The evolution of corotating stream fronts near the ecliptic plane in the inner solar system: 2. Three‐dimensional tilted‐dipole fronts

Abstract

We develop a numerical model that treats the three‐dimensional (3‐D) magnetohydrodynamic interactions taking place at interplanetary corotating stream fronts near the heliographic equator during those periods of the solar cycle when the large‐scale coronal magnetic structure is in the tilted‐dipole configuration. The dynamic simplicity of the tilted‐dipole geometry permits the formulation of a local approximation valid over a limited range of heliographic latitudes and longitudes centered about the stream front. We show that one component of the spatial orientation of the stream interface is fixed by the implied coronal configuration and that this orientation varies in a systematic and predictable way with heliocentric distance. This is important, since the orientation of the interface regulates the pace of the interplanetary evolution by determining the obliquity of the dynamical interaction between fast and slow flows at the stream front. We find that the appearance of 3‐D tilted‐dipole interfaces near the Sun (0.3–0.6 AU) may differ considerably from the classic 2‐D structures described previously in the literature: they should be subtle features, in which the amplitudes of the nonradial flow deflections are only a few degrees and the north‐south component dominates the east‐west component. In particular, we expect that the amplitude of the total nonradial deflection should go roughly as sin α (where α is the dipole tilt angle), whereas the ratio between the north‐south and east‐west deflections should vary as cot α. We also show that magnetic torques should have strong influence on the interface structure near the Sun (inside ∼0.5 AU); the amplitudes and phases of the nonradial flows there are subject to details of the large‐scale magnetic properties of the flow, which can only be assessed observationally. In addition, magnetic stresses carried in the solar wind may inhibit the formation of discontinuous interfaces in this region. The effects of the tilted‐dipole geometry on the evolution of corotating interaction regions and the formation of shocks at larger heliocentric distances will be addressed in a subsequent paper.

Formation, shape, and evolution of magnetic structures in CIRs at 1 AU

“…They found qualitative agreement with Gosling et al [1993] in that solar wind is deflected eastward and equatorward ahead of the CIR and westward and poleward behind the CIR. Clack et al [2000] also observed that a CIR from the Northern coronal hole had a Southern tilt out of the ecliptic, while a CIR from the Southern coronal hole had a Northern tilt out of the ecliptic, as predicted for a simple dipole model of coronal hole structure [Lee, 2000;Pizzo, 1991].…”

“…Previous observational studies [Clack et al, 2000;Gosling et al, 1978Gosling et al, , 1993Riley et al, 1996] of CIRs' three dimensional structure used data from Ulysses. This paper for the first time studies the local three dimensional tilt of CIRs at 1 AU over an entire solar cycle and compares observations to the predictions of Lee [2000] and Pizzo [1991] regarding CIR formation, shape, and evolution.…”

Global MHD Modeling of the Solar Corona and Inner Heliosphere for the Whole Heliosphere Interval

“…Much of the basic structure had been predicted by global MHD simulations performed by Pizzo (1991). However, it was not until Ulysses measurements began to uncover a systematic picture of the properties of CIRs at mid latitudes during the declining phase of Solar Cycle 22, that these earlier numerical results began to be appreciated (Pizzo and Gosling, 1994).…”

Diagnostics of corotating interaction regions with the kinetic properties of iron ions as determined with STEREO/PLASTIC

“…These compression waves steepen and eventually form a shock pair in the forward and in the reverse direction. Increasingly sophisticated models of stream interfaces have been developed over the past decades, beginning from purely hydrodynamic models (Carovillano and Siscoe, 1969;Hundhausen, 1973;Pizzo, 1978) to models including the full set of MHD-equations in three dimensions (Pizzo, 1991). In about three quarters of cases, shocks are formed outside 1 AU, when the angle of the Parker spiral has become sufficiently oblique relative to the radial flow direction.…”