Evolution of the outer plasmasphere during low geomagnetic activity observed by the EUV scanner onboard Planet‐B

The extreme ultraviolet scanner onboard the Planet‐B spacecraft has taken EUV images (He II 304 Å) of both the duskside plasmasphere and nearby outer magnetosphere on September 9–10, 1998. Even during a relatively quiet period the images have revealed that there is a significant amount of escape of plasmaspheric ions from the outer plasmasphere toward the magnetosphere. Such a phenomenon is not expected from the conventional teardrop model of the plasmasphere. Plasmaspheric materials of at least 2.9×10 28 He + ions were peeled off the main body of the plasmasphere within 12 hours. This quantity corresponds to an ion flux of 6.7 × 10 24 ions/s under the assumption that the He + /H + ratio is 10%. If geomagnetic storms occur twice a month, this flux is comparable to the predictions for a stormtime transport. This suggests that the plasmasphere may provide the magnetosphere with significant amount of plasmas even when the geomagnetic activity is low.