| The ionosphere is an important part of the Earth’s upper atmosphere and an important hub connecting the near-Earth atmosphere and outer space in the solar-terrestrial environment.It is also closely related to satellite navigation,wireless communication and human space activities.In addition,various particles in the ionosphere undergo complicated changes due to the influence of instability factors such as neutral wind and electric fields,forming the irregular structures of ionospheric electron density(abbreviated ionospheric irregularities).In the low and middle latitude regions,the occurrence of ionospheric irregularities is the largest and the impact is the most serious.Therefore,it is meaningful to study the distribution characteristics of ionospheric irregularities in the low and middle latitude regions.The ESA Swarm constellation is composed of three near-polar orbit satellites,and the Langmuir probe it carries provides rich in-situ observations data for studying the characteristics of the spatiotemporal changes in the irregular structures of ionospheric Ne in the low and middle latitude.As a near-polar orbit satellite,the Swarm satellite helps to study the variation characteristics of the irregular structures of ionosphere electron density in the low and middle latitude regions along the meridional direction.At present,there are few reports on the change characteristics of the irregular structures of ionospheric in the low and middle latitude regions along the geomagnetic meridian direction(i.e.,the meridional direction).In view of this,the following work was carried out here by using the electron density observations data provided by the in-situ observations of swarm satellites from January 2014 to December 2016.First,the flight range of each Swarm satellite is geographic latitudes of 87.35°S–87.35°N,and the time required to fly over this geographic latitudes range is about 47 minutes.The electron density data obtained by the in-situ observations of the Swarm satellites Langmuir probe is stored in different data files in units of days.This paper used Matlab software to develop a program for segmented extraction of electron density data from Swarm satellite in-situ observations in the low and middle latitude regions(referring to the geographic latitude range of 40°S–40°N)and verified the correctness of the program.Second,in order to study the distribution characteristics of the low and middle latitude ionospheric irregularities,a program for automatically identifying and classifying the low and middle latitude ionospheric irregularities were developed.The program was then used to automatically identify and classify the electron density data obtained from the in-situ observations of three Swarm satellites in the geographic latitude 40°S–40°N between January 2014 and December 2016.Finally,the results of automatic program identification and classification were manually verified.The results of manual verification prove the applicability of the program that developed in this paper to automatically identify and classify the low and middle latitude ionospheric irregularities.Third,since there are few reports on the change characteristics of the irregular structures of ionospheric in the low and middle latitude regions along the geomagnetic meridian direction(i.e.,the meridional direction),this article analyzed the electron density observations from in-situ observations by the three Swarm satellites from January 2014 to December 2016 in the Asia-Pacific region(i.e.,the region with a geographic longitude ranging from 90 ° E to 150 ° E)and South America(i.e.,the region with a geographic longitude ranging from-81 ° E to-63 ° E).The analysis results show that the low and middle latitude ionospheric irregularities mainly occur at night(i.e.,local time 18h–6h),which are consistent with the conclusion that the low and middle latitude ionospheric irregularities produced by the generalized R-T mechanism.From 2014 to 2016,with the gradual weakening of solar activity intensity,the occurrence of the low and middle latitude ionospheric irregularities also decreased year by year,which is the same as the occurrence of ionospheric irregularities and solar activity intensity obtained by previous studies.The relationship between them is consistent.From 2014 to 2016,in most cases(the occurrence rate in the Asia-Pacific region is 52% –83%,and the occurrence rate in the South America region is 54% –69%),the low and middle latitude ionospheric irregularities exhibit northsouth symmetry.Fourth,due to oceanic and geopolitical reasons,there are large areas on the earth where there are not possible to establish ground-based GNSS(Global Navigation Satellite System)stations to study the L-band ionospheric scintillation caused by the ionospheric irregularities.The in-situ observations data of electron density of the Swarm satellites provide a rich data source for studying the distribution characteristics of the ionospheric irregularities in the above regions.This paper has compared the ionospheric irregularities between the in-situ observations of the Swarm satellites from January 2014 to December 2016 and the groundbased GPS ionospheric scintillation observations of the Sanya station that observed simultaneously and in the same place.The comparison results are divided into three cases,namely: the cases of the ionospheric irregularities detected by the in-situ observations of Swarm satellites and the ground-based GPS ionospheric scintillation observations of the Sanya station,the cases of the ionospheric irregularities detected only by the in-situ observations of Swarm satellites,and the cases of the ionospheric irregularities detected only by the ground-based GPS ionospheric scintillation observations of the Sanya station.The analysis results in this paper show that the probability of detecting the ionospheric irregularities by both means is 27.27%,the probability of detecting the ionospheric irregularities by the in-situ observations of Swarm satellites is 60.61%,and the probability of detecting the ionospheric irregularities by the ground-based GPS ionospheric scintillation observations of the Sanya station is 12.12%.These results show that although the simultaneous and co-location monitoring by using different observation method is a new way to deeply study the distribution characteristics of the low and middle latitude ionospheric irregularities,the results obtained by combining the two methods of in-situ electron density observations of Swarm satellites and the ground-based GPS ionospheric scintillation observations are not very good.This paper analyzed the possible reasons for the poor consistency of the results obtained by these two observation methods. |
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