Fig. 1

Evolution of the a longitudinal separation and b time lag between Swarm spacecrafts when they are passing the geographic equator during the early mission phase: December 09, 2013–January 22, 2014. The three spacecrafts were divided into three pairs and presented with different colors. During this period, the local times covered by the three spacecrafts were almost the same, which decreased from 0100 to 2100 LT for the descending node. Therefore, we only present the local time for Swarm A, as indicated by the gray dashed line in frame (b). Frames c and d are the same as a and b, but for Swarm A and C during the final constellation phase: April 17, 2014–September 27, 2015. The local time for the descending node of Swarm A has also been presented by gray dashed line in frame (c)