A two‐dimensional simulation of the radial and latitudinal evolution of a solar wind disturbance driven by a fast, high‐pressure coronal mass ejection

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Abstract
Using a hydrodynamic simulation, we have studied the two‐dimensional (symmetry in the azimuthal direction) evolution of a fast, high‐pressure coronal mass ejection (CME) ejected into a solar wind with latitudinal variations similar to those observed by Ulysses. Specifically, the latitudinal structure of the ambient solar wind in the meridional plane is approximated by two zones: At low latitudes (< 20°) the solar wind is slow and dense, while at higher latitudes the solar wind is fast and tenuous. The CME is introduced into this ambient wind as a bell‐shaped pressure pulse in time, spanning from the equator to 45° with a speed and temperature equal to that of the high‐latitude solar wind. We find that such an ejection profile produces radically different disturbance profiles at low and high latitudes. In particular, the low‐latitude portion of the ejecta material drives a highly asymmetric disturbance because of the relative difference in speed between the fast CME and slower ambient solar wind ahead. In contrast, the high‐latitude portion of the same ejecta material drives a much more radially symmetric disturbance because the relative difference in pressure between the CME and ambient background plasma dominates the dynamics. The simulations reveal a number of other interesting features. First, there is significant distortion of the CME in the interplanetary medium. By ∼ 1 AU the CME has effectively separated (in radius as well as latitude) into two pieces. The radial separation is due to the strong velocity shear between the slow and fast ambient solar wind. The latitudinal separation arises from pressure gradients associated with rarefaction regions that develop as the CME propagates outward. Second, there is significant poleward motion of the highest‐latitude portion of the CME and its associated disturbance. The main body of the CME expands poleward by ∼ 18°, while the forward and reverse waves (produced by the overexpanding portion of the CME) propagate all the way to the pole. Third, the simulations show that the high‐pressure region, which develops at low latitudes as the fast CME ploughs through the slow ambient solar wind, penetrates significantly (∼ 10°) into the high‐latitude fast solar wind. We compare the simulation results with a CME‐driven interplanetary disturbance observed at both low and high latitudes and find that the simulation reproduces many of the essential features of the observations.