Nascap-2k Self-Consistent Simulations of a VLF Plasma Antenna
We simulate the plasma response through tens of cycles of the Demonstration and Science Experiments (DSX) kilovolt near-square-wave very low frequency (VLF) (0.1-50 kHz) antenna in Medium Earth Orbit plasma with Nascap-2k. DSX is an Air Force Research Laboratory experiment that will explore the physics of electromagnetic wave injection from space transmitters, wave propagation, and wave-particle interactions in the magnetosphere. DSX will transmit and receive VLF waves and quantify their effect on the trapped electron populations in the magnetosphere. In these preflight calculations, the plasma is modeled with a hybrid particle-in-cell (PIC) approach with PIC ions and fluid barometric electron densities. The plasma response, collected ion currents, and chassis floating potential are computed self-consistently with a near-square-wave bias applied to the antennas. Particle injection and splitting are used to replenish the plasma depleted at the boundary, represent the thermal distribution, and maintain appropriately sized macroparticles. Therefore, limitation of current due to the thermal distribution of ions and the resulting angular momentum barrier is included. Above the ion plasma frequency, the plasma current lags the voltage by about 10, while below the ion plasma frequency, the current leads the voltage by about 7 . The volume ion current shows periodic launch of blocks of energetic ions radially outward.
Evidence for the resonance precipitation of energetic electrons from the slot region of the radiation belts
An investigation is made of the energy spectra of energetic electrons precipitating from the slot region between the inner and outer radiation zones (L ≈ 2 to ≈3.5). This represents an extension of a previous study in which we reported the discovery of peaks (having strongly L-dependent central energies) often occurring in the spectra of electrons precipitating from the outer edge (L ≈ 1.4 to ≈ 1.8) of the inner radiation zone. Quasi-trapped electrons (hmin < 0) are often observed to have exponential spectral shapes in the slot region, but a variety of other spectral shapes can also occur. On certain passes, L-dependent peaks are found in the spectra of the precipitating electrons. However, the peaks are generally broader and less pronounced than those found at the outer edge of the inner belt. In the slot region up to L ≈ 3.5 the dependences of the central energies of the peaks upon L, on a given satellite pass, are generally consistent with the energy variations calculated for first-order cyclotron resonance at the equator with waves of a constant frequency in the range of a few hundred hertz to about 2 kHz propagating parallel to the magnetic field. This frequency band is consistent with the measurements by Thorne et al. (1973) of plasmaspheric hiss occurring over a broad range of L shells throughout the plasmasphere. Within the limits of available data on equatorial electron densities the frequencies required for cyclotron resonance can be put into agreement with the wave band assumed by Lyons et al. (1972) to account for the high electron loss rates in the slot region.
Enhancing whistler wave-Electron interaction by the use of specially modulated VLF wave injection
It is shown that interactions between artificially injected whistler mode waves and electrons can be strongly enhanced if the frequency and amplitude of the injected signal are appropriately programmed. The frequency variation produces efficient and steady changes in the particles energy and pitch angle through an extension of the cyclotron-resonance interval. The amplitude modulation might prove necessary in certain interactions to avoid instability of the phase trapping mechanism. Sixteen interactions evolving in a duct centered at a geomagnetic field line with L=4 are studied by following the (test particle) evolution of the equatorial pitch angle and energy, and obtaining the required (ground) transmitter spectrogram format. Transmission using the proposed modulation with sufficient accuracy to match the real magnetosphere appears feasible and could be attempted in the near future.