Plasmaspheric material on high‐latitude open field lines

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During periods of increased geomagnetic activity, cold and dense plasmaspheric material is observed to drain from the inner magnetosphere toward the day side magnetopause. Geosynchronous observations have shown that plasmaspheric material may participate in the dayside reconnection. However, predictions of plasmaspheric material passing through the polar region on recently opened field lines have not yet been confirmed by observations. We present evidence for the existence of such plasmaspheric material on high‐latitude open magnetic field lines based on our investigations of 20 months of Interball/Hyperboloid observations and 11 months of Polar/Thermal Ion Dynamics Experiment (TIDE) observations. In order to distinguish plasmaspheric material from the low‐energy portion of entering magnetosheath plasma from the solar wind, observed phase‐space densities are compared to modeled magnetosheath and plasmaspheric phasespace densities. The phase‐space density distribution function of magnetosheath ions is estimated from upstream solar wind parameters using the gasdynamic theory. Twenty‐one events were found in which Interball passed through open field lines on the dayside during periods of increased geomagnetic activity, two of which show evidence for the presence of plasmaspheric material. Additionally, six such events were identified from the Polar/TIDE database. Although the occurrence frequency is low, evidence of cold plasmaspheric material being transported on high‐latitude open field lines does exist.