The Whistler Traveling Wave Parametric Amplifier Driven by an Ion-Ring Beam Distribution from a Neutral Gas Injection in Space Plasmas

Abstract

A new parametric amplifier model describes the observations of intensified whistler waves produced by a dedicated burn of the BT-4 engine on the Cygnus spacecraft during the NG-13 mission. Ground very low frequency (VLF) radio emissions at 25.2 kHz from a Navy NML transmitter in North Dakota were amplified by 20-30 dB during the Cygnus burn at 480-km altitude and recorded at 1060 km by the e-POP/RRI plasma wave receiver on the SWARM-E satellite. The amplification process starts with charge exchange between the exhaust molecules and the ambient O+ in the ionosphere to produce water vapor ions that spiral around the earth's magnetic field lines. This ion-velocity-ring distribution generates broadband, oblique-lower-hybrid (OLH) waves, which act as a pump for the parametric amplifier. The nonlinear ponderomotive force on the electrons causes the high-amplitude OLH pump to mix with the input whistler signal, yielding an OLH idler wave. Resonant mixing of the pump and idler electric fields promotes temporal growth in the amplitude of the whistler waves as they propagate through the exhaust cloud. The key features to rocket exhaust-driven amplification (REDA) process are broadband gain, bi-directionality along the magnetic field, pump depletion, phasing, and feedback. Pump depletion limits the intensity of the output whistler waves by wave energy conservation. The total wave energy is the electrostatic energy of the pump and idler lower hybrid waves plus the energy extracted by the propagating whistler signal. The whistler traveling wave parametric amplifier provides an efficient mechanism for active amplification of signals in space.

Radiation Belt Electron Acceleration Driven by Very‐Low‐Frequency Transmitter Waves in Near‐Earth Space

“…Using two-dimensional Fokker-Planck diffusion simulations over L = 1.5-3.0, we quantify the role of VLF transmitter waves and other naturally occurring plasma waves in electron acceleration at hundreds of keV in the inner belt and slot region. Based on the achievable amplification of VLF transmitter waves during the REDA process (Bernhardt, 2021;, we adopt various amplified wave intensities to determine the critical wave amplitude of VLF transmitter waves for electron acceleration to occur under the combined scattering of Coulomb collisions and all waves including VLF transmitter waves, LGW, plasmaspheric hiss, and magnetosonic waves. Our principle conclusions are as follows:…”

“…Using two‐dimensional Fokker–Planck diffusion simulations over L = 1.5–3.0, we quantify the role of VLF transmitter waves and other naturally occurring plasma waves in electron acceleration at hundreds of keV in the inner belt and slot region. Based on the achievable amplification of VLF transmitter waves during the REDA process (Bernhardt, 2021; Bernhardt, Bougas, et al., 2021; Bernhardt, Griffin, et al., 2021), we adopt various amplified wave intensities to determine the critical wave amplitude of VLF transmitter waves for electron acceleration to occur under the combined scattering of Coulomb collisions and all waves including VLF transmitter waves, LGW, plasmaspheric hiss, and magnetosonic waves. Our principle conclusions are as follows: VLF transmitter waves play a dual role in inner radiation belt electron dynamics by causing both acceleration of electrons at higher energies from ∼200 to ∼700 keV through energy diffusion, and precipitating electrons at lower energies from tens to hundreds of keV through pitch angle scattering. The electron acceleration driven by VLF transmitter waves is usually small and is suppressed by the strong pitch angle scattering due to LGW and plasmaspheric hiss, leading to overall electron flux decay due to the combined scattering effects. By amplifying the VLF transmitter waves by a factor of 5, the electrons can be accelerated under the action of the combined scattering effects of Coulomb collisions and all waves.…”

“…Although the combined scattering effects by using the statistical wave amplitude of VLF transmitter waves cannot accelerate electrons due to the faster electron loss driven by LGW, plasmaspheric hiss, and Coulomb collisions, the acceleration can nevertheless occur when the VLF transmitter waves are intensified sufficiently, which has been demonstrated in recent studies (Bernhardt, 2021; Bernhardt, Bougas, et al., 2021; Bernhardt, Griffin, et al., 2021) using REDA. Figure 3 shows the temporal evolution of the simulated electron spin‐averaged fluxes at L = 2.4 as a function of electron energy by considering (a) VLF transmitter waves only, (b) the combined scattering effects, (c) LGW only, (d) plasmaspheric hiss only, and (e) Coulomb collisions only.…”

“…Recent studies reported the detection of rocket exhaust driven amplification (REDA) of VLF transmitter waves from NML at 25.2 kHz by 20–30 dB, which is local intensification and lasts for more than 1 min (Bernhardt, 2021; Bernhardt, Bougas, et al., 2021; Bernhardt, Griffin, et al., 2021). The amplification level can easily reach higher than 30 dB in future experiment as a small amount of the exhaust kinetic energy is needed to provide substantial amplification of whistler waves in space (Bernhardt, 2021). At the same time, the preexisting extremely low‐frequency (ELF) waves with frequency of ∼300 Hz were amplified by 50 dB, which were monochromatic within the first 52 s of amplification and then broke into four sidebands around 300 Hz from 52 to 98 s. These amplified ELF waves are distinct from the plasmaspheric hiss, which is generally regarded as a broadband, structureless waves with frequency between tens of hertz to several kilohertz (e.g., W. Li et al., 2015).…”

“…Recent studies reported the detection of rocket exhaust driven amplification (REDA) of VLF transmitter waves from NML at 25.2 kHz by 20–30 dB, which is local intensification and lasts for more than 1 min (Bernhardt, 2021; Bernhardt, Bougas, et al., 2021; Bernhardt, Griffin, et al., 2021). The amplification level can easily reach higher than 30 dB in future experiment as a small amount of the exhaust kinetic energy is needed to provide substantial amplification of whistler waves in space (Bernhardt, 2021).…”