Ion streams in the magnetotail

Ion mass spectrometer observations of low‐temperature streaming plasmas in the earth's magnetotail are reported. Measurements in the energy per charge range 0 ≤ E/q ≤17 keV/ e were made at geocentric radial distances <23 R E from the ISEE 1 spacecraft. Ion streams of solar wind origin in the magnetotail boundary layer and of ionospheric origin in the tail lobes and plasma sheet are described. A statistical study of some of the characteristics of the streams allows us to infer that the central tail lobe plasmas are primarily constituted of ion streams of ionospheric origin and that the ionosphere is a significant contributor to the hot plasmas that form the plasma sheet.