A Correlation Study Of The Day-to-day Variability Of F2-layer Peak Electron Density Over Europe

The ionosphere has strong coupling processes and effects to regions below and above. Its interface at low latitudes is to a dense neutral atmosphere, itself modulated by tropospheric climate and weather. At high latitudes, space plasma processes in the magnetosphere, instigated by its coupling to the solar wind and interplanetary field, provide an interface with highly variable input of energetic particles and electrodynamic energy. Such complex coupling processes, not only decide the regular changes of ionospheric morphology, but also the extraordinary complicated random changes (the variabilities), the time scale ranges of which are very wide. Among these variabilities, the day-to-day variability is very representative, the time range of which is one to several days. The affirmed causes of it mainly comprise the solar ionizing radiation, the solar wind and geomagnetic activity, the disturbances from neutral atmosphere below, the seismic shock and electrodynamics, and so on.In this paper, we carry out a statistical correlation study of the day-to-day variability. We adopt the ionospheric F₂-layer peak electron density, NmF₂, obtained from 22 European stations, to study the standard deviationσand correlation distance L of its day-to-day variability, especially the diurnal and seasonal behavior ofσand L, and how they change with solar and geomagnetic activities. At first, we compute the absolute and relative changes,⊿NmF₂ and RNmF₂ of F₂-layer peak electron density NmF₂, in one day step, including all the stations and all the time. Then we use two exponential functions, and , to describe how the covariance, Cov of the day-to-day variability between any two stations, changes with its distance d, and obtain the standard deviationσ_Δandσ_R, and correlation distance L_Δand L_R , and we consider them as average. Further more, according to the ranges of the parameters characterizing its possible causes, we divide all the data into small groups, and study the diurnal changes of the standard deviation and correlation distance in each group, versus local time at different geomagnetic active levels (quiet and disturbed), different solar active levels (low, medium and high) and different seasons (spring, summer, autumn and winter) in great detail. We find the following results:â’ˆIn all groups according to different parameters, the shapes of the standard deviation and correlation distance, are quite similar with its corresponding average. In general,σ_Δis larger at daytime than nighttime, and reaches its maximum at 10:00 or 11:00, its minimum at 04:00 or 05:00;σ_R is smaller at daytime than nighttime, reach its maximum at 01:00 and minimum at 16:00 or 17:00; both of L_Δand L_R are larger at daytime than nighttime, and reach their maximum at 06:00 or 07:00 and minimum at 01:00.â’‰The standard deviation and correlation distance, become larger as the geomagnetic activity become more intense, namely get larger than the average when the level of geomagnetic activity is disturbed, get smaller when it is quiet. As forσ_Δ, the range of increase gets wider at daytime; as forσ_R, it changes little verse LT; As for L_Δand L_R , it gets widest at sunrise.â’ŠThe standard deviation and correlation distance, become larger as the solar activity become more intense, namely get smaller than the average when the level of solar activity is low, get larger when it is high or medium. As for the standard deviation, the ranges of increase get widest at 10:00; as for correlation distance, they get widest at sunrise.â’‹There are seasonal changes of standard deviation and correlation distance. As forσ_Δ, during the day, it is largest in spring and autumn, and smallest in winter; At night, it is largest in summer, smallest in winter, and the difference between spring and autumn are quiet small. As forσ_R, during the day, it is largest in spring and autumn, and smallest in winter; At night, it is largest in winter, smallest in summer, and the difference between spring and autumn are quiet small. As for L_Δand L_R, they are largest in summer, and smallest in winter. In spring and autumn, the correlation distance has a remarkable increase at sunrise.â’ŒAt around mid-night, the standard deviation change little with the solar activity levels, the correlation distance change little with the geomagnetic and solar activity levels or seasons.Based on the above results, we conclude that:â‘´The influences of solar ionizing radiation to the ionospheric day-to-day variability are of large-scale, and are larger at daytime and when the solar activity level is high than at nighttime and when the solar activity level is low.⑵The influences of geomagnetic activities are also of large-scale, and are larger when the geomagnetic activity level is disturbed than it is quiet.⑶The meteorological influences are relatively of small-scale, and the day-to-day variability has seasonal trends.From the standpoint of the correlation scales, we validate the conclusion that the causes of ionospheric day-to-day variability are the solar ionizing radiation, the geomagnetic activity, and disturbances from the meteorological activities.