In situ observations of low-density regions inside the plasmasphere

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Thermal plasma measurements performed on MAGION 5 (subsatellite of INTERBALL 1) in the plasmasphere of the Earth are analyzed in conjunction with simultaneous solar wind data and ground-based ionospheric measurements in quiet geomagnetic conditions, during and after geomagnetic storms. In situ satellite observations reveal the existence of long-lived (2–3 days) depleted regions (MLT width ≳ 1.5–2 hours) in the plasmasphere that extend out from L ∼ 3. These observations well correspond to ‘notch’ or ‘bite-out’ regions found by the IMAGE spacecraft. Possible reasons for the formation of low-density regions are discussed.