Deriving CME density from remote sensing data and comparison to in-situ measurements

TL;DR

This study combines remote sensing and in-situ data to estimate CME densities near the Sun and at 1AU, revealing correlations and uncertainties in mass and density evolution during propagation.

Contribution

It introduces a method to derive CME density from combined white-light observations and compares these estimates with in-situ measurements, highlighting the role of solar wind in sheath formation.

Findings

Moderate correlation between CME magnetic structure mass and in-situ proton density (~0.56-0.59)

Weak correlation for sheath density with in-situ measurements (~0.26)

Strong correlation between sheath density and solar wind parameters (~ -0.73 with density, 0.56 with speed)

Abstract

We determine the 3D geometry and deprojected mass of 29 well-observed coronal mass ejections (CMEs) and their interplanetary counterparts (ICMEs) using combined STEREO-SOHO white-light data. From the geometry parameters we calculate the volume of the CME for the magnetic ejecta (flux-rope type geometry) and sheath structure (shell-like geometry resembling the (I)CME frontal rim). Working under the assumption that the CME mass is roughly equally distributed within a specific volume, we expand the CME self-similarly and calculate the CME density for distances close to the Sun (15-30 Rs) and at 1AU. Specific trends are derived comparing calculated and in-situ measured proton densities at 1AU, though large uncertainties are revealed due to the unknown mass and geometry evolution: i) a moderate correlation for the magnetic structure having a mass that stays rather constant (~0.56-0.59), and ii) a weak correlation for the sheath density (~0.26) by assuming the sheath region is an extra mass - as expected for a mass pile-up process - that is in its amount comparable to the initial CME deprojected mass. High correlations are derived between in-situ measured sheath density and the solar wind density (~ -0.73) and solar wind speed (~0.56) as measured 24 hours ahead of the arrival of the disturbance. This gives additional confirmation that the sheath-plasma indeed stems from piled-up solar wind material. While the CME interplanetary propagation speed is not related to the sheath density, the size of the CME may play some role in how much material could be piled up.