Generation of ELF (30-3000 Hz) and VLF (3-30 kHz) electromagnetic radiation by conventional techniques requires the use of very large and expensive antennas. However, it is possible ot generate ELF/VLF radiowaves indirectly by heating the ionosphere with a high powered HF (3-30 MHz) radio 'heater'. Modulating the heater power at ELF/VLF frequencies leads to a modulation in the ionospheric conductivity. This in turn modulates the natural currents flowing in the ionosphere such that the heated region radiates at the frequency of modulation.

QinetiQ has developed a model that simulates the ELF/VLF radiation emitted from the lower ionosphere as a result of square wave modulated-heating from a ground-based HF transmitter. The model determines height profiles of the electron temperature variations based on calculations of the characteristic heating and cooling times of the ionospheric plasma. The total radiated power and the magnetic field strength of the ELF/VLF radiation at the ground are then calculated by modelling the heated region as a Hertzian dipole antenna.

The model assumes a circular beam with a Gaussian power profile (see figure below). The beam may be launched at any angle of elevation and azimuth. In the model, the power is cycled on and off at the frequency of the desired ELF/VLF signal. QinetiQ is also developing a model of ionospheric heating by continuous-wave, dual frequency heaters.

For a flow chart of the model, click here.

Click on images to enlarge

The graph below shows an example of the electron temperatures produced using typical model input parameters. The lowest solid curve represents the electron temperature profile (Te_min) when the heater is off. This is equal to the neutral temperature (Tn). The upper solid curve represents the maximum temperature profile (Te_Max) corresponding to the heater being permanently on. The dashed lines represent the range of electron temperatures while the heater output is modulated at 10 kHz.

Parameters produced by the ionospheric model

• Height profiles of the electron temperature variations, characteristic heating and cooling times of the electron gas
• Conductivity and current densities in the ionosphere
• Total radiated power and the magnetic field strength of the ELF/VLF radiation on the ground

Benefits of the ionospheric model

• Enables estimates of efficiency of ELF/VLF production by ionospheric heating
• Allows optimum waveforms to be found for data communication
• Furthers understanding of ionospheric heating