Occurrence statistics of cold, streaming ions in the near‐Earth magnetotail: Survey of Polar‐TIDE observations

Abstract

Results are presented from a survey of cold ion observations in the near‐Earth magnetotail using data from the Polar Thermal Ion Dynamics Experiment (TIDE). During the interval from July to December of 2001, Polar had its apogee (∼9.5RE) near the equatorial plane in the tail region of the magnetosphere. It is shown that a lobal wind is ubiquitous in the inner tail, with low‐energy (<300 eV) ions streaming from the ionosphere downtail. These lobal winds often pass through the plasma sheet, forming bidirectional streams, in addition to the unidirectional beams seen at higher magnetic latitudes. The observance of bidirectional streams is inversely, although weakly, correlated with geomagnetic activity. Bidirectional streams are interpreted as indicating the minimum size of the closed flux tube region. The reduced frequency of bidirectional streams with activity level times is consistent with the thinning of the plasma sheet during these times. It is inferred from the universality of these observations during Polar's passage through the inner tail region that the ionosphere is a continuous supplier of plasma to the near‐Earth magnetosphere. The high occurrence rate of these streams means that during geomagnetic disturbances, it is not necessary to wait for outflow and magnetospheric circulation in order to supply the inner magnetosphere with ionospheric ions; these cold streams are an immediately available supply of ionospheric‐origin particles.