A Study Of Polar Ionospheric Heating Experiments And Abnormal Incoherent Scattering Spectra

As an important part of the earth space environment, the ionosphere can be studied by artificial active experiments with the progress of science and technology. Ionosphere can be modified by high powerful HF electromagnetic waves, and many nonlinear phenomena can be induced in ionospheric heating experiments. Incoherent scatter radars have been the major diagnostic tools for its favorable accuracy and multiple parameters. In this paper, the enhanced ion- and plasma line spectra are present, which observed by both VHF and UHF incoherent scatter radar in polar ionospheric heating experiments. The physical mechanisms of abnormal spectra are discussed and the inversion method of ionospheric parameters in strongly disturbed region is studied. The main achievements are as followed:1. The enhanced ion and plasma line spectra are analyzed,which were observed in polar ionospheric heating experiments carried out on 13 September 2010. The long-lasting enhanced ion and plasma-lines are obtained with the VHF incoherent scatter radar. In previous experiments, the enhanced ion lines observed by VHF radar only lasted 200 ms. Observations of enhanced ion and plasma lines lasted 8 minutes in the VHF radar have not been reported before. The shape of enhanced ion line spectra is asymmetry double-humped structure, and the right one is larger than the left. The enhanced lines may be caused by Parametric Decay Instability(PDI) due to the analysis of the plasma spectra, and the possible requirement for the long lasting PDI is given through the comparison of the experimental conditions. Then another long lasting PDI experiments are achieved in on 20 November 2011. In addition, the VHF data show that the enhanced ion lines are descending in altitude with time during the heater-on period. Likewise, the temporal and spatial evolutions of the enhanced plasma lines also show a similar morphology. A possible mechanism for the altitude evolution is given in this paper.2. An obviously increased electron density is observed by EISCAT incoherent scatter radar in the polar ionospheric heating experiments. The increasement is up to 269.3% above the background level around the reflection height, and it could be sustained 30-50% at the altitude range of 300-500 km where is far away from the heater interaction region. The increased electron density observed at 300-500 km is verified by the analysis of ion lines and residual error in this experiment. Through the analysis of plasma line spectra, a double-peaked spectrum is obtained. The suprathemal electrons may be the reason for the large increasement of electron density in this experiment based on the relationship between the electron velocity distribution function and the plasma line spectrum. In addation, the temperal evolutions of enhanced ion and plasma lines are studied, and the suprathemal electrons may be caused by electron gyroharmonic effects.3.The ion line spectra are simulated with Bump-on-tail distribution, and the asymmetry double-humped spectra observed in 2010 could be obtained from the simulation. The temperal evolation of reflection height is simulated with ohmic heating theory neglecting the influence of electron temperature, and the reflection height can be descent with time due to the enhanced diffusion. Thus the altitude evolution of enhanced lines observed in 2010 may be caused by the enhanced plasma diffusion in ionospheric heating experiments. The plasma line spectra are simulated with the suprathemal electrons distribution, the simulation result is good agree with the observations in ionospheric heating experiments in 2011. Then the onization ability, horizontal and vertical free path of suprathemal are calculated using the velocity parameters used in the plasma lines simulation, and the hypothesis that the large increasement of electron density in this experiment may be caused by suprathemal electrons is proved to be reliable.4. An inversion method for ionospheric parameters of strongly disturbed region in ionospheric heating experiments is developed by using the cascade spectrum induced in PDI. Electron density inversion method is given under the condition of ion plasma wave resonance, which is verified by a quantitative comparison with the traditional Grand Unified Incoherent Scatter Design and Analysis Package(GUISDAP) inversion that utilizes the reliable UHF radar data to hold an average deviation of ~ 1% for the inversion results. In addition, electron temperature and density could be inversed under the condition of ion acoustic wave resonance. The method using the cascade spectrum is of significant importance to evaluating the parameters of strongly modified ionosphere region and improving the understanding of the nonlinear process of PDI.