Strong Amplification of ELF/VLF Signals in Space Using Neutral Gas Injections From a Satellite Rocket Engine

Abstract

The first demonstration of rocket exhaust driven amplification (REDA) of whistler mode waves occurred on May 26, 2020 by transferring energy from pickup ions in a rocket exhaust plume to EM waves. The source of coherent VLF waves was the Navy NML Transmitter at 25.2 kHz located in LaMoure, North Dakota. The topside ionosphere at 480 km altitude became an amplifying medium with a 60 s firing of the Cygnus BT‐4 engine. The rocket engine injected exhaust as a neutral cloud moving perpendicular to field lines that connected the NML transmitter to the VLF Radio Receiver Instrument (RRI) on e‐POP/SWARM‐E. Charge exchange between the ambient O+ ions and the hypersonic water molecules in the exhaust produced H2O+ ions in a ring‐beam velocity distribution. The 25.2 kHz VLF signal from NML was amplified by 30 dB for a period 77 s as observed by the RRI. Simultaneously, preexisting coherent ELF waves at 300 Hz were amplified by 50 dB during and after the Cygnus burn. Extremely strong coherent emissions and quasiperiodic bursts in the 300–310 Hz frequency range lasted for 200 s after the release. The excitation of an ELF whistler cavity may have lasted even longer, but the orbit of the SWARM‐E/e‐POP moved the RRI sensor away from the wave emission region. The amplified 300 Hz ELF waves may have gained even more energy by cyclotron resonance with radiation belt electrons as they were ducted between geomagnetic‐conjugate hemispheres.

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

“…Whistler waves passing though this region are parametrically amplified by converting the energy of the gyrating ions into intense EM signals. This process has been demonstrated by measurements with the e-POP/RRI instrument on SWARM-E using a dedicated burn of the BT-4 engine from Cygnus on May 26, 2020, during the NG-13 Mission [1]. During this experiment, amplifications between 20 and 30 dB of the 25.2-kHz signal from the Navy NML transmitter in North Dakota were recorded (Fig.…”

“…The fraction of activated ions α = n r /n e is a critical parameter for driving the REDA process in the plasma. This fraction is computed using the time-dependent models [1], [18], which couples the neutral expansion of the exhaust cloud with the background plasma using the chemical reactions from (1). A simulation of the NG-13 Cygnus experiment is used to generate the plasma densities illustrated in Fig.…”

“…Representative experimental parameters for these distributions are given as electron thermal speed v e ≈ 100 km/s, ion perpendicular and parallel thermal speed v i⊥ ≈ v i⊥ ≈ 1 km/s, ion-ring velocity V r ≈ 4 km/s, ion beam parallel velocity V z ≈ 1 km/s, and ion-ring beam thermal speed v m ≈ 0.1 km/s for a retrograde (i.e., wake) Cygnus engine burn that subtracts the exhaust speed from the satellite orbit motion [1].…”

“…1). An ELF signal at 300 Hz was also amplified by more than 50 dB during the burn [1]. These two frequencies are examined with the REDA theory.…”

“…The SWARM-E/e-POP satellite made the observations within the whistler propagation cone around the magnetic field relative to the Cygnus exhaust cloud. The measured potential on the 6-m booms comprising the RRI antennas can be converted in low-frequency electric fields with division by the boom sample distance of 3 m. The differences in the signal strengths on the two dipole-antenna channels are attributed to orientation of the dipoles and to different saturations of the input amplifiers [1].…”