We present a 2.5D MHD simulation of a magnetic flux rope (FR) propagating in
the heliosphere and investigate the cause of the observed sharp plasma beta
transition. Specifically, we consider a strong internal magnetic field and an
explosive fast start, such that the plasma beta is significantly lower in the
FR than the sheath region that is formed ahead. This leads to an unusual FR
morphology in the first stage of propagation, while the more traditional view
(e.g. from space weather simulations like Enlil) of a `pancake' shaped FR is
observed as it approaches 1 AU. We investigate how an equipartition line,
defined by a magnetic Weber number, surrounding a core region of a propagating
FR can demarcate a boundary layer where there is a sharp transition in the
plasma beta. The substructure affects the distribution of toroidal flux, with
the majority of the flux remaining in a small core region which maintains a
quasi-cylindrical structure. Quantitatively, we investigate a locus of points
where the kinetic energy density of the relative inflow field is equal to the
energy density of the transverse magnetic field (i.e. effective tension force).
The simulation provides compelling evidence that at all heliocentric distances
the distribution of toroidal magnetic flux away from the FR axis is not linear;
with 80% of the toroidal flux occurring within 40% of the distance from the FR
axis. Thus our simulation displays evidence that the competing ideas of a
pancaking structure observed remotely can coexist with a quasi-cylindrical
magnetic structure seen in situ.
