A Drag-force Analysis of Solar Wind White-light Tracers in the Inner Heliosphere

Abstract

We studied the propagation of blobs, which are a subset of density mesoscale structures observed in the solar corona. The detection of blobs in white-light data was performed during Solar Cycle 23. Blobs are tracers of the solar wind and an important source of its variability. We analyzed the deprojected blob radial elongation and kinematics as they evolved in the inner heliosphere using a dynamical “drag-force” model. We characterized 13 blob-like structures detected by Large Angle Spectrometric Coronagraph and SECCHI coronagraphs aboard Solar and Heliospheric Observatory and Solar TErrestrial RElations Observatory, respectively. We applied, for the first time, analysis techniques that were typically used for coronal mass ejections to these compact plasma blobs that seem to propagate “passively” with the solar wind. This is the first time that the mass of blobs has been reported, with a mean value of 3.32 ± 0.19 × 10¹² g. In addition, blobs show a mean radial expansion rate of 1.10 ± 0.96 × 10⁻¹ R⊙ hr⁻¹. We assumed that the blob movement is governed by a force that is active at a heliocentric distance ~5 R⊙, “dragging” blobs near the Sun outward until they reached a mean final velocity of 427 ± 55 km s⁻¹ at ~15 R⊙. According to the physical parameters involved in this “drag-force” model, the best estimate of the dynamic viscosity of the ambient solar wind is 1.27 ± 0.98 × 10⁻⁴ g cm⁻¹ s⁻¹. This is also the first time that this crucial parameter for aerodynamical studies has been reported close to the Sun.