Vlf‐hf Heating of the Lower Ionosphere and Elf Wave Generation

For incident wave power densities of 10−6 – 10−2 W/m2 (at 30 km altitude), VLF heating of the D‐region (< 90 km) is found to be 2–10 times more effective (depending on power) than HF heating, resulting in comparable perturbations of subionospheric VLF probe waves in spite of up to 103 times larger power density utilized in HF heating and at least as efficient in ELF wave generation. In view of generally larger (100×100 km) areas of the ionosphere illuminated by VLF transmitters, ELF wave generation by modulated VLF heating is estimated to produce ELF power levels of ∼100 mW, comparable with or larger than those produced in typical HF heating experiments. ELF wave generation in a typical midlatitude ambient ionosphere occurs primarily via the modulation of Pedersen current whereas in a typical auroral ionosphere Hall current is dominant for pump wave frequencies up to ∼6 MHz. For 10–30 MHz and power densities > 10−4 W/m2, Pedersen current modulation is again dominant, potentially providing up to 2–15 times higher ELF dipole moment than those found in recent experiments using 3–5 MHz heaters.