We present results of simulations of a magnetic cloud's evolution during its passage from the solar vicinity (18 solar radii) to approximately 1 AU using a two‐dimensional MHD code. The cloud is a cylinder perpendicular to the ecliptic plane. The external flow is explicitly considered self‐consistently. Our results show that the magnetic cloud retains its basic topology up to 1 AU, although it is distorted due to radially expanding solar wind and magnetic field lines bending. The magnetic cloud expands, faster near the Sun, and faster in the azimuthal direction than in the radial one; its extent is approximately 1.5–2× larger in the azimuthal direction. Magnetic clouds reach approximately the same asymptotic propagation velocity (higher than the background solar wind velocity) despite our assumptions of various initial conditions for their release. Recorded time profiles of the magnetic field magnitude, velocity, and temperature at one point, which would be measured by a hypothetical spacecraft, are qualitatively in agreement with observed profiles. The simulations qualitatively confirm the behavior of magnetic clouds derived from some observations, so they support the interpretations of some magnetic cloud phenomena as magnetically closed regions in the solar wind.

Abstract.A force-free magnetic field with constant alpha for a circular cylindrical flux rope (Lundquist solution) is widely used to describe the magnetic field configuration in interplanetary flux ropes. Observations as well as MHD simulations indicate that interplanetary flux ropes are not circular but have an oblate shape. Here we present an analytical solution for a force-free magnetic field with constant alpha in an elliptic flux rope which may be regarded as a direct generalization of the Lundquist solution. An alternative simpler solution for a force-free magnetic field with constant alpha in an oblate flux rope is discussed.

[1] A magnetic cloud ejected from the Sun is simulated as a part of a toroid. Its evolution and propagation through interplanetary space are studied using three-dimensional magnetohydrodynamic self-consistent numerical simulations. The flux rope deforms as it moves from the Sun, and this deformation causes a hypothetical spacecraft to observe the flux rope two times at some locations; that is, it could be crossed at its apex and a flank. Simulated observations at the apex give all three signatures of a magnetic cloud following from its definition. The apex is rather flat and can be approximated by a prolate (in latitude) cylinder. At the flanks of the flux rope the temperature drop is not pronounced, but the magnetic field increase has a double-peak profile.

This paper continues studies of the cylindrical magnetic clouds' propagation in the interplanetary medium. In our first paper devoted to this topic (Vandas et al., 1995) we dealt with the cloud with the axis perpendicular to the ecliptic plane and derived time dependencies of its velocity, field magnitude, and temperature as well as its shape for different initial conditions. Here, analogously, we present simulations for the cloud with the axis parallel to the ecliptic plane and show that the propagation of these clouds practically does not depend on the inclination of their axes to the ecliptic plane. We made a new conclusion concerning the helicity of the magnetic field inside the cloud. Because of the magnetic interaction with the background field, the cloud is shifted to the side where it meets with the external interplanetary magnetic field (IMF) polarity that is opposite to that within the cloud. The net effect of the time dependent Lorentz, inertial, and pressure gradient forces probably causes the complementary deformation of the whole cloud.

Abstract. Interplanetary shock waves, propagating in the heliosphere faster than earlier-emitted coronal ejecta, penetrate them and modify their parameters during this interaction. Using two and one half dimensional MHD simulations, we show how a magnetic cloud (flux rope) propagating with a speed 3 times higher than the ambient solar wind is affected by an even faster traveling shock wave overtaking the cloud. The magnetic field increases inside the cloud during the interaction as it is compressed in the radial direction and becomes very oblate. The cloud is also accelerated and moves faster, as a whole, while both shocks (driven by the cloud and the faster interplanetary shock) merge upstream of the cloud. This interaction may be a rather common phenomenon due to the frequency of coronal mass ejections and occurrence of shock waves during periods of high solar activity.