An estimate of the maximum speed of the solar wind, 1938–1989

Abstract

Beginning with a list of severe geomagnetic storms, we obtain an estimate of the maximum solar wind flow speed at Earth during the past ∼50 years. We do this by (1) focusing on the subset of severe storms that followed major proton flares (since large proton events are strongly associated with fast coronal mass ejections), (2) calculating the average speed of the associated interplanetary shocks from the time intervals between the flares and the onsets of the storms, and (3) using an empirical relationship between the average shock speed and the maximum flow speed of the associated transient stream to infer a peak flow velocity for each event. We find no evidence for bulk flow velocities greater than the ≳2000‐km/s value deduced from Prognoz 2 and HEOS 2 in situ plasma measurements for the August 4, 1972, event. The ≳2000‐km/s speed for that event does not appear to be anomalously high, however; there are other credible events, occurring before 1960, with inferred flow speeds approaching this value. The ≳2000‐km/s value lies at the high‐speed edge of a reasonable single distribution of peak solar wind speeds for a representative sample of the most energetic solar‐terrestrial events observed from 1938 to 1989.