Characteristics Of Electron Density Variation In Ionospheric F2 Layer At Mid-high Latitude In The Northern Hemisphere

The Earth's ionosphere is a region about 60-1000km above the ground in which neutral atmospheric elements are partially ionized by solar radiation.This region is the link between near-earth space and outer space.It is the key region for the energy transmission of the solar-earth system,and it is also an important place for the effects of space weather.In the ionosphere,the electron density varies with the height,and F2 layer is usually the region with the highest electron density.The peak electron density of F2 layer(NmF2)is often used to characterize the diurnal variation characteristics of the ionosphere.Under the influence of electrodynamics and other processes in the ionosphere,the evolution process of NmF2 often deviates from Chapman's theory,that is,the phenomenon of"anomaly" appears,which shows that NmF2 abnormally increases and decreases under different solar illumination conditions.These ionospheric "anomalies" can directly affect over-the-horizon radar detection,wireless communication and navigation,etc.,so they have been the focus of research at home and abroad.At present,previous studies mainly focus on the distribution characteristics and generation mechanism of the ionospheric "anomalies" in the middle and low latitudes.However,due to the complex ionospheric dynamics process,the distribution characteristics and generation mechanism of the ionospheric"anomalies" in the middle and high latitudes are far from clear.In this paper,we focus on the ionospheric "anomalies" in mid-high latitudes.According to the "anomalies" in different solar illumination conditions(photoionization)associated with the diurnal variation of ionospheric NmF2,these anomalies can be classified as:nighttime enhancement,dusk enhancement and forenoon "bite-out".Based on 16 years' continuous ionospheric digital altimeter data in the middle and high latitudes of the Northern Hemisphere,the seasonal distribution and variation characteristics of the above "anomaly"phenomena with the level of solar activity are statistically studied,and these"anomaly" phenomena are simulated by using the empirical model E-CHAIM in the middle and high latitudes.To explore its regional distribution characteristics and the influencing factors of these characteristics.The main research results of this paper are as follows:1.Nighttime enhancement.In the middle and high latitudes of the northern hemisphere,the photoionization disappears completely because there is no light at night.Under normal circumstances,the ionospheric NmF2 will decrease rapidly at night,but sometimes it will stop decreasing and turn to increase at night and reach the peak near midnight,which is the phenomenon of Nighttime enhancement.The Nighttime enhancement mainly occurred in November,December,January and February,and its incidence and relative enhancement amplitude were negatively correlated with the solar activity.This phenomenon is mainly caused by the injection of plasma from the plasma layer and the top ionosphere into the F2 layer under the action of electrodynamic drift.The simulation results of E-CHAIM model are in agreement with the observation,which reproduces the Nighttime enhancement and captures the spatiotemporal distribution characteristics of nocturnal enhancement in the middle and high latitudes of the Northern Hemisphere:mainly distributed in eastern Europe and northeastern Asia(around 60° N,60° E)and along the geomagnetic 50° N with a width of about 10°.This distribution may be related to the variation of downward plasma flux caused by the tilt of geomagnetic field at different longitudes,and the variation of radial wind and electric field drift caused by the meridional variation of geomagnetic field.2.Dusk enhancement.In the middle and high latitudes of the northern hemisphere,the photoionization decreases as the light decreases in the afternoon.Normally,the ionospheric NmF2 decreases gradually in the afternoon,but sometimes it stops decreasing and turns to increasing in the afternoon near dusk,and reaches the peak near sunset,which is the dusk enhancement phenomenon.Dusk enhancement mainly occurred in May,June,July and August,and its incidence and relative intensification amplitude were negatively correlated with the solar activity.This phenomenon is mainly caused by the combined action of photoionization near sunset and F2 layer uplift caused by equatorial neutral wind.The simulation results of E-CHAIM model are consistent with the observation,which reproduces the dusk intensification phenomenon,and obtains the temporal and spatial distribution characteristics of dusk intensification in the middle and high latitudes of the Northern Hemisphere:Mainly distributed in central North America(around 45° N,90° W),western Europe(around 55° N,15° E),and northeastern Asia(around 60° N,150° E),and along the geomagnetic 50 ° N line.This distribution is closely related to the regional distribution of the significant increase of hmF2 near sunset in the Northern Hemisphere.3.Forenoon "bite-out".In the middle and high latitudes of the northern hemisphere,morning due to enhanced light illumination ionization effect is enhanced,under normal circumstances the ionosphere NmF2 will gradually increase,in the morning and at noon near the peak,but sometimes NmF2 stop increase and in the morning to dropped significantly,and in the noon of reaching a minimum,this is forenoon "bite-out".The forenoon "bite-out" mainly occurred in May,June,July and August in local summer,and the incidence and bite loss degree increased with the increase of solar activity.This phenomenon is mainly due to the joint action of plasma downward transport caused by neutral wind and plasma upward diffusion caused by rising electron temperature.