level down is the E layer, it seems to respond and recombine more rapidly to Soft and Hard X radiation and particles as well. The topic of Sporadic E causation may be finally solved or at least better understood within the next few years I feel. It is dealt with very well by @KW2P elsewhere here.  The lowest layer us radio amateurs are concerned with is the D layer.   It is ionised rapidly by soft and hard X radiation and probably gamma radiation as well  These effects are immediately noticed on our frequencies lower than about 10 mhz. Increased noise level is also readily apparent as high as 30 mhz or higher. 
The D-RAP shown in several areas depicts only the additional absorption of our transmissions not normally attenuated or reduced in strength.  I am studying the possible contribution of m flares and above to radio wave propagation for fun and interest, I hope to be able to make a minor contribution to the study of Sporadic E myself as I find it really interesting. 73 Mike N7ORL   Edit: I should add here that another thing I discovered during researching this subject matter that while 10.7 cm flux is a good predictor of F layer behaviour in general, the relationship between it and UV radiation is not linear unfortunately further complicating predictions.  In the 50mhz referred article he made reference to the fact the the very instruments used to detect UV are ultimately degraded or destroyed by them in space.  Edit  Not two days after beginning this I witnessed live on dx maps a contact between Oklahoma and Jordan.  On 50 mhz. Likely they used FT-8 our popular digital mode and likely double hop Es.  A very long duration minor M class (1.5) from region 3590 was quite likely the enabling ioniser that permitted this. There appears to be a lot of 50 mhz activity also within the states   It will probably show up in either QST  or CQ magazine soon.  Pretty cool stuff. 

Edited February 29 by hamateur 1953
Gamma and postscripts