A Sub‐Alfvénic solar wind: Interplanetary and magnetosheath observations

Many years of observation have established that the solar wind flow at 1 AU normally is both supersonic and super‐Alfvénic. However, for portions of an ∼5‐hour period on November 22, 1979, the solar wind flow speed (∼320 km s −1 ) observed at ISEE 3 was considerably less than the Alfvén speed (∼540 km s −1 ) that resulted from an abnormally low ion density (∼0.07 cm −3 ). The origin of this sub‐Alfvénic flow remains uncertain: the flow was not associated in any obvious way with the rarefaction region of a high‐speed stream or shock wave disturbance. Intermittent observations of the flow within the earth’s magnetosheath were made with instruments aboard ISEE 1 and 2 during the period when the solar wind flow was sub‐Alfvénic. The bulk of the magnetosheath plasma was compressed, heated, and slowed relative to the upstream solar wind plasma and was directed around the magnetosphere. Superposed on this flow was a hot, suprathermal ion beam with mean energy ∼1 keV directed along the magnetic field towards the magnetosphere. These observations suggest that the bow shock neither dissipated nor disappeared when the solar wind flow was sub‐Alfvénic.