Principal Investigator Manuel Martinez-Sanchez
Co-investigators Jeffrey Hoffman , David Miller , Kerri Cahoy
Project Website http://web.mit.edu/aeroastro/labs/spl/research_maria.htm
The high energy particles of the Van Allen belts coming from cosmic rays, solar storms, high altitude nuclear explosions (HANEs) and other processes represent a significant danger to humans and spacecraft operating in those regions, as well as an obstacle to exploration and development of space technologies. The "Radiation Belt Remediation" (RBR) concept has been proposed as a way to try to solve this problem through VLF/ELF transmissions in the ionosphere, which will create a pitch-angle scattering of these energetic particles with some of them falling into their loss cone, thus reentering the Earth.
The aim of this project is to determine the radiation pattern from an Electromagnetic Ion Cyclotron Wave (EMIC) antenna located in a space-based RBR system, which would probably involve a constellation of spacecraft radiating EMIC waves from very long tethers (several kilometers).
An analytical model of the propagation and radiation problem is being developed. The plasma is considered anisotropic due to the external Earth's magnetic field and is assumed to have sufficiently low density, temperature and degree of ionization so that thermal velocities and collisions can be neglected. Using the stationary phase method, an asymptotic analysis is developed to calculate the fields and power flux radiated by the tether that reach a specified observation point located in the far field region.
The effect of the antenna-plasma interaction is being studied by adding to the conventional triangular source current distribution along the antenna a radial current arising from the sheath region. The impedance of the tether is being determined and compared to previous models. A methodology to study the near field case close to the tether is also being developed, which is applicable in the limits of the linear approximation.
The next milestone is to couple the linear approximation detailed above with a numerical model of the sheath region surrounding the antenna. This coupling will allow obtaining the self-consistent source current distribution and a closed picture of the radiation and propagation problem.
Results -- The far-field Poynting vector radiated from an EMIC tether for two orientations of the antenna, a = 0°„ and ¶¡ = 90°„, where ¶¡ is the angle with respect to the magnetic field of the Earth. ¶»x represents the spherical angle of the observation point and Y is the ratio between the driving frequency and the cyclotron frequency of protons. If the tether is aligned with the external magnetic field, we show that the radiation is concentrated around the angle corresponding to the resonant condition of EMIC waves. On the other hand, if the antenna is located perpendicular to the external magnetic field, the radiation is concentrated in the region parallel to the field. The addition of the current arising from the sheath does not have a noticeable effect in the far-field region, except for the appearance of a second harmonic. In the EMIC range of interest, this second harmonic does not propagate, but it does in the Whistler range. The model developed so far taken to the very low frequency limit (w << wci) shows good agreement with previous studies that calculated the radiated fields from a dipole antenna working in the Alfv®¶n range. In the close-field case (outside the sheath), the addition of the radial current arising from the sheath seems to have an important role in the determination of the radiation pattern, although its behavior is not well understood yet. It is part of the future work to interpret these results and improve our understanding of the radiation pattern in the close-field region.