Fig. 4

Seven EUV images obtained by the TEX instrument on KAGUYA. The image was taken at 2-h intervals with the exception of the period between (d) and (e). The Earth is located near the upper edge of each image. The dashed arc on the Earth represents the Earth’s geomagnetic equator, and the dot on the Earth is the sub-solar point. The inside and outside contour lines are drawn at count rates of 2.0 and 1.0 count/min, respectively. The two white circles are in place to mask the circular ghosts due to an instrument problem. The geomagnetic field lines (L = 3) are indicated at magnetic local time (MLT) to align the enhanced density structure. With the exception of image (a), an enhanced density structure, indicated by the black arrow, is clearly seen in each image. It is noted that the magnetic field line in the consecutive image from (b) to (d) is identical, i.e., it co-rotates with the Earth. The magnetic field line in the series of images from (e) to (g) is also identical, but has a time lag of about 2 h from that in (b) to (d).