Comparison of Doppler scintillation and in situ spacecraft plasma measurements of interplanetary disturbances

Abstract

Since 1979 the radio path of the Pioneer Venus orbiter (PVO) spacecraft has spent considerable time probing the near‐Sun solar wind off the limbs of the Sun. On occasion, Helios 1 has also been in position above the Sun's limb at the same solar longitude to simultaneously observe in situ the solar wind plasma. These fortuitous circumstances, along with the availability of near‐continuous measurements, have made it possible for the first time to carry out detailed comparisons between Doppler scintillation and in situ plasma measurements and to improve our understanding of Doppler scintillation transients. During a combined observing period of nearly 3 months in 1981–1982 near solar maximum, 22 transients were observed by PVO and 23 shocks were observed by Helios 1. On the basis of a comparison of mass flux density and rms Doppler scintillations, we find that at least 84% of the transients are shocks, while at least 90% of the shocks are transients. Hence there is a near one‐to‐one correspondence between transients and interplanetary shocks. Although the temporal profiles of Doppler scintillation and mass flux density are similar, the magnitudes of the Doppler scintillation transients may not simply reflect those of mass flux density. Only one pronounced solar wind event that was observed in the mass flux density measurements showed no signature in the scintillation data; field and particle measurements by Helios 1 suggest that it is a noncompressive density enhancement and/or a magnetic cloud. One of the scintillation transients that is not a fast‐mode shock appears to correspond to a slow shock. However, when scintillations alone are available, slow shocks may be difficult to identify. Shock speeds based on transit times between the PVO radio scintillation path and the Helios 1 spacecraft are consistent with those from the in situ plasma measurements and indicate shock deceleration in essentially all cases. A significant consequence of this investigation is that Doppler scintillation measurements can now be used by themselves to detect and locate interplanetary shocks near the Sun with a relatively high degree of confidence and hence can be used to conduct useful correlative studies in the future with other solar and interplanetary observations.