It seems to be an obsession of the tin foil crowd.
Someday I wouldn’t mind knowing what it really is, but the wild theories just get in the way,
They use ELF to communicate with submerged submarines I think, I have no idea about HAARP though
Ultra low frequency submarine communication.
High Frequency Active Auroral Research Program (HAARP)
They use large antenna arrays to pump energy (high freq radio signals) into the atmosphere. Many theories as to what it does; everything from a sensing array for stealth aircraft, weather control, mind control, communications, long range sensors and more.
Practical and basic research. It is a phased-array RF beam designed to manipulate the ionosphere, put simply, to enable over-the-horizon radar among other things - adapting to changes in the "maximum usable frequency" as needed.