Tampering with the Van Allen Belts is easy and fun; and I wonder what precipitating electrons look like.
Electromagnetic Ion Cyclotron (EMIC) Waves for Radiation Belt Remediation Applications
Maria de Soria-Santacruz (PhD Candidate), Committee: Prof. Manuel Martinez-Sanchez (Professor), Dr. Gregory Ginet, Prof. David Miller, Prof. Jeffrey Hoffman and Prof. Kerri Cahoy
Motivation and Objectives
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 solve this problem through ULF/VLF transmissions in the magnetosphere, which will create a pitch-angle scattering of these energetic particles. A portion of the particles would then fall into their loss cone, lowering the altitude of their mirror point to a level where they are absorbed by the atmosphere.
The possible utilization of Whistler waves for precipitation of high-energy trapped electrons has been studied extensively [e.g. Inan et al, 2003], and a space test of a linear antenna for this purpose is in preparation [Spangers et al, 2006]. The lower frequency EMIC band has also been studied in the context of electron precipitation [e.g. Albert and Bortnik, 2009], but much less study has been devoted to the use of the left-hand polarized branch of EMIC waves for ion precipitation.
The possible utilization of Whistler waves for precipitation of high-energy trapped electrons has been studied extensively [Inan et al., 2003], and a space test of a linear antenna for this purpose is in preparation [Ginet et al., 2009]. The lower frequency EMIC band has also been studied in the context of electron precipitation [Albert and Bortnik, 2009], but much less work has been devoted to the use of the left-hand polarized branch of EMIC waves for proton precipitation. This study aims at characterizing the ability of Electromagnetic Ion Cyclotron (EMIC) waves to precipitate energetic protons and electrons trapped in the Van Allen belts, and to translate these findings into engineering specifications of a spaceborne RBR system able to significantly reduce this energetic radiation. In order to fulfill this goal, the following objectives have been defined:
1. Determine the type of antenna able to radiate EMIC waves in the magnetospheric plasma. This is a largely unexplored territory that should be addressed, given its potential practical importance. Linear full-wave models to calculate the radiation pattern and radiation impedance in the far-field region due to a spaceborne electric dipole have been developed [De Soria-Santacruz, 2011; Wang and Bell, 1969]. However, the sheath around a space-based RBR antenna is very thick, and so its capacitance is almost the vacuum capacitance, which is nearly independent of the frequency and proportional to the transmitter length. The associated reactance is extremely high for the EMIC band to the point that it is not possible to use an electric dipole to radiate these waves without the help of any other device. On the other hand, in terms of the radiation resistance, a short antenna would be ideal, because the relevant wavelengths (those near the resonance cone) are indeed very short; unfortunately, short antennas suffer the most from the small capacitance problem, although even a multi-km antenna would have too much reactance at the EMIC regime. This study deals with two innovative ways to emit these waves; the first option involves plasma contactors at both ends of a linear dipole, thus avoiding oscillatory charge accumulation. The second case under consideration consists of a magnetic dipole working as an EMIC transmitter.
2. Characterize the radiation impedance and radiation pattern of this antenna in the far-field region.
3. Study the cold plasma wave-propagation of the EMIC band radiated from the proposed transmitter. In order to do that we will need to modify previously developed ray-tracing codes, which are able to handle Whistler waves.
4. Characterize the interaction of EMIC waves with the energetic population of particles in the belts. The waves are considered monochromatic and propagating at an angle to the geomagnetic field. Similar studies have been previously developed for Whistlers interacting with electrons, but no attention has been paid to the lower frequency and its interaction with both high-energy protons and electrons. This model uses a Lagrangian formulation involving a test particle simulation of the nonlinear equations of motion [Inan et al., 1978] to reproduce the interaction between the distribution of energetic particles and EMIC waves. This formulation allows one to deal with coherent and narrow-band waves, which are fundamentally different from those produced by incoherent signals. In the later case the particles perform a random walk in velocity space, whereas during the interaction with a coherent wave individual particles are not scattered randomly, but they stay in resonance long enough for the particle's pitch angle to be substantially changed through non-linear interactions. This model takes into account the oblique propagation of coherent EMIC pulsed waves in a multi-ion plasma and their interaction with the energetic protons and electrons in the Van Allen belts. i. Study the scattering of the magnetospheric energetic distribution using a test particle method.ii. Study the scattering of a single particle. This analysis determines the region in velocity space that includes all particles that can resonantly interact with the waves, which is an input to the distribution function.
5. Characterize the feasibility in terms of power levels, frequencies, voltages, currents and mass of a potential spaceborne RBR antennae capable of significantly reduce the energetic radiation in the belts. 6. Estimate precipitated fluxes and compare them with the values of typical background precipitation.
“Oh for shame, how these mortals put the blame upon us gods, for they say evils come from us, when it is they rather who by their own recklessness win sorrow beyond what is given…”
~Homer
