Three-dimensional and twisted: An MHD interpretation of on-disk observational characteristics of coronal mass ejections

A physical interpretation of observed coronal "on-disk" manifestations of a n Earth-directed coronal mass ejection (CME) is presented. The fundamental question of how the CME;'s magnetic field and its plasma distribution are r elated is largely unanswered, because a crucial piece of the puzzle, that i s the three-dimensional (3-D) morphology of the CME, remains difficult to a scertain so long as coronal observations are limited to projections onto a single plane of the sky. In order to understand the relationship between ob servations of CMEs projected at the solar limb and those projected on the s olar disk, some sort of model of the 3-D CME is required. In this paper we address both the question of the 3-D morphology of the CME and the more fun damental question of the nature of the plasma-magnetic field relationship, by comparing the limb and on-disk CME representations of an analytic 3-D MH D model based on a spheromak-type flue; rope magnetic field configuration. In particular, we show that the morphology of twin dimmings (also referred to as transient coronal holes) observed in X ray and EUV can be reproduced by the CME model as the on-disk projection of the prominence cavity modeled for limb CMEs. Moreover, the bright core of a limb CME, generally correspo nding to the material in an erupting prominence, may be interpreted to be t he S-shaped central core of the modeled on-disk CME, splitting the cavity i nto twin dimmings when observed head-on without obstruction. The magnetic f ield structure of this central core exhibits many of a filament's magnetic field features required to match observations. Finally, we consider the nat ure of S-shaped filaments and X-ray "sigmoids" in the context of the model, in terms of localized heating and cooling acting on the modeled CME magnet ic field structure.