Numerical Simulation of VLF Antennas in Space Plasma

Energetic electrons (100 – 2000 keV), due to either natural processes or a High Altitude Nuclear Explosion (HANE), become trapped in the Earth’s magnetic field where they are a threat to satellites in low Earth orbit (LEO). One technique for removing these electrons is to mimic nature with electromagnetic (EM) Whistler waves that are known to scatter electrons onto trajectories aligned with the magnetic field, where they are lost into the atmosphere. The recent Demonstration and Science Experiment (DSX) mission successfully demonstrated transmission by an 80 m dipole antenna and reception by a remote satellite. While DSX was able to measure the dissipated power, it was unable to quantify the efficiency of radiation versus local dissipation i.e., plasma heating. Since follow-on systems must be precision engineered for a required size, weight, and power, this topic calls for a numerical model capable of representing antenna concepts in realistic detail while modeling both the local plasma response and the distance radiation including the wave mode. Whistler waves can exist in a range of modes from electrostatic (ES) to electromagnetic (EM). Current thinking holds that EM waves are more effective at modifying the electron pitch angle since the magnetic interaction consumes no energy; while antennas that are small compared to the free space VLF wavelength (100 km) preferentially excite ES waves due to the high index of refraction that matches the wavelength in plasma to the antenna size. Current antenna concepts include: the electric dipole such as with DSX, the magnetic loop antenna, the magnetic rod antenna; and engineered plasma modification for such (near the antenna) to produce non-linear beat wave interference and conversion from ES to EM radiation. The end product will be a computer plasma model capable of aiding the design of a space-based antenna immersed in the low or near Earth plasma. This model will be capable of simulating the performance of the recent DSX antenna, a 100A 20 m diameter magnetic loop, and a compact magnetic rod producing a similar dipole moment. In Phase I, EMA will identify and demonstrate the numerical methods to be used, and at conclusion have demonstrated the viability of the approach. This effort is expected to be computationally intensive so a limited demonstration will be acceptable i.e., it will not be necessary to obtain production levels of super-computer access. Never-the-less, since super-computing is an anticipated requirement, such capability must be convincingly demonstrated. In Phase II, the capability to effectively model one or more antenna concepts is expected, and a fully functional design tool should be delivered at the conclusion of Phase II. In Phase III the antenna-plasma design tool will be used to design candidate systems for the radiation of radio waves at Very Low Frequency.