We have compared two quantities which were simultaneously observed by two different instruments at the same point of the plasmaspheric plume. One quantity is the field-line eigenfrequency, which was obtained by applying the dual-station H-component power ratio method to geomagnetic data obtained from two ground magnetometers (at Tixie and Chokurdakh) of the Circum-pan Pacific Magnetometer Network (CPMN). The other is the He+ column abundance obtained by the IMAGE satellite Extreme Ultraviolet Imager (IMAGE/EUV). We used the EUV He+ column abundance data mapped to the equatorial footpoint of the field-line whose eigenfrequency we obtained from the ground data. The mapped EUV He+ column abundance is roughly proportional to the equatorial density of He+. The result of the IMAGE/EUV analysis showed an increase-then-decrease pattern of the He+ column abundance. On the other hand, as a result of analyzing the simultaneously observed ground magnetometer data, we found that the eigenfrequency showed a coherent decrease-then-increase pattern. Since this pattern takes place if the equatorial plasma density along the field line increased and then decreased, these two results are qualitatively consistent. In addition, the H-power ratio showed an offset when either Tixie or Chokurdakh (these stations are longitudinally separated) stayed inside the plume. This feature can be explained if the overall ULF wave power was weaker in the plume than in the surrounding trough. With the above findings, this paper is the first to identify a plume from both the spacecraft image and the ground magnetometer ULF waves.