Daytime ionospheric longitudinal gradients seen in the observations from a regional BeiDou GEO receiver network

Many studies have devoted to the longitudinal variations of the ionosphere globally. However, the ionospheric longitudinal variations in a small region are rarely reported. In this paper, we for the first time use total electron content (TEC) data from a BeiDou geostationary orbit (GEO) receiver network to observe and investigate ionospheric longitudinal variations within the zonal scale of 1000km over Central China (112 degrees-122 degrees E, 27 degrees-31 degrees N; 20 degrees-24 degrees N magnetic latitudes) during the period from June 2015 to December 2016. The BeiDou GEO TEC provides a good data set to study longitudinal variations, compared with non-GEO TEC, without contaminating the spatial variations and elevation changes due to satellite motion. Pronounced daytime longitudinal gradients within the distance of 1000km are present in BeiDou GEO TEC. It was found that the TEC is generally larger in the west than in the east. The maximum TEC longitudinal gradient can reach 45 total electron content unit (TECU; 1TECU=10(16)elm(-2)). For most events, the obvious daytime longitudinal gradients are accompanied by the TEC enhancement. The occurrence rate of daytime longitudinal gradients under different geomagnetic activities is similar, whereas strong daytime longitudinal gradients mainly occur under the moderate and strong disturbance geomagnetic activities. These observations suggest that the electric field disturbances could have significant effects on producing the observed ionospheric longitudinal gradients.