Abstract
Efficient (~1%) electron cyclotron radio emissions are known to originate in the X mode from regions of locally depleted plasma in the Earths polar magnetosphere. These emissions are commonly referred to as the Auroral Kilometric Radiation (AKR). AKR occurs naturally in these polar regions where electrons are accelerated by electric fields into the increasing planetary magnetic dipole. Here conservation of the magnetic moment converts axial to rotational momentum forming a horseshoe distribution in velocity phase space. This distribution is unstable to cyclotron emission with radiation emitted in the X-mode. Initial studies were conducted in the form of 2D PiC code simulations [1] and a scaled laboratory experiment that was constructed to reproduce the mechanism of AKR. As studies progressed, 3D PiC code simulations were conducted to enable complete investigation of the complex interaction dimensions. A maximum efficiency of 1.25% is predicted from these simulations in the same mode and frequency as measured in the experiment. This is also consistent with geophysical observations and the predictions of theory.
Highlights
Electrons precipitating into the Earth’s magnetosphere, Figure 1, are subject to increasing magnetic field with decreasing altitude
In the absence of collisions and given that the field increases slowly compared to the electron oscillation period, the adiabatic conservation of the magnetic moment comes into effect
Those electrons having a small initial component of velocity perpendicular to the magnetic flux lines experience an increase in their rotational component of velocity as they descend towards the atmosphere. The effect of this process is that an initially primarily rectilinear electron beam assumes a horseshoe formation in electron velocity space with a significant number of electrons having high pitch angles θ = arctan(v⊥/v// ) and a region with a positive gradient in number density vs perpendicular velocity, dn/dv Figure 2. Such horseshoe electron distributions have been measured in ⊥, the Auroral Kilometric Radiation (AKR) source regions within the polar magnetosphere [2,3]
Summary
Electrons precipitating into the Earth’s magnetosphere, Figure 1, are subject to increasing magnetic field with decreasing altitude. Those simulations have demonstrated the formation of horseshoe distribution in an electron-beam subject to significant magnetic compression and subsequent cyclotron maser emission within an interaction waveguide.
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