Concurrent observations of solar wind oxygen by Geotail in the magnetosphere and wind in interplanetary space

We present an initial report on the first direct, concurrent measurements of solar wind oxygen (O) charge state variations inside and outside Earth's magnetosphere. On 10-11 Jan 1997 a magnetic cloud, with a spatial scale of at least ∼100 RE engulfed the magnetosphere. At ∼1200 UT on 10 Jan 1997, the Wind/SWICS ion charge-state spectrometer, at ∼90 RE upstream of Earth, detected an increase in the solar wind O charge state ratio Γo = (O+7 + O+8) / O+6 at ∼0.5-30 keV/e which persisted until ∼2400 UT on 10 Jan 1997. After ∼1110 UT on 10 Jan 1997, Geotail was in and near the mid-afternoon to dusk magnetosphere. The ∼9.4-210 keV/e Γo value from the Geotail/STICS spectrometer showed a longer-term, gradual increase over ∼8–10 hr. Then, after ∼0600 UT on 11 Jan 1997, Wind observed a Γo decrease at a high speed solar wind stream onset. About 6 hr later Γo decreased promptly at Geotail during a plasma sheet substorm injection. Both instances are long durations compared to nominal 3-hr substorm time-scales.