A Laboratory Investigation On Plasma Instabilities In Ionospheric Heating By High Frequency Radio Waves

The Earth’s ionosphere,the partially ionized part of the upper atmosphere,dynamically interacts with both the lower atmosphere and the magnetosphere.Meanwhile,the ionospheric plasma and its activities also significantly impact several human activities,such as radio communication and GPS/GNSS ionospheric correction.The artificial ionospheric heating experiments take the ionosphere as a natural laboratory to investigate the basic plasma physics,which has been recognized as a powerful tool to modify the ionosphere and taken as a long-term hotspot in space physics research.In active ionospheric heating experiments,ground-based high-power antenna arrays are used to launch high-frequency radio waves to the ionosphere.Through the interactions of electromagnetic waves and ionospheric plasma,the energy of high-power microwaves is transferred to the ionosphere,and the subsequent response of the ionospheric plasma can be observed.Various effects are produced by the artificial ionospheric heating experiments,such as large-scale electron density changes,which have important national defense applications.At the same time,the ionospheric heating experiments provide a good opportunity for theoretical research on space physics,without waiting for a specific occurrence of space weather events.Based on the observation of the experimental phenomena,various physical processes of space plasma,such as the excitation of instability and the mechanism of particle acceleration,have been elucidated.Since the 1970s,the heating experiments by ground-based powerful HF facilities sprang up all over the world,such as Arecibo,HAARP,EISCAT,and SUDA.Based on these HF facilities,scientists obtained many observation data and discovered a series of plasma physics phenomena.However,the diagnostic equipments are limited in the artificially ionospheric heating experiments,and it is difficult to shed light on the detailed physical processes just by the ground-based instruments or satellites.The laboratory experiments hold the advantages of controllable parameters,reproducibility,diagnosability,and reconfigurability.Hence,laboratory modeled ionospheric heating experiments could be extended to understanding the physical mechanism of the observations and validating the theoretical modeling.In this work,controlled experiments have been designed and conducted in a laboratory plasma device to study the plasma instabilities and their evolutions during ionospheric heating,especially,the parametric decay instability(PDI)was focused.The main experimental results are summarized as follows:1.Laboratory modeling of the background ionospheric plasma processes without the pump waveA large-area oxide cathode was used to generate plasma,and the ionospheric-like collisional plasma environment was modeled via releasing neutrals into the background plasma to enhance the collisions.The plasma processes of the ionospheric plasma also have been considered,for example,an inhomogeneous magnetic field-aligned current flow was generated in the ionospheric-like plasma,and the collisional electrostatic mode in the ion cyclotron frequency range indicated by Basu and Coppi was excited and observed.The collisional electrostatic mode was usually observed in the high latitude ionosphere,and it was studied in detail as the neutral-ion collisions increase.Furthermore,we found that this pre-existing background instability can significantly influence the nonlinear process in the heating experiments.2.Laboratory investigation of the parametric decay instability during HF ionospheric heatingWe carried out experimental studies on the excitation and evolution of PDI during ionospheric heating in an ionospheric-like laboratory environment.PDI is a common wave-wave interaction,which is one of the most fundamental physical processes generated in ionospheric heating experiments.In laboratory experiments,how to realize the transmission and reception of high-frequency signals are difficult technical problems.The key instruments such as the high-power amplifier,matching network,antenna,and down-conversion mixer were designed and equipped to solve these problems.The wave-wave interactions during the heating processes were investigated by spectral analysis of the probe signals.The results indicated that high-and lowfrequency modes were generated during the heating process,and the spectral characteristics were similar to the observations by radars in the active heating experiments.In addition,under different experimental conditions,the frequency matching relations of the high-frequency-wave mode,low-frequency-wave mode,and the pump waves were always satisfied,which confirmed the generation of the PDI in the ionosphere-like plasma.Furthermore,our results suggest that the existing background wave mode can significantly decrease the excitation threshold of the parametric instability,indicating that the PDI can be much more easily excited.3.Laboratory study of enhanced airglow during the heating processesThe high-power high-frequency radio waves can result in electron acceleration,and further excites the neutral atmospheric components,which results in photons are released with wavelengths similar to those of the natural aurora.Based on the existing ionospheric heating experiments,oxygen plasma was produced to study the airglow during the HF heating.The optical emission spectrometer was adopted to observe the simultaneous plasma response after the pump waves were injected.The enhanced airglow was recreated under low-pressure experimental conditions.Combined with the spectral analysis of the probe signals,the corresponding PDI could be simultaneously observed.Direct evidence was proposed that the excitation of parametric instability can accelerate electrons,eventually leading to the induced oxygen atomic airglow.In conclusion,we have experimentally investigated the generation and evolution of the plasma instabilities encountered during HF ionospheric heating.A novel experimental platform for simulating ionospheric heating has been designed and constructed.Meanwhile,various diagnostic tools including probes and optical emission spectrometer were used to observe the plasma response.We focused on the wave-wave interactions,especially PDI during the heating processes.The excitation threshold for PDI could significantly be decreased when an electrostatic wave mode was pre-existing in the background plasma.In addition,the induced airglow was observed accompanied with the PDI processes,which suggests that the electron should be accelerated by the PDI processes.