I’ve stumbled on two pieces of info related to this in the past couple of days so reckon it’s worth making a note.
The first is NASA’s Van Allen Probes Spot Man-Made Barrier Shrouding Earth, actually about high-powered VLF transmitters for ground-submarine comms, probably affecting the near-space environment. “A number of experiments and observations have figured out that, under the right conditions, radio communications signals in the VLF frequency range can in fact affect the properties of the high-energy radiation environment around the Earth”. The main reference paper is on a pay-for site, so little detail is at hand.
The second is Why I Quit Natural Radio, a post by a Radio Nature enthusiast who’s been monitoring ELV/VLF for decades, noting a massive drop off in the ‘interesting’ natural signals he receives. He suggests the cause may be the rise in the use of mobile phones, associated UHF/microwave emissions (‘Cellular frequencies‘) affecting the magnetosphere and/or ionosphere thus impacting VLF propagation. A term he’s coined is rather disturbing ‘electromagnetic smog’.
He also refers to HAARP, a research system (and favourite of conspiracy theorists) that has historically blasted the ionosphere with high power HF. According to official sources it hasn’t been used for a long time. I have my doubts that relatively brief, localised high-energy signals like these would have any lasting impact – similar events might well occur in nature, and these natural systems tend to be very resilient.