Statistical Analysis And Research Of Polar Mesospheric Winter Echoes

The polar mesosphere winter echoes(PMWE)are radar echoes detected from the height region 50-80 km in the polar during winter month.Its physical mechanism has not yet conclusive.The main purpose of this paper is to analyze the average characteristics of PMWE by using statistical data of PMWE from 2003 to 2014 based on the EISCAT VHF radar.PMWE are extremely rare phenomenon with mean occurrence rate of about 3.26% in this paper.Diurnal variation of PMWE show that the highest rate of time is 11-12 am,and its proportion is up to 21.8%.92% of PMWE appears in the time period between 8am and 15 pm,that is,most of PMWE appear in the daytime,the lowest incidence at night,which indicating that solar radiation is essential to the formation of PMWE;As for the height distribution of PMWE,the height region of PMWE from 2003 to 2014 is 57-77 km,and the proportion of PMWE within height range 66-67 km is the highest;about 55.8% of PMWE lies in 65-68 km,that is,more than half of PMWE lies in 65-68km;PMWE within height region 62-70 km account for about 86.6%,and below 62 km is very small,in other word,PMWE mainly occurs in height range 62-70 km.Seasonal variation of PMWE display that incidence of PMWE during November is the highest,up to 9.4%,the second is during October,the lowest rate is during December.Annual variation of PMWE reveal that PMWE is characterized by high incidence,strong radar reflectivity,long duration and enhanced height range during solar maximum.The correlation analysis between AP/F10.7 and annual incidence rate of PMWE respectively show that annual occurrence of PMWE has a strong positive correlation with AP value,which suggest that high-energy particle play an important role in the formation of PMWE;However,annual occurrence of PMWE has no correlation with F10.7.From the general meaning of F10.7,PMWE is related to solar activity,but it is not a simple linear relationship,which further indicates that PMWE formation is affected by many factors;From the narrow sense of F10.7,there is no direct relationship between the solar Lyman ? radiation and PMWE.This paper also study PMWE of five consecutive days during the solar proton event in November 2004,and find that enhanced ionization in D-region is a prerequisite for the formation of PMWE.The incidence of PMWE during the day is high and the night is very small,because the ionization level of D-region during daytime is higher than night.Two reasons can explain this phenomenon.First,Since the largest amount of ionization is caused by solar irradiance,the night-side of the earth,and the pole pointed away from the sun(depending on the season)have much less ionization than the day-side of the earth,and the pole pointing towards the sun.Second,the increasing attachment rate of electrons to neutral molecules forming negative ions during nighttime,resulting in a largely decrease in electrons density.When the D-region of ionosphere is illuminated,electrons begin to separate from negative ions,and free electrons increase slowly.