Numerical Simulations Of Abnormal Incoherent Scatter Spectra And Inversion Of Perturbed Parameters During Ionospheric Heating

During ionospheric heating campaigns,high power HF radio waves are transmitted into the ionosphere,causing the parametric excitation of strong Langmuir turbulence near the reflection altitude,which will result in the change of ion and electron distribution characteristics and affect the backscatter spectra obtained by incoherent scatter radar markedly.Thus the analysis of these spectra using incoherent scatter theory based on equilibrium plasma leads to serious error in estimation of perturbed ionospheric parameters.Based on the basic theory of ionospheric heating,the anomalous incoherent scatter spectra,especially the triple-humped spectra with zero-frequency peak near the reflection altitude are studied and the generation mechanism of them is analyzed.The Langmuir turbulence excited by ionospheric heating is numerically simulated and the ionospheric parameters are inversed by an improved inversion method.The main works are as follows:1?Based on the ionospheric heating experiment conducted near Troms? on September 6,2010,the anomalous incoherent scatter spectra around the reflection altitude are extracted and analyzed by using GUISDAP and RTG,which are two kinds of standard incoherent scatter data analysis software.The triple-humped spectra with zero-frequency peak are especially studied,and the time law of their occurrence and the temporal and spatial evolution characteristics in heating are investigated statistically.It is found that these triple-humped spectra generally appear in the period when the heater is switched on,and maintain one or two data dumps.Sometimes these spectra appear in the ?heater on‘ period because of the continuing interaction of the plasma with the strong heating waves.Besides,it shows that the triple-humped spectra can only occur within a narrow altitude region,about a few kilometers or even less.2?With the ionospheric parameters obtained from the International Reference Ionosphere 2012 and the MSIS-E-90 Atmospheric Model,the threshold fields of the instability at different times at different heating frequencies for Arecibo,Troms? and HAARP are calculated.It indicates that the parametric decay instability(PDI)has the lower threshold field than the oscillating two-stream instability(OTSI),and the threshold field will increase with the heating frequency rises.With the threshold field computed,PDI and OTSI are verified to be excited in the heating experiment mentioned above.3?Based on Zakharov model,Troms? in high latitude and Arecibo in low latitude are selected as the heating sites,and the Langmuir turbulence is numerically simulated.The wave number spectra calculated from Zakharov model can reflect the coupling relationship between Langmuir wave and ion acoustic wave apparently.It is found that the larger the driving electric field is,the faster the Langmuir turbulence saturates and the less obvious the cascade structure will be.With the simulation results from the Zakharov model,the equivalent incoherent scatter spectra are calculated,and obvious asymmetric double-humped spectra,cascade spectra and the spectra with zero-frequency peak are obtained.4?Concerning on the triple-humped spectra with zero-frequency peak,an improved method for estimating ionospheric parameters is proposed,based on the traditional incoherent scatter inversion and analysis method.By removing the PGM feature of triple-humped spectra and employing the empirical heating electron velocity distribution function,reliable parameters are obtained,which proves that the improved method is feasible and the standard GUISDAP analysis program will lead to a poor estimate of parameters on VHF data.Besides,It is found that the electron temperatures will be underestimated,with the effect of PGM peak on the incoherent scatter spectra.