Design of a space-borne antenna for controlled removal of energetic Van Allen belt protons

Abstract

The energetic protons trapped in the inner Van Allen belt pose a risk to humans and spacecraft operating in Low Earth Orbit (LEO). These particles come from cosmic rays, solar storms and other processes, and they are a hindrance to development of space technologies. The Radiation Belt Remediation (RBR) idea has been proposed as a way to solve this problem through Ultra/Very Low Frequency (VLF/ULF) transmissions in the magnetosphere capable of inducing pitch angle scattering of the hazardous particles and precipitating them into the atmosphere. Whistler-type emissions (VLF band, tens of kHz) have been extensively studied for precipitation of energetic trapped electrons, but much less work has been devoted to the controlled removal of inner belt protons. The latter would require the man-made radiation of Electromagnetic Ion Cyclotron (EMIC) waves into the magnetosphere (ULF band, less than 10 Hz), the frequency of which is close to the cyclotron frequency of the trapped protons. In this paper we first identify the space-borne transmitter capable of radiating EMIC waves, and we estimate its radiation impedance and radiation pattern. The selected antenna configuration consists of a DC rotating coil, which is equivalent to two AC phased-orthogonal coils but with negligible self-inductance. However, the radiation resistance of magnetic dipoles is very small. For this reason, we propose a design based on superconductors and multiple turn arrangements. One of the most challenging aspects of using superconductors in space is their cooling system. This paper presents a preliminary thermal and mechanical design of a superconducting coil antenna capable of radiating EMIC waves into the magnetosphere. The coil is composed of high temperature superconducting tapes (HTS), which have to be kept below 77 K. Active thermal control and the use of cryogenics are therefore required to reject the heat coming from environmental sources. This preliminary design is used to calculate the power radiated from the antenna, its radiation pattern and its effect on the energetic proton population of the inner Van Allen belt. The feasibility of the remediation concept, as well as a scientific mission scaled down to detectability of the proton precipitating fluxes are finally addressed at the end of the paper.