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 A) 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.

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